//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // Peephole optimize the CFG. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Utils/Local.h" #include "llvm/ADT/DenseMap.h" #include "llvm/ADT/STLExtras.h" #include "llvm/ADT/SetVector.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/SmallVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/ConstantFolding.h" #include "llvm/Analysis/InstructionSimplify.h" #include "llvm/Analysis/TargetTransformInfo.h" #include "llvm/Analysis/ValueTracking.h" #include "llvm/IR/CFG.h" #include "llvm/IR/ConstantRange.h" #include "llvm/IR/Constants.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/DerivedTypes.h" #include "llvm/IR/GlobalVariable.h" #include "llvm/IR/IRBuilder.h" #include "llvm/IR/Instructions.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/IR/LLVMContext.h" #include "llvm/IR/MDBuilder.h" #include "llvm/IR/Metadata.h" #include "llvm/IR/Module.h" #include "llvm/IR/NoFolder.h" #include "llvm/IR/Operator.h" #include "llvm/IR/PatternMatch.h" #include "llvm/IR/Type.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include #include #include using namespace llvm; using namespace PatternMatch; #define DEBUG_TYPE "simplifycfg" static cl::opt PHINodeFoldingThreshold("phi-node-folding-threshold", cl::Hidden, cl::init(1), cl::desc("Control the amount of phi node folding to perform (default = 1)")); static cl::opt DupRet("simplifycfg-dup-ret", cl::Hidden, cl::init(false), cl::desc("Duplicate return instructions into unconditional branches")); static cl::opt SinkCommon("simplifycfg-sink-common", cl::Hidden, cl::init(true), cl::desc("Sink common instructions down to the end block")); static cl::opt HoistCondStores( "simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true), cl::desc("Hoist conditional stores if an unconditional store precedes")); STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps"); STATISTIC(NumLookupTables, "Number of switch instructions turned into lookup tables"); STATISTIC(NumLookupTablesHoles, "Number of switch instructions turned into lookup tables (holes checked)"); STATISTIC(NumSinkCommons, "Number of common instructions sunk down to the end block"); STATISTIC(NumSpeculations, "Number of speculative executed instructions"); namespace { /// ValueEqualityComparisonCase - Represents a case of a switch. struct ValueEqualityComparisonCase { ConstantInt *Value; BasicBlock *Dest; ValueEqualityComparisonCase(ConstantInt *Value, BasicBlock *Dest) : Value(Value), Dest(Dest) {} bool operator<(ValueEqualityComparisonCase RHS) const { // Comparing pointers is ok as we only rely on the order for uniquing. return Value < RHS.Value; } bool operator==(BasicBlock *RHSDest) const { return Dest == RHSDest; } }; class SimplifyCFGOpt { const TargetTransformInfo &TTI; const DataLayout *const DL; Value *isValueEqualityComparison(TerminatorInst *TI); BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI, std::vector &Cases); bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, BasicBlock *Pred, IRBuilder<> &Builder); bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI, IRBuilder<> &Builder); bool SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder); bool SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder); bool SimplifyUnreachable(UnreachableInst *UI); bool SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder); bool SimplifyIndirectBr(IndirectBrInst *IBI); bool SimplifyUncondBranch(BranchInst *BI, IRBuilder <> &Builder); bool SimplifyCondBranch(BranchInst *BI, IRBuilder <>&Builder); public: SimplifyCFGOpt(const TargetTransformInfo &TTI, const DataLayout *DL) : TTI(TTI), DL(DL) {} bool run(BasicBlock *BB); }; } /// SafeToMergeTerminators - Return true if it is safe to merge these two /// terminator instructions together. /// static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) { if (SI1 == SI2) return false; // Can't merge with self! // It is not safe to merge these two switch instructions if they have a common // successor, and if that successor has a PHI node, and if *that* PHI node has // conflicting incoming values from the two switch blocks. BasicBlock *SI1BB = SI1->getParent(); BasicBlock *SI2BB = SI2->getParent(); SmallPtrSet SI1Succs(succ_begin(SI1BB), succ_end(SI1BB)); for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I) if (SI1Succs.count(*I)) for (BasicBlock::iterator BBI = (*I)->begin(); isa(BBI); ++BBI) { PHINode *PN = cast(BBI); if (PN->getIncomingValueForBlock(SI1BB) != PN->getIncomingValueForBlock(SI2BB)) return false; } return true; } /// isProfitableToFoldUnconditional - Return true if it is safe and profitable /// to merge these two terminator instructions together, where SI1 is an /// unconditional branch. PhiNodes will store all PHI nodes in common /// successors. /// static bool isProfitableToFoldUnconditional(BranchInst *SI1, BranchInst *SI2, Instruction *Cond, SmallVectorImpl &PhiNodes) { if (SI1 == SI2) return false; // Can't merge with self! assert(SI1->isUnconditional() && SI2->isConditional()); // We fold the unconditional branch if we can easily update all PHI nodes in // common successors: // 1> We have a constant incoming value for the conditional branch; // 2> We have "Cond" as the incoming value for the unconditional branch; // 3> SI2->getCondition() and Cond have same operands. CmpInst *Ci2 = dyn_cast(SI2->getCondition()); if (!Ci2) return false; if (!(Cond->getOperand(0) == Ci2->getOperand(0) && Cond->getOperand(1) == Ci2->getOperand(1)) && !(Cond->getOperand(0) == Ci2->getOperand(1) && Cond->getOperand(1) == Ci2->getOperand(0))) return false; BasicBlock *SI1BB = SI1->getParent(); BasicBlock *SI2BB = SI2->getParent(); SmallPtrSet SI1Succs(succ_begin(SI1BB), succ_end(SI1BB)); for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I) if (SI1Succs.count(*I)) for (BasicBlock::iterator BBI = (*I)->begin(); isa(BBI); ++BBI) { PHINode *PN = cast(BBI); if (PN->getIncomingValueForBlock(SI1BB) != Cond || !isa(PN->getIncomingValueForBlock(SI2BB))) return false; PhiNodes.push_back(PN); } return true; } /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will /// now be entries in it from the 'NewPred' block. The values that will be /// flowing into the PHI nodes will be the same as those coming in from /// ExistPred, an existing predecessor of Succ. static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred, BasicBlock *ExistPred) { if (!isa(Succ->begin())) return; // Quick exit if nothing to do PHINode *PN; for (BasicBlock::iterator I = Succ->begin(); (PN = dyn_cast(I)); ++I) PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred); } /// ComputeSpeculationCost - Compute an abstract "cost" of speculating the /// given instruction, which is assumed to be safe to speculate. 1 means /// cheap, 2 means less cheap, and UINT_MAX means prohibitively expensive. static unsigned ComputeSpeculationCost(const User *I, const DataLayout *DL) { assert(isSafeToSpeculativelyExecute(I, DL) && "Instruction is not safe to speculatively execute!"); switch (Operator::getOpcode(I)) { default: // In doubt, be conservative. return UINT_MAX; case Instruction::GetElementPtr: // GEPs are cheap if all indices are constant. if (!cast(I)->hasAllConstantIndices()) return UINT_MAX; return 1; case Instruction::ExtractValue: case Instruction::Load: case Instruction::Add: case Instruction::Sub: case Instruction::And: case Instruction::Or: case Instruction::Xor: case Instruction::Shl: case Instruction::LShr: case Instruction::AShr: case Instruction::ICmp: case Instruction::Trunc: case Instruction::ZExt: case Instruction::SExt: case Instruction::BitCast: case Instruction::ExtractElement: case Instruction::InsertElement: return 1; // These are all cheap. case Instruction::Call: case Instruction::Select: return 2; } } /// DominatesMergePoint - If we have a merge point of an "if condition" as /// accepted above, return true if the specified value dominates the block. We /// don't handle the true generality of domination here, just a special case /// which works well enough for us. /// /// If AggressiveInsts is non-null, and if V does not dominate BB, we check to /// see if V (which must be an instruction) and its recursive operands /// that do not dominate BB have a combined cost lower than CostRemaining and /// are non-trapping. If both are true, the instruction is inserted into the /// set and true is returned. /// /// The cost for most non-trapping instructions is defined as 1 except for /// Select whose cost is 2. /// /// After this function returns, CostRemaining is decreased by the cost of /// V plus its non-dominating operands. If that cost is greater than /// CostRemaining, false is returned and CostRemaining is undefined. static bool DominatesMergePoint(Value *V, BasicBlock *BB, SmallPtrSet *AggressiveInsts, unsigned &CostRemaining, const DataLayout *DL) { Instruction *I = dyn_cast(V); if (!I) { // Non-instructions all dominate instructions, but not all constantexprs // can be executed unconditionally. if (ConstantExpr *C = dyn_cast(V)) if (C->canTrap()) return false; return true; } BasicBlock *PBB = I->getParent(); // We don't want to allow weird loops that might have the "if condition" in // the bottom of this block. if (PBB == BB) return false; // If this instruction is defined in a block that contains an unconditional // branch to BB, then it must be in the 'conditional' part of the "if // statement". If not, it definitely dominates the region. BranchInst *BI = dyn_cast(PBB->getTerminator()); if (!BI || BI->isConditional() || BI->getSuccessor(0) != BB) return true; // If we aren't allowing aggressive promotion anymore, then don't consider // instructions in the 'if region'. if (!AggressiveInsts) return false; // If we have seen this instruction before, don't count it again. if (AggressiveInsts->count(I)) return true; // Okay, it looks like the instruction IS in the "condition". Check to // see if it's a cheap instruction to unconditionally compute, and if it // only uses stuff defined outside of the condition. If so, hoist it out. if (!isSafeToSpeculativelyExecute(I, DL)) return false; unsigned Cost = ComputeSpeculationCost(I, DL); if (Cost > CostRemaining) return false; CostRemaining -= Cost; // Okay, we can only really hoist these out if their operands do // not take us over the cost threshold. for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) if (!DominatesMergePoint(*i, BB, AggressiveInsts, CostRemaining, DL)) return false; // Okay, it's safe to do this! Remember this instruction. AggressiveInsts->insert(I); return true; } /// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr /// and PointerNullValue. Return NULL if value is not a constant int. static ConstantInt *GetConstantInt(Value *V, const DataLayout *DL) { // Normal constant int. ConstantInt *CI = dyn_cast(V); if (CI || !DL || !isa(V) || !V->getType()->isPointerTy()) return CI; // This is some kind of pointer constant. Turn it into a pointer-sized // ConstantInt if possible. IntegerType *PtrTy = cast(DL->getIntPtrType(V->getType())); // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*). if (isa(V)) return ConstantInt::get(PtrTy, 0); // IntToPtr const int. if (ConstantExpr *CE = dyn_cast(V)) if (CE->getOpcode() == Instruction::IntToPtr) if (ConstantInt *CI = dyn_cast(CE->getOperand(0))) { // The constant is very likely to have the right type already. if (CI->getType() == PtrTy) return CI; else return cast (ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false)); } return nullptr; } /// GatherConstantCompares - Given a potentially 'or'd or 'and'd together /// collection of icmp eq/ne instructions that compare a value against a /// constant, return the value being compared, and stick the constant into the /// Values vector. static Value * GatherConstantCompares(Value *V, std::vector &Vals, Value *&Extra, const DataLayout *DL, bool isEQ, unsigned &UsedICmps) { Instruction *I = dyn_cast(V); if (!I) return nullptr; // If this is an icmp against a constant, handle this as one of the cases. if (ICmpInst *ICI = dyn_cast(I)) { if (ConstantInt *C = GetConstantInt(I->getOperand(1), DL)) { Value *RHSVal; ConstantInt *RHSC; if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ:ICmpInst::ICMP_NE)) { // (x & ~2^x) == y --> x == y || x == y|2^x // This undoes a transformation done by instcombine to fuse 2 compares. if (match(ICI->getOperand(0), m_And(m_Value(RHSVal), m_ConstantInt(RHSC)))) { APInt Not = ~RHSC->getValue(); if (Not.isPowerOf2()) { Vals.push_back(C); Vals.push_back( ConstantInt::get(C->getContext(), C->getValue() | Not)); UsedICmps++; return RHSVal; } } UsedICmps++; Vals.push_back(C); return I->getOperand(0); } // If we have "x ult 3" comparison, for example, then we can add 0,1,2 to // the set. ConstantRange Span = ConstantRange::makeICmpRegion(ICI->getPredicate(), C->getValue()); // Shift the range if the compare is fed by an add. This is the range // compare idiom as emitted by instcombine. bool hasAdd = match(I->getOperand(0), m_Add(m_Value(RHSVal), m_ConstantInt(RHSC))); if (hasAdd) Span = Span.subtract(RHSC->getValue()); // If this is an and/!= check then we want to optimize "x ugt 2" into // x != 0 && x != 1. if (!isEQ) Span = Span.inverse(); // If there are a ton of values, we don't want to make a ginormous switch. if (Span.getSetSize().ugt(8) || Span.isEmptySet()) return nullptr; for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp) Vals.push_back(ConstantInt::get(V->getContext(), Tmp)); UsedICmps++; return hasAdd ? RHSVal : I->getOperand(0); } return nullptr; } // Otherwise, we can only handle an | or &, depending on isEQ. if (I->getOpcode() != (isEQ ? Instruction::Or : Instruction::And)) return nullptr; unsigned NumValsBeforeLHS = Vals.size(); unsigned UsedICmpsBeforeLHS = UsedICmps; if (Value *LHS = GatherConstantCompares(I->getOperand(0), Vals, Extra, DL, isEQ, UsedICmps)) { unsigned NumVals = Vals.size(); unsigned UsedICmpsBeforeRHS = UsedICmps; if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, DL, isEQ, UsedICmps)) { if (LHS == RHS) return LHS; Vals.resize(NumVals); UsedICmps = UsedICmpsBeforeRHS; } // The RHS of the or/and can't be folded in and we haven't used "Extra" yet, // set it and return success. if (Extra == nullptr || Extra == I->getOperand(1)) { Extra = I->getOperand(1); return LHS; } Vals.resize(NumValsBeforeLHS); UsedICmps = UsedICmpsBeforeLHS; return nullptr; } // If the LHS can't be folded in, but Extra is available and RHS can, try to // use LHS as Extra. if (Extra == nullptr || Extra == I->getOperand(0)) { Value *OldExtra = Extra; Extra = I->getOperand(0); if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, DL, isEQ, UsedICmps)) return RHS; assert(Vals.size() == NumValsBeforeLHS); Extra = OldExtra; } return nullptr; } static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) { Instruction *Cond = nullptr; if (SwitchInst *SI = dyn_cast(TI)) { Cond = dyn_cast(SI->getCondition()); } else if (BranchInst *BI = dyn_cast(TI)) { if (BI->isConditional()) Cond = dyn_cast(BI->getCondition()); } else if (IndirectBrInst *IBI = dyn_cast(TI)) { Cond = dyn_cast(IBI->getAddress()); } TI->eraseFromParent(); if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond); } /// isValueEqualityComparison - Return true if the specified terminator checks /// to see if a value is equal to constant integer value. Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) { Value *CV = nullptr; if (SwitchInst *SI = dyn_cast(TI)) { // Do not permit merging of large switch instructions into their // predecessors unless there is only one predecessor. if (SI->getNumSuccessors()*std::distance(pred_begin(SI->getParent()), pred_end(SI->getParent())) <= 128) CV = SI->getCondition(); } else if (BranchInst *BI = dyn_cast(TI)) if (BI->isConditional() && BI->getCondition()->hasOneUse()) if (ICmpInst *ICI = dyn_cast(BI->getCondition())) if (ICI->isEquality() && GetConstantInt(ICI->getOperand(1), DL)) CV = ICI->getOperand(0); // Unwrap any lossless ptrtoint cast. if (DL && CV) { if (PtrToIntInst *PTII = dyn_cast(CV)) { Value *Ptr = PTII->getPointerOperand(); if (PTII->getType() == DL->getIntPtrType(Ptr->getType())) CV = Ptr; } } return CV; } /// GetValueEqualityComparisonCases - Given a value comparison instruction, /// decode all of the 'cases' that it represents and return the 'default' block. BasicBlock *SimplifyCFGOpt:: GetValueEqualityComparisonCases(TerminatorInst *TI, std::vector &Cases) { if (SwitchInst *SI = dyn_cast(TI)) { Cases.reserve(SI->getNumCases()); for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i) Cases.push_back(ValueEqualityComparisonCase(i.getCaseValue(), i.getCaseSuccessor())); return SI->getDefaultDest(); } BranchInst *BI = cast(TI); ICmpInst *ICI = cast(BI->getCondition()); BasicBlock *Succ = BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_NE); Cases.push_back(ValueEqualityComparisonCase(GetConstantInt(ICI->getOperand(1), DL), Succ)); return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ); } /// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries /// in the list that match the specified block. static void EliminateBlockCases(BasicBlock *BB, std::vector &Cases) { Cases.erase(std::remove(Cases.begin(), Cases.end(), BB), Cases.end()); } /// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as /// well. static bool ValuesOverlap(std::vector &C1, std::vector &C2) { std::vector *V1 = &C1, *V2 = &C2; // Make V1 be smaller than V2. if (V1->size() > V2->size()) std::swap(V1, V2); if (V1->size() == 0) return false; if (V1->size() == 1) { // Just scan V2. ConstantInt *TheVal = (*V1)[0].Value; for (unsigned i = 0, e = V2->size(); i != e; ++i) if (TheVal == (*V2)[i].Value) return true; } // Otherwise, just sort both lists and compare element by element. array_pod_sort(V1->begin(), V1->end()); array_pod_sort(V2->begin(), V2->end()); unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size(); while (i1 != e1 && i2 != e2) { if ((*V1)[i1].Value == (*V2)[i2].Value) return true; if ((*V1)[i1].Value < (*V2)[i2].Value) ++i1; else ++i2; } return false; } /// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a /// terminator instruction and its block is known to only have a single /// predecessor block, check to see if that predecessor is also a value /// comparison with the same value, and if that comparison determines the /// outcome of this comparison. If so, simplify TI. This does a very limited /// form of jump threading. bool SimplifyCFGOpt:: SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, BasicBlock *Pred, IRBuilder<> &Builder) { Value *PredVal = isValueEqualityComparison(Pred->getTerminator()); if (!PredVal) return false; // Not a value comparison in predecessor. Value *ThisVal = isValueEqualityComparison(TI); assert(ThisVal && "This isn't a value comparison!!"); if (ThisVal != PredVal) return false; // Different predicates. // TODO: Preserve branch weight metadata, similarly to how // FoldValueComparisonIntoPredecessors preserves it. // Find out information about when control will move from Pred to TI's block. std::vector PredCases; BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(), PredCases); EliminateBlockCases(PredDef, PredCases); // Remove default from cases. // Find information about how control leaves this block. std::vector ThisCases; BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases); EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases. // If TI's block is the default block from Pred's comparison, potentially // simplify TI based on this knowledge. if (PredDef == TI->getParent()) { // If we are here, we know that the value is none of those cases listed in // PredCases. If there are any cases in ThisCases that are in PredCases, we // can simplify TI. if (!ValuesOverlap(PredCases, ThisCases)) return false; if (isa(TI)) { // Okay, one of the successors of this condbr is dead. Convert it to a // uncond br. assert(ThisCases.size() == 1 && "Branch can only have one case!"); // Insert the new branch. Instruction *NI = Builder.CreateBr(ThisDef); (void) NI; // Remove PHI node entries for the dead edge. ThisCases[0].Dest->removePredecessor(TI->getParent()); DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); EraseTerminatorInstAndDCECond(TI); return true; } SwitchInst *SI = cast(TI); // Okay, TI has cases that are statically dead, prune them away. SmallPtrSet DeadCases; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) DeadCases.insert(PredCases[i].Value); DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI); // Collect branch weights into a vector. SmallVector Weights; MDNode* MD = SI->getMetadata(LLVMContext::MD_prof); bool HasWeight = MD && (MD->getNumOperands() == 2 + SI->getNumCases()); if (HasWeight) for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e; ++MD_i) { ConstantInt* CI = dyn_cast(MD->getOperand(MD_i)); assert(CI); Weights.push_back(CI->getValue().getZExtValue()); } for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) { --i; if (DeadCases.count(i.getCaseValue())) { if (HasWeight) { std::swap(Weights[i.getCaseIndex()+1], Weights.back()); Weights.pop_back(); } i.getCaseSuccessor()->removePredecessor(TI->getParent()); SI->removeCase(i); } } if (HasWeight && Weights.size() >= 2) SI->setMetadata(LLVMContext::MD_prof, MDBuilder(SI->getParent()->getContext()). createBranchWeights(Weights)); DEBUG(dbgs() << "Leaving: " << *TI << "\n"); return true; } // Otherwise, TI's block must correspond to some matched value. Find out // which value (or set of values) this is. ConstantInt *TIV = nullptr; BasicBlock *TIBB = TI->getParent(); for (unsigned i = 0, e = PredCases.size(); i != e; ++i) if (PredCases[i].Dest == TIBB) { if (TIV) return false; // Cannot handle multiple values coming to this block. TIV = PredCases[i].Value; } assert(TIV && "No edge from pred to succ?"); // Okay, we found the one constant that our value can be if we get into TI's // BB. Find out which successor will unconditionally be branched to. BasicBlock *TheRealDest = nullptr; for (unsigned i = 0, e = ThisCases.size(); i != e; ++i) if (ThisCases[i].Value == TIV) { TheRealDest = ThisCases[i].Dest; break; } // If not handled by any explicit cases, it is handled by the default case. if (!TheRealDest) TheRealDest = ThisDef; // Remove PHI node entries for dead edges. BasicBlock *CheckEdge = TheRealDest; for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI) if (*SI != CheckEdge) (*SI)->removePredecessor(TIBB); else CheckEdge = nullptr; // Insert the new branch. Instruction *NI = Builder.CreateBr(TheRealDest); (void) NI; DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); EraseTerminatorInstAndDCECond(TI); return true; } namespace { /// ConstantIntOrdering - This class implements a stable ordering of constant /// integers that does not depend on their address. This is important for /// applications that sort ConstantInt's to ensure uniqueness. struct ConstantIntOrdering { bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const { return LHS->getValue().ult(RHS->getValue()); } }; } static int ConstantIntSortPredicate(ConstantInt *const *P1, ConstantInt *const *P2) { const ConstantInt *LHS = *P1; const ConstantInt *RHS = *P2; if (LHS->getValue().ult(RHS->getValue())) return 1; if (LHS->getValue() == RHS->getValue()) return 0; return -1; } static inline bool HasBranchWeights(const Instruction* I) { MDNode* ProfMD = I->getMetadata(LLVMContext::MD_prof); if (ProfMD && ProfMD->getOperand(0)) if (MDString* MDS = dyn_cast(ProfMD->getOperand(0))) return MDS->getString().equals("branch_weights"); return false; } /// Get Weights of a given TerminatorInst, the default weight is at the front /// of the vector. If TI is a conditional eq, we need to swap the branch-weight /// metadata. static void GetBranchWeights(TerminatorInst *TI, SmallVectorImpl &Weights) { MDNode* MD = TI->getMetadata(LLVMContext::MD_prof); assert(MD); for (unsigned i = 1, e = MD->getNumOperands(); i < e; ++i) { ConstantInt *CI = cast(MD->getOperand(i)); Weights.push_back(CI->getValue().getZExtValue()); } // If TI is a conditional eq, the default case is the false case, // and the corresponding branch-weight data is at index 2. We swap the // default weight to be the first entry. if (BranchInst* BI = dyn_cast(TI)) { assert(Weights.size() == 2); ICmpInst *ICI = cast(BI->getCondition()); if (ICI->getPredicate() == ICmpInst::ICMP_EQ) std::swap(Weights.front(), Weights.back()); } } /// Keep halving the weights until all can fit in uint32_t. static void FitWeights(MutableArrayRef Weights) { uint64_t Max = *std::max_element(Weights.begin(), Weights.end()); if (Max > UINT_MAX) { unsigned Offset = 32 - countLeadingZeros(Max); for (uint64_t &I : Weights) I >>= Offset; } } /// FoldValueComparisonIntoPredecessors - The specified terminator is a value /// equality comparison instruction (either a switch or a branch on "X == c"). /// See if any of the predecessors of the terminator block are value comparisons /// on the same value. If so, and if safe to do so, fold them together. bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI, IRBuilder<> &Builder) { BasicBlock *BB = TI->getParent(); Value *CV = isValueEqualityComparison(TI); // CondVal assert(CV && "Not a comparison?"); bool Changed = false; SmallVector Preds(pred_begin(BB), pred_end(BB)); while (!Preds.empty()) { BasicBlock *Pred = Preds.pop_back_val(); // See if the predecessor is a comparison with the same value. TerminatorInst *PTI = Pred->getTerminator(); Value *PCV = isValueEqualityComparison(PTI); // PredCondVal if (PCV == CV && SafeToMergeTerminators(TI, PTI)) { // Figure out which 'cases' to copy from SI to PSI. std::vector BBCases; BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases); std::vector PredCases; BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases); // Based on whether the default edge from PTI goes to BB or not, fill in // PredCases and PredDefault with the new switch cases we would like to // build. SmallVector NewSuccessors; // Update the branch weight metadata along the way SmallVector Weights; bool PredHasWeights = HasBranchWeights(PTI); bool SuccHasWeights = HasBranchWeights(TI); if (PredHasWeights) { GetBranchWeights(PTI, Weights); // branch-weight metadata is inconsistent here. if (Weights.size() != 1 + PredCases.size()) PredHasWeights = SuccHasWeights = false; } else if (SuccHasWeights) // If there are no predecessor weights but there are successor weights, // populate Weights with 1, which will later be scaled to the sum of // successor's weights Weights.assign(1 + PredCases.size(), 1); SmallVector SuccWeights; if (SuccHasWeights) { GetBranchWeights(TI, SuccWeights); // branch-weight metadata is inconsistent here. if (SuccWeights.size() != 1 + BBCases.size()) PredHasWeights = SuccHasWeights = false; } else if (PredHasWeights) SuccWeights.assign(1 + BBCases.size(), 1); if (PredDefault == BB) { // If this is the default destination from PTI, only the edges in TI // that don't occur in PTI, or that branch to BB will be activated. std::set PTIHandled; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) if (PredCases[i].Dest != BB) PTIHandled.insert(PredCases[i].Value); else { // The default destination is BB, we don't need explicit targets. std::swap(PredCases[i], PredCases.back()); if (PredHasWeights || SuccHasWeights) { // Increase weight for the default case. Weights[0] += Weights[i+1]; std::swap(Weights[i+1], Weights.back()); Weights.pop_back(); } PredCases.pop_back(); --i; --e; } // Reconstruct the new switch statement we will be building. if (PredDefault != BBDefault) { PredDefault->removePredecessor(Pred); PredDefault = BBDefault; NewSuccessors.push_back(BBDefault); } unsigned CasesFromPred = Weights.size(); uint64_t ValidTotalSuccWeight = 0; for (unsigned i = 0, e = BBCases.size(); i != e; ++i) if (!PTIHandled.count(BBCases[i].Value) && BBCases[i].Dest != BBDefault) { PredCases.push_back(BBCases[i]); NewSuccessors.push_back(BBCases[i].Dest); if (SuccHasWeights || PredHasWeights) { // The default weight is at index 0, so weight for the ith case // should be at index i+1. Scale the cases from successor by // PredDefaultWeight (Weights[0]). Weights.push_back(Weights[0] * SuccWeights[i+1]); ValidTotalSuccWeight += SuccWeights[i+1]; } } if (SuccHasWeights || PredHasWeights) { ValidTotalSuccWeight += SuccWeights[0]; // Scale the cases from predecessor by ValidTotalSuccWeight. for (unsigned i = 1; i < CasesFromPred; ++i) Weights[i] *= ValidTotalSuccWeight; // Scale the default weight by SuccDefaultWeight (SuccWeights[0]). Weights[0] *= SuccWeights[0]; } } else { // If this is not the default destination from PSI, only the edges // in SI that occur in PSI with a destination of BB will be // activated. std::set PTIHandled; std::map WeightsForHandled; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) if (PredCases[i].Dest == BB) { PTIHandled.insert(PredCases[i].Value); if (PredHasWeights || SuccHasWeights) { WeightsForHandled[PredCases[i].Value] = Weights[i+1]; std::swap(Weights[i+1], Weights.back()); Weights.pop_back(); } std::swap(PredCases[i], PredCases.back()); PredCases.pop_back(); --i; --e; } // Okay, now we know which constants were sent to BB from the // predecessor. Figure out where they will all go now. for (unsigned i = 0, e = BBCases.size(); i != e; ++i) if (PTIHandled.count(BBCases[i].Value)) { // If this is one we are capable of getting... if (PredHasWeights || SuccHasWeights) Weights.push_back(WeightsForHandled[BBCases[i].Value]); PredCases.push_back(BBCases[i]); NewSuccessors.push_back(BBCases[i].Dest); PTIHandled.erase(BBCases[i].Value);// This constant is taken care of } // If there are any constants vectored to BB that TI doesn't handle, // they must go to the default destination of TI. for (std::set::iterator I = PTIHandled.begin(), E = PTIHandled.end(); I != E; ++I) { if (PredHasWeights || SuccHasWeights) Weights.push_back(WeightsForHandled[*I]); PredCases.push_back(ValueEqualityComparisonCase(*I, BBDefault)); NewSuccessors.push_back(BBDefault); } } // Okay, at this point, we know which new successor Pred will get. Make // sure we update the number of entries in the PHI nodes for these // successors. for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i) AddPredecessorToBlock(NewSuccessors[i], Pred, BB); Builder.SetInsertPoint(PTI); // Convert pointer to int before we switch. if (CV->getType()->isPointerTy()) { assert(DL && "Cannot switch on pointer without DataLayout"); CV = Builder.CreatePtrToInt(CV, DL->getIntPtrType(CV->getType()), "magicptr"); } // Now that the successors are updated, create the new Switch instruction. SwitchInst *NewSI = Builder.CreateSwitch(CV, PredDefault, PredCases.size()); NewSI->setDebugLoc(PTI->getDebugLoc()); for (unsigned i = 0, e = PredCases.size(); i != e; ++i) NewSI->addCase(PredCases[i].Value, PredCases[i].Dest); if (PredHasWeights || SuccHasWeights) { // Halve the weights if any of them cannot fit in an uint32_t FitWeights(Weights); SmallVector MDWeights(Weights.begin(), Weights.end()); NewSI->setMetadata(LLVMContext::MD_prof, MDBuilder(BB->getContext()). createBranchWeights(MDWeights)); } EraseTerminatorInstAndDCECond(PTI); // Okay, last check. If BB is still a successor of PSI, then we must // have an infinite loop case. If so, add an infinitely looping block // to handle the case to preserve the behavior of the code. BasicBlock *InfLoopBlock = nullptr; for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i) if (NewSI->getSuccessor(i) == BB) { if (!InfLoopBlock) { // Insert it at the end of the function, because it's either code, // or it won't matter if it's hot. :) InfLoopBlock = BasicBlock::Create(BB->getContext(), "infloop", BB->getParent()); BranchInst::Create(InfLoopBlock, InfLoopBlock); } NewSI->setSuccessor(i, InfLoopBlock); } Changed = true; } } return Changed; } // isSafeToHoistInvoke - If we would need to insert a select that uses the // value of this invoke (comments in HoistThenElseCodeToIf explain why we // would need to do this), we can't hoist the invoke, as there is nowhere // to put the select in this case. static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2, Instruction *I1, Instruction *I2) { for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { PHINode *PN; for (BasicBlock::iterator BBI = SI->begin(); (PN = dyn_cast(BBI)); ++BBI) { Value *BB1V = PN->getIncomingValueForBlock(BB1); Value *BB2V = PN->getIncomingValueForBlock(BB2); if (BB1V != BB2V && (BB1V==I1 || BB2V==I2)) { return false; } } } return true; } /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and /// BB2, hoist any common code in the two blocks up into the branch block. The /// caller of this function guarantees that BI's block dominates BB1 and BB2. static bool HoistThenElseCodeToIf(BranchInst *BI, const DataLayout *DL) { // This does very trivial matching, with limited scanning, to find identical // instructions in the two blocks. In particular, we don't want to get into // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As // such, we currently just scan for obviously identical instructions in an // identical order. BasicBlock *BB1 = BI->getSuccessor(0); // The true destination. BasicBlock *BB2 = BI->getSuccessor(1); // The false destination BasicBlock::iterator BB1_Itr = BB1->begin(); BasicBlock::iterator BB2_Itr = BB2->begin(); Instruction *I1 = BB1_Itr++, *I2 = BB2_Itr++; // Skip debug info if it is not identical. DbgInfoIntrinsic *DBI1 = dyn_cast(I1); DbgInfoIntrinsic *DBI2 = dyn_cast(I2); if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) { while (isa(I1)) I1 = BB1_Itr++; while (isa(I2)) I2 = BB2_Itr++; } if (isa(I1) || !I1->isIdenticalToWhenDefined(I2) || (isa(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))) return false; BasicBlock *BIParent = BI->getParent(); bool Changed = false; do { // If we are hoisting the terminator instruction, don't move one (making a // broken BB), instead clone it, and remove BI. if (isa(I1)) goto HoistTerminator; // For a normal instruction, we just move one to right before the branch, // then replace all uses of the other with the first. Finally, we remove // the now redundant second instruction. BIParent->getInstList().splice(BI, BB1->getInstList(), I1); if (!I2->use_empty()) I2->replaceAllUsesWith(I1); I1->intersectOptionalDataWith(I2); I2->eraseFromParent(); Changed = true; I1 = BB1_Itr++; I2 = BB2_Itr++; // Skip debug info if it is not identical. DbgInfoIntrinsic *DBI1 = dyn_cast(I1); DbgInfoIntrinsic *DBI2 = dyn_cast(I2); if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) { while (isa(I1)) I1 = BB1_Itr++; while (isa(I2)) I2 = BB2_Itr++; } } while (I1->isIdenticalToWhenDefined(I2)); return true; HoistTerminator: // It may not be possible to hoist an invoke. if (isa(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) return Changed; for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { PHINode *PN; for (BasicBlock::iterator BBI = SI->begin(); (PN = dyn_cast(BBI)); ++BBI) { Value *BB1V = PN->getIncomingValueForBlock(BB1); Value *BB2V = PN->getIncomingValueForBlock(BB2); if (BB1V == BB2V) continue; if (isa(BB1V) && !isSafeToSpeculativelyExecute(BB1V, DL)) return Changed; if (isa(BB2V) && !isSafeToSpeculativelyExecute(BB2V, DL)) return Changed; } } // Okay, it is safe to hoist the terminator. Instruction *NT = I1->clone(); BIParent->getInstList().insert(BI, NT); if (!NT->getType()->isVoidTy()) { I1->replaceAllUsesWith(NT); I2->replaceAllUsesWith(NT); NT->takeName(I1); } IRBuilder Builder(NT); // Hoisting one of the terminators from our successor is a great thing. // Unfortunately, the successors of the if/else blocks may have PHI nodes in // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI // nodes, so we insert select instruction to compute the final result. std::map, SelectInst*> InsertedSelects; for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { PHINode *PN; for (BasicBlock::iterator BBI = SI->begin(); (PN = dyn_cast(BBI)); ++BBI) { Value *BB1V = PN->getIncomingValueForBlock(BB1); Value *BB2V = PN->getIncomingValueForBlock(BB2); if (BB1V == BB2V) continue; // These values do not agree. Insert a select instruction before NT // that determines the right value. SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)]; if (!SI) SI = cast (Builder.CreateSelect(BI->getCondition(), BB1V, BB2V, BB1V->getName()+"."+BB2V->getName())); // Make the PHI node use the select for all incoming values for BB1/BB2 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2) PN->setIncomingValue(i, SI); } } // Update any PHI nodes in our new successors. for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) AddPredecessorToBlock(*SI, BIParent, BB1); EraseTerminatorInstAndDCECond(BI); return true; } /// SinkThenElseCodeToEnd - Given an unconditional branch that goes to BBEnd, /// check whether BBEnd has only two predecessors and the other predecessor /// ends with an unconditional branch. If it is true, sink any common code /// in the two predecessors to BBEnd. static bool SinkThenElseCodeToEnd(BranchInst *BI1) { assert(BI1->isUnconditional()); BasicBlock *BB1 = BI1->getParent(); BasicBlock *BBEnd = BI1->getSuccessor(0); // Check that BBEnd has two predecessors and the other predecessor ends with // an unconditional branch. pred_iterator PI = pred_begin(BBEnd), PE = pred_end(BBEnd); BasicBlock *Pred0 = *PI++; if (PI == PE) // Only one predecessor. return false; BasicBlock *Pred1 = *PI++; if (PI != PE) // More than two predecessors. return false; BasicBlock *BB2 = (Pred0 == BB1) ? Pred1 : Pred0; BranchInst *BI2 = dyn_cast(BB2->getTerminator()); if (!BI2 || !BI2->isUnconditional()) return false; // Gather the PHI nodes in BBEnd. std::map > MapValueFromBB1ToBB2; Instruction *FirstNonPhiInBBEnd = nullptr; for (BasicBlock::iterator I = BBEnd->begin(), E = BBEnd->end(); I != E; ++I) { if (PHINode *PN = dyn_cast(I)) { Value *BB1V = PN->getIncomingValueForBlock(BB1); Value *BB2V = PN->getIncomingValueForBlock(BB2); MapValueFromBB1ToBB2[BB1V] = std::make_pair(BB2V, PN); } else { FirstNonPhiInBBEnd = &*I; break; } } if (!FirstNonPhiInBBEnd) return false; // This does very trivial matching, with limited scanning, to find identical // instructions in the two blocks. We scan backward for obviously identical // instructions in an identical order. BasicBlock::InstListType::reverse_iterator RI1 = BB1->getInstList().rbegin(), RE1 = BB1->getInstList().rend(), RI2 = BB2->getInstList().rbegin(), RE2 = BB2->getInstList().rend(); // Skip debug info. while (RI1 != RE1 && isa(&*RI1)) ++RI1; if (RI1 == RE1) return false; while (RI2 != RE2 && isa(&*RI2)) ++RI2; if (RI2 == RE2) return false; // Skip the unconditional branches. ++RI1; ++RI2; bool Changed = false; while (RI1 != RE1 && RI2 != RE2) { // Skip debug info. while (RI1 != RE1 && isa(&*RI1)) ++RI1; if (RI1 == RE1) return Changed; while (RI2 != RE2 && isa(&*RI2)) ++RI2; if (RI2 == RE2) return Changed; Instruction *I1 = &*RI1, *I2 = &*RI2; // I1 and I2 should have a single use in the same PHI node, and they // perform the same operation. // Cannot move control-flow-involving, volatile loads, vaarg, etc. if (isa(I1) || isa(I2) || isa(I1) || isa(I2) || isa(I1) || isa(I2) || isa(I1) || isa(I2) || I1->mayHaveSideEffects() || I2->mayHaveSideEffects() || I1->mayReadOrWriteMemory() || I2->mayReadOrWriteMemory() || !I1->hasOneUse() || !I2->hasOneUse() || MapValueFromBB1ToBB2.find(I1) == MapValueFromBB1ToBB2.end() || MapValueFromBB1ToBB2[I1].first != I2) return Changed; // Check whether we should swap the operands of ICmpInst. ICmpInst *ICmp1 = dyn_cast(I1), *ICmp2 = dyn_cast(I2); bool SwapOpnds = false; if (ICmp1 && ICmp2 && ICmp1->getOperand(0) != ICmp2->getOperand(0) && ICmp1->getOperand(1) != ICmp2->getOperand(1) && (ICmp1->getOperand(0) == ICmp2->getOperand(1) || ICmp1->getOperand(1) == ICmp2->getOperand(0))) { ICmp2->swapOperands(); SwapOpnds = true; } if (!I1->isSameOperationAs(I2)) { if (SwapOpnds) ICmp2->swapOperands(); return Changed; } // The operands should be either the same or they need to be generated // with a PHI node after sinking. We only handle the case where there is // a single pair of different operands. Value *DifferentOp1 = nullptr, *DifferentOp2 = nullptr; unsigned Op1Idx = 0; for (unsigned I = 0, E = I1->getNumOperands(); I != E; ++I) { if (I1->getOperand(I) == I2->getOperand(I)) continue; // Early exit if we have more-than one pair of different operands or // the different operand is already in MapValueFromBB1ToBB2. // Early exit if we need a PHI node to replace a constant. if (DifferentOp1 || MapValueFromBB1ToBB2.find(I1->getOperand(I)) != MapValueFromBB1ToBB2.end() || isa(I1->getOperand(I)) || isa(I2->getOperand(I))) { // If we can't sink the instructions, undo the swapping. if (SwapOpnds) ICmp2->swapOperands(); return Changed; } DifferentOp1 = I1->getOperand(I); Op1Idx = I; DifferentOp2 = I2->getOperand(I); } // We insert the pair of different operands to MapValueFromBB1ToBB2 and // remove (I1, I2) from MapValueFromBB1ToBB2. if (DifferentOp1) { PHINode *NewPN = PHINode::Create(DifferentOp1->getType(), 2, DifferentOp1->getName() + ".sink", BBEnd->begin()); MapValueFromBB1ToBB2[DifferentOp1] = std::make_pair(DifferentOp2, NewPN); // I1 should use NewPN instead of DifferentOp1. I1->setOperand(Op1Idx, NewPN); NewPN->addIncoming(DifferentOp1, BB1); NewPN->addIncoming(DifferentOp2, BB2); DEBUG(dbgs() << "Create PHI node " << *NewPN << "\n";); } PHINode *OldPN = MapValueFromBB1ToBB2[I1].second; MapValueFromBB1ToBB2.erase(I1); DEBUG(dbgs() << "SINK common instructions " << *I1 << "\n";); DEBUG(dbgs() << " " << *I2 << "\n";); // We need to update RE1 and RE2 if we are going to sink the first // instruction in the basic block down. bool UpdateRE1 = (I1 == BB1->begin()), UpdateRE2 = (I2 == BB2->begin()); // Sink the instruction. BBEnd->getInstList().splice(FirstNonPhiInBBEnd, BB1->getInstList(), I1); if (!OldPN->use_empty()) OldPN->replaceAllUsesWith(I1); OldPN->eraseFromParent(); if (!I2->use_empty()) I2->replaceAllUsesWith(I1); I1->intersectOptionalDataWith(I2); I2->eraseFromParent(); if (UpdateRE1) RE1 = BB1->getInstList().rend(); if (UpdateRE2) RE2 = BB2->getInstList().rend(); FirstNonPhiInBBEnd = I1; NumSinkCommons++; Changed = true; } return Changed; } /// \brief Determine if we can hoist sink a sole store instruction out of a /// conditional block. /// /// We are looking for code like the following: /// BrBB: /// store i32 %add, i32* %arrayidx2 /// ... // No other stores or function calls (we could be calling a memory /// ... // function). /// %cmp = icmp ult %x, %y /// br i1 %cmp, label %EndBB, label %ThenBB /// ThenBB: /// store i32 %add5, i32* %arrayidx2 /// br label EndBB /// EndBB: /// ... /// We are going to transform this into: /// BrBB: /// store i32 %add, i32* %arrayidx2 /// ... // /// %cmp = icmp ult %x, %y /// %add.add5 = select i1 %cmp, i32 %add, %add5 /// store i32 %add.add5, i32* %arrayidx2 /// ... /// /// \return The pointer to the value of the previous store if the store can be /// hoisted into the predecessor block. 0 otherwise. static Value *isSafeToSpeculateStore(Instruction *I, BasicBlock *BrBB, BasicBlock *StoreBB, BasicBlock *EndBB) { StoreInst *StoreToHoist = dyn_cast(I); if (!StoreToHoist) return nullptr; // Volatile or atomic. if (!StoreToHoist->isSimple()) return nullptr; Value *StorePtr = StoreToHoist->getPointerOperand(); // Look for a store to the same pointer in BrBB. unsigned MaxNumInstToLookAt = 10; for (BasicBlock::reverse_iterator RI = BrBB->rbegin(), RE = BrBB->rend(); RI != RE && (--MaxNumInstToLookAt); ++RI) { Instruction *CurI = &*RI; // Could be calling an instruction that effects memory like free(). if (CurI->mayHaveSideEffects() && !isa(CurI)) return nullptr; StoreInst *SI = dyn_cast(CurI); // Found the previous store make sure it stores to the same location. if (SI && SI->getPointerOperand() == StorePtr) // Found the previous store, return its value operand. return SI->getValueOperand(); else if (SI) return nullptr; // Unknown store. } return nullptr; } /// \brief Speculate a conditional basic block flattening the CFG. /// /// Note that this is a very risky transform currently. Speculating /// instructions like this is most often not desirable. Instead, there is an MI /// pass which can do it with full awareness of the resource constraints. /// However, some cases are "obvious" and we should do directly. An example of /// this is speculating a single, reasonably cheap instruction. /// /// There is only one distinct advantage to flattening the CFG at the IR level: /// it makes very common but simplistic optimizations such as are common in /// instcombine and the DAG combiner more powerful by removing CFG edges and /// modeling their effects with easier to reason about SSA value graphs. /// /// /// An illustration of this transform is turning this IR: /// \code /// BB: /// %cmp = icmp ult %x, %y /// br i1 %cmp, label %EndBB, label %ThenBB /// ThenBB: /// %sub = sub %x, %y /// br label BB2 /// EndBB: /// %phi = phi [ %sub, %ThenBB ], [ 0, %EndBB ] /// ... /// \endcode /// /// Into this IR: /// \code /// BB: /// %cmp = icmp ult %x, %y /// %sub = sub %x, %y /// %cond = select i1 %cmp, 0, %sub /// ... /// \endcode /// /// \returns true if the conditional block is removed. static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB, const DataLayout *DL) { // Be conservative for now. FP select instruction can often be expensive. Value *BrCond = BI->getCondition(); if (isa(BrCond)) return false; BasicBlock *BB = BI->getParent(); BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0); // If ThenBB is actually on the false edge of the conditional branch, remember // to swap the select operands later. bool Invert = false; if (ThenBB != BI->getSuccessor(0)) { assert(ThenBB == BI->getSuccessor(1) && "No edge from 'if' block?"); Invert = true; } assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block"); // Keep a count of how many times instructions are used within CondBB when // they are candidates for sinking into CondBB. Specifically: // - They are defined in BB, and // - They have no side effects, and // - All of their uses are in CondBB. SmallDenseMap SinkCandidateUseCounts; unsigned SpeculationCost = 0; Value *SpeculatedStoreValue = nullptr; StoreInst *SpeculatedStore = nullptr; for (BasicBlock::iterator BBI = ThenBB->begin(), BBE = std::prev(ThenBB->end()); BBI != BBE; ++BBI) { Instruction *I = BBI; // Skip debug info. if (isa(I)) continue; // Only speculatively execution a single instruction (not counting the // terminator) for now. ++SpeculationCost; if (SpeculationCost > 1) return false; // Don't hoist the instruction if it's unsafe or expensive. if (!isSafeToSpeculativelyExecute(I, DL) && !(HoistCondStores && (SpeculatedStoreValue = isSafeToSpeculateStore(I, BB, ThenBB, EndBB)))) return false; if (!SpeculatedStoreValue && ComputeSpeculationCost(I, DL) > PHINodeFoldingThreshold) return false; // Store the store speculation candidate. if (SpeculatedStoreValue) SpeculatedStore = cast(I); // Do not hoist the instruction if any of its operands are defined but not // used in BB. The transformation will prevent the operand from // being sunk into the use block. for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) { Instruction *OpI = dyn_cast(*i); if (!OpI || OpI->getParent() != BB || OpI->mayHaveSideEffects()) continue; // Not a candidate for sinking. ++SinkCandidateUseCounts[OpI]; } } // Consider any sink candidates which are only used in CondBB as costs for // speculation. Note, while we iterate over a DenseMap here, we are summing // and so iteration order isn't significant. for (SmallDenseMap::iterator I = SinkCandidateUseCounts.begin(), E = SinkCandidateUseCounts.end(); I != E; ++I) if (I->first->getNumUses() == I->second) { ++SpeculationCost; if (SpeculationCost > 1) return false; } // Check that the PHI nodes can be converted to selects. bool HaveRewritablePHIs = false; for (BasicBlock::iterator I = EndBB->begin(); PHINode *PN = dyn_cast(I); ++I) { Value *OrigV = PN->getIncomingValueForBlock(BB); Value *ThenV = PN->getIncomingValueForBlock(ThenBB); // FIXME: Try to remove some of the duplication with HoistThenElseCodeToIf. // Skip PHIs which are trivial. if (ThenV == OrigV) continue; HaveRewritablePHIs = true; ConstantExpr *OrigCE = dyn_cast(OrigV); ConstantExpr *ThenCE = dyn_cast(ThenV); if (!OrigCE && !ThenCE) continue; // Known safe and cheap. if ((ThenCE && !isSafeToSpeculativelyExecute(ThenCE, DL)) || (OrigCE && !isSafeToSpeculativelyExecute(OrigCE, DL))) return false; unsigned OrigCost = OrigCE ? ComputeSpeculationCost(OrigCE, DL) : 0; unsigned ThenCost = ThenCE ? ComputeSpeculationCost(ThenCE, DL) : 0; if (OrigCost + ThenCost > 2 * PHINodeFoldingThreshold) return false; // Account for the cost of an unfolded ConstantExpr which could end up // getting expanded into Instructions. // FIXME: This doesn't account for how many operations are combined in the // constant expression. ++SpeculationCost; if (SpeculationCost > 1) return false; } // If there are no PHIs to process, bail early. This helps ensure idempotence // as well. if (!HaveRewritablePHIs && !(HoistCondStores && SpeculatedStoreValue)) return false; // If we get here, we can hoist the instruction and if-convert. DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *ThenBB << "\n";); // Insert a select of the value of the speculated store. if (SpeculatedStoreValue) { IRBuilder Builder(BI); Value *TrueV = SpeculatedStore->getValueOperand(); Value *FalseV = SpeculatedStoreValue; if (Invert) std::swap(TrueV, FalseV); Value *S = Builder.CreateSelect(BrCond, TrueV, FalseV, TrueV->getName() + "." + FalseV->getName()); SpeculatedStore->setOperand(0, S); } // Hoist the instructions. BB->getInstList().splice(BI, ThenBB->getInstList(), ThenBB->begin(), std::prev(ThenBB->end())); // Insert selects and rewrite the PHI operands. IRBuilder Builder(BI); for (BasicBlock::iterator I = EndBB->begin(); PHINode *PN = dyn_cast(I); ++I) { unsigned OrigI = PN->getBasicBlockIndex(BB); unsigned ThenI = PN->getBasicBlockIndex(ThenBB); Value *OrigV = PN->getIncomingValue(OrigI); Value *ThenV = PN->getIncomingValue(ThenI); // Skip PHIs which are trivial. if (OrigV == ThenV) continue; // Create a select whose true value is the speculatively executed value and // false value is the preexisting value. Swap them if the branch // destinations were inverted. Value *TrueV = ThenV, *FalseV = OrigV; if (Invert) std::swap(TrueV, FalseV); Value *V = Builder.CreateSelect(BrCond, TrueV, FalseV, TrueV->getName() + "." + FalseV->getName()); PN->setIncomingValue(OrigI, V); PN->setIncomingValue(ThenI, V); } ++NumSpeculations; return true; } /// \returns True if this block contains a CallInst with the NoDuplicate /// attribute. static bool HasNoDuplicateCall(const BasicBlock *BB) { for (BasicBlock::const_iterator I = BB->begin(), E = BB->end(); I != E; ++I) { const CallInst *CI = dyn_cast(I); if (!CI) continue; if (CI->cannotDuplicate()) return true; } return false; } /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch /// across this block. static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) { BranchInst *BI = cast(BB->getTerminator()); unsigned Size = 0; for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { if (isa(BBI)) continue; if (Size > 10) return false; // Don't clone large BB's. ++Size; // We can only support instructions that do not define values that are // live outside of the current basic block. for (User *U : BBI->users()) { Instruction *UI = cast(U); if (UI->getParent() != BB || isa(UI)) return false; } // Looks ok, continue checking. } return true; } /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value /// that is defined in the same block as the branch and if any PHI entries are /// constants, thread edges corresponding to that entry to be branches to their /// ultimate destination. static bool FoldCondBranchOnPHI(BranchInst *BI, const DataLayout *DL) { BasicBlock *BB = BI->getParent(); PHINode *PN = dyn_cast(BI->getCondition()); // NOTE: we currently cannot transform this case if the PHI node is used // outside of the block. if (!PN || PN->getParent() != BB || !PN->hasOneUse()) return false; // Degenerate case of a single entry PHI. if (PN->getNumIncomingValues() == 1) { FoldSingleEntryPHINodes(PN->getParent()); return true; } // Now we know that this block has multiple preds and two succs. if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false; if (HasNoDuplicateCall(BB)) return false; // Okay, this is a simple enough basic block. See if any phi values are // constants. for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { ConstantInt *CB = dyn_cast(PN->getIncomingValue(i)); if (!CB || !CB->getType()->isIntegerTy(1)) continue; // Okay, we now know that all edges from PredBB should be revectored to // branch to RealDest. BasicBlock *PredBB = PN->getIncomingBlock(i); BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue()); if (RealDest == BB) continue; // Skip self loops. // Skip if the predecessor's terminator is an indirect branch. if (isa(PredBB->getTerminator())) continue; // The dest block might have PHI nodes, other predecessors and other // difficult cases. Instead of being smart about this, just insert a new // block that jumps to the destination block, effectively splitting // the edge we are about to create. BasicBlock *EdgeBB = BasicBlock::Create(BB->getContext(), RealDest->getName()+".critedge", RealDest->getParent(), RealDest); BranchInst::Create(RealDest, EdgeBB); // Update PHI nodes. AddPredecessorToBlock(RealDest, EdgeBB, BB); // BB may have instructions that are being threaded over. Clone these // instructions into EdgeBB. We know that there will be no uses of the // cloned instructions outside of EdgeBB. BasicBlock::iterator InsertPt = EdgeBB->begin(); DenseMap TranslateMap; // Track translated values. for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { if (PHINode *PN = dyn_cast(BBI)) { TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB); continue; } // Clone the instruction. Instruction *N = BBI->clone(); if (BBI->hasName()) N->setName(BBI->getName()+".c"); // Update operands due to translation. for (User::op_iterator i = N->op_begin(), e = N->op_end(); i != e; ++i) { DenseMap::iterator PI = TranslateMap.find(*i); if (PI != TranslateMap.end()) *i = PI->second; } // Check for trivial simplification. if (Value *V = SimplifyInstruction(N, DL)) { TranslateMap[BBI] = V; delete N; // Instruction folded away, don't need actual inst } else { // Insert the new instruction into its new home. EdgeBB->getInstList().insert(InsertPt, N); if (!BBI->use_empty()) TranslateMap[BBI] = N; } } // Loop over all of the edges from PredBB to BB, changing them to branch // to EdgeBB instead. TerminatorInst *PredBBTI = PredBB->getTerminator(); for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i) if (PredBBTI->getSuccessor(i) == BB) { BB->removePredecessor(PredBB); PredBBTI->setSuccessor(i, EdgeBB); } // Recurse, simplifying any other constants. return FoldCondBranchOnPHI(BI, DL) | true; } return false; } /// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry /// PHI node, see if we can eliminate it. static bool FoldTwoEntryPHINode(PHINode *PN, const DataLayout *DL) { // Ok, this is a two entry PHI node. Check to see if this is a simple "if // statement", which has a very simple dominance structure. Basically, we // are trying to find the condition that is being branched on, which // subsequently causes this merge to happen. We really want control // dependence information for this check, but simplifycfg can't keep it up // to date, and this catches most of the cases we care about anyway. BasicBlock *BB = PN->getParent(); BasicBlock *IfTrue, *IfFalse; Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse); if (!IfCond || // Don't bother if the branch will be constant folded trivially. isa(IfCond)) return false; // Okay, we found that we can merge this two-entry phi node into a select. // Doing so would require us to fold *all* two entry phi nodes in this block. // At some point this becomes non-profitable (particularly if the target // doesn't support cmov's). Only do this transformation if there are two or // fewer PHI nodes in this block. unsigned NumPhis = 0; for (BasicBlock::iterator I = BB->begin(); isa(I); ++NumPhis, ++I) if (NumPhis > 2) return false; // Loop over the PHI's seeing if we can promote them all to select // instructions. While we are at it, keep track of the instructions // that need to be moved to the dominating block. SmallPtrSet AggressiveInsts; unsigned MaxCostVal0 = PHINodeFoldingThreshold, MaxCostVal1 = PHINodeFoldingThreshold; for (BasicBlock::iterator II = BB->begin(); isa(II);) { PHINode *PN = cast(II++); if (Value *V = SimplifyInstruction(PN, DL)) { PN->replaceAllUsesWith(V); PN->eraseFromParent(); continue; } if (!DominatesMergePoint(PN->getIncomingValue(0), BB, &AggressiveInsts, MaxCostVal0, DL) || !DominatesMergePoint(PN->getIncomingValue(1), BB, &AggressiveInsts, MaxCostVal1, DL)) return false; } // If we folded the first phi, PN dangles at this point. Refresh it. If // we ran out of PHIs then we simplified them all. PN = dyn_cast(BB->begin()); if (!PN) return true; // Don't fold i1 branches on PHIs which contain binary operators. These can // often be turned into switches and other things. if (PN->getType()->isIntegerTy(1) && (isa(PN->getIncomingValue(0)) || isa(PN->getIncomingValue(1)) || isa(IfCond))) return false; // If we all PHI nodes are promotable, check to make sure that all // instructions in the predecessor blocks can be promoted as well. If // not, we won't be able to get rid of the control flow, so it's not // worth promoting to select instructions. BasicBlock *DomBlock = nullptr; BasicBlock *IfBlock1 = PN->getIncomingBlock(0); BasicBlock *IfBlock2 = PN->getIncomingBlock(1); if (cast(IfBlock1->getTerminator())->isConditional()) { IfBlock1 = nullptr; } else { DomBlock = *pred_begin(IfBlock1); for (BasicBlock::iterator I = IfBlock1->begin();!isa(I);++I) if (!AggressiveInsts.count(I) && !isa(I)) { // This is not an aggressive instruction that we can promote. // Because of this, we won't be able to get rid of the control // flow, so the xform is not worth it. return false; } } if (cast(IfBlock2->getTerminator())->isConditional()) { IfBlock2 = nullptr; } else { DomBlock = *pred_begin(IfBlock2); for (BasicBlock::iterator I = IfBlock2->begin();!isa(I);++I) if (!AggressiveInsts.count(I) && !isa(I)) { // This is not an aggressive instruction that we can promote. // Because of this, we won't be able to get rid of the control // flow, so the xform is not worth it. return false; } } DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond << " T: " << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"); // If we can still promote the PHI nodes after this gauntlet of tests, // do all of the PHI's now. Instruction *InsertPt = DomBlock->getTerminator(); IRBuilder Builder(InsertPt); // Move all 'aggressive' instructions, which are defined in the // conditional parts of the if's up to the dominating block. if (IfBlock1) DomBlock->getInstList().splice(InsertPt, IfBlock1->getInstList(), IfBlock1->begin(), IfBlock1->getTerminator()); if (IfBlock2) DomBlock->getInstList().splice(InsertPt, IfBlock2->getInstList(), IfBlock2->begin(), IfBlock2->getTerminator()); while (PHINode *PN = dyn_cast(BB->begin())) { // Change the PHI node into a select instruction. Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse); Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue); SelectInst *NV = cast(Builder.CreateSelect(IfCond, TrueVal, FalseVal, "")); PN->replaceAllUsesWith(NV); NV->takeName(PN); PN->eraseFromParent(); } // At this point, IfBlock1 and IfBlock2 are both empty, so our if statement // has been flattened. Change DomBlock to jump directly to our new block to // avoid other simplifycfg's kicking in on the diamond. TerminatorInst *OldTI = DomBlock->getTerminator(); Builder.SetInsertPoint(OldTI); Builder.CreateBr(BB); OldTI->eraseFromParent(); return true; } /// SimplifyCondBranchToTwoReturns - If we found a conditional branch that goes /// to two returning blocks, try to merge them together into one return, /// introducing a select if the return values disagree. static bool SimplifyCondBranchToTwoReturns(BranchInst *BI, IRBuilder<> &Builder) { assert(BI->isConditional() && "Must be a conditional branch"); BasicBlock *TrueSucc = BI->getSuccessor(0); BasicBlock *FalseSucc = BI->getSuccessor(1); ReturnInst *TrueRet = cast(TrueSucc->getTerminator()); ReturnInst *FalseRet = cast(FalseSucc->getTerminator()); // Check to ensure both blocks are empty (just a return) or optionally empty // with PHI nodes. If there are other instructions, merging would cause extra // computation on one path or the other. if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator()) return false; if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator()) return false; Builder.SetInsertPoint(BI); // Okay, we found a branch that is going to two return nodes. If // there is no return value for this function, just change the // branch into a return. if (FalseRet->getNumOperands() == 0) { TrueSucc->removePredecessor(BI->getParent()); FalseSucc->removePredecessor(BI->getParent()); Builder.CreateRetVoid(); EraseTerminatorInstAndDCECond(BI); return true; } // Otherwise, figure out what the true and false return values are // so we can insert a new select instruction. Value *TrueValue = TrueRet->getReturnValue(); Value *FalseValue = FalseRet->getReturnValue(); // Unwrap any PHI nodes in the return blocks. if (PHINode *TVPN = dyn_cast_or_null(TrueValue)) if (TVPN->getParent() == TrueSucc) TrueValue = TVPN->getIncomingValueForBlock(BI->getParent()); if (PHINode *FVPN = dyn_cast_or_null(FalseValue)) if (FVPN->getParent() == FalseSucc) FalseValue = FVPN->getIncomingValueForBlock(BI->getParent()); // In order for this transformation to be safe, we must be able to // unconditionally execute both operands to the return. This is // normally the case, but we could have a potentially-trapping // constant expression that prevents this transformation from being // safe. if (ConstantExpr *TCV = dyn_cast_or_null(TrueValue)) if (TCV->canTrap()) return false; if (ConstantExpr *FCV = dyn_cast_or_null(FalseValue)) if (FCV->canTrap()) return false; // Okay, we collected all the mapped values and checked them for sanity, and // defined to really do this transformation. First, update the CFG. TrueSucc->removePredecessor(BI->getParent()); FalseSucc->removePredecessor(BI->getParent()); // Insert select instructions where needed. Value *BrCond = BI->getCondition(); if (TrueValue) { // Insert a select if the results differ. if (TrueValue == FalseValue || isa(FalseValue)) { } else if (isa(TrueValue)) { TrueValue = FalseValue; } else { TrueValue = Builder.CreateSelect(BrCond, TrueValue, FalseValue, "retval"); } } Value *RI = !TrueValue ? Builder.CreateRetVoid() : Builder.CreateRet(TrueValue); (void) RI; DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:" << "\n " << *BI << "NewRet = " << *RI << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc); EraseTerminatorInstAndDCECond(BI); return true; } /// ExtractBranchMetadata - Given a conditional BranchInstruction, retrieve the /// probabilities of the branch taking each edge. Fills in the two APInt /// parameters and return true, or returns false if no or invalid metadata was /// found. static bool ExtractBranchMetadata(BranchInst *BI, uint64_t &ProbTrue, uint64_t &ProbFalse) { assert(BI->isConditional() && "Looking for probabilities on unconditional branch?"); MDNode *ProfileData = BI->getMetadata(LLVMContext::MD_prof); if (!ProfileData || ProfileData->getNumOperands() != 3) return false; ConstantInt *CITrue = dyn_cast(ProfileData->getOperand(1)); ConstantInt *CIFalse = dyn_cast(ProfileData->getOperand(2)); if (!CITrue || !CIFalse) return false; ProbTrue = CITrue->getValue().getZExtValue(); ProbFalse = CIFalse->getValue().getZExtValue(); return true; } /// checkCSEInPredecessor - Return true if the given instruction is available /// in its predecessor block. If yes, the instruction will be removed. /// static bool checkCSEInPredecessor(Instruction *Inst, BasicBlock *PB) { if (!isa(Inst) && !isa(Inst)) return false; for (BasicBlock::iterator I = PB->begin(), E = PB->end(); I != E; I++) { Instruction *PBI = &*I; // Check whether Inst and PBI generate the same value. if (Inst->isIdenticalTo(PBI)) { Inst->replaceAllUsesWith(PBI); Inst->eraseFromParent(); return true; } } return false; } /// FoldBranchToCommonDest - If this basic block is simple enough, and if a /// predecessor branches to us and one of our successors, fold the block into /// the predecessor and use logical operations to pick the right destination. bool llvm::FoldBranchToCommonDest(BranchInst *BI, const DataLayout *DL) { BasicBlock *BB = BI->getParent(); Instruction *Cond = nullptr; if (BI->isConditional()) Cond = dyn_cast(BI->getCondition()); else { // For unconditional branch, check for a simple CFG pattern, where // BB has a single predecessor and BB's successor is also its predecessor's // successor. If such pattern exisits, check for CSE between BB and its // predecessor. if (BasicBlock *PB = BB->getSinglePredecessor()) if (BranchInst *PBI = dyn_cast(PB->getTerminator())) if (PBI->isConditional() && (BI->getSuccessor(0) == PBI->getSuccessor(0) || BI->getSuccessor(0) == PBI->getSuccessor(1))) { for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { Instruction *Curr = I++; if (isa(Curr)) { Cond = Curr; break; } // Quit if we can't remove this instruction. if (!checkCSEInPredecessor(Curr, PB)) return false; } } if (!Cond) return false; } if (!Cond || (!isa(Cond) && !isa(Cond)) || Cond->getParent() != BB || !Cond->hasOneUse()) return false; // Only allow this if the condition is a simple instruction that can be // executed unconditionally. It must be in the same block as the branch, and // must be at the front of the block. BasicBlock::iterator FrontIt = BB->front(); // Ignore dbg intrinsics. while (isa(FrontIt)) ++FrontIt; // Allow a single instruction to be hoisted in addition to the compare // that feeds the branch. We later ensure that any values that _it_ uses // were also live in the predecessor, so that we don't unnecessarily create // register pressure or inhibit out-of-order execution. Instruction *BonusInst = nullptr; if (&*FrontIt != Cond && FrontIt->hasOneUse() && FrontIt->user_back() == Cond && isSafeToSpeculativelyExecute(FrontIt, DL)) { BonusInst = &*FrontIt; ++FrontIt; // Ignore dbg intrinsics. while (isa(FrontIt)) ++FrontIt; } // Only a single bonus inst is allowed. if (&*FrontIt != Cond) return false; // Make sure the instruction after the condition is the cond branch. BasicBlock::iterator CondIt = Cond; ++CondIt; // Ignore dbg intrinsics. while (isa(CondIt)) ++CondIt; if (&*CondIt != BI) return false; // Cond is known to be a compare or binary operator. Check to make sure that // neither operand is a potentially-trapping constant expression. if (ConstantExpr *CE = dyn_cast(Cond->getOperand(0))) if (CE->canTrap()) return false; if (ConstantExpr *CE = dyn_cast(Cond->getOperand(1))) if (CE->canTrap()) return false; // Finally, don't infinitely unroll conditional loops. BasicBlock *TrueDest = BI->getSuccessor(0); BasicBlock *FalseDest = (BI->isConditional()) ? BI->getSuccessor(1) : nullptr; if (TrueDest == BB || FalseDest == BB) return false; for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { BasicBlock *PredBlock = *PI; BranchInst *PBI = dyn_cast(PredBlock->getTerminator()); // Check that we have two conditional branches. If there is a PHI node in // the common successor, verify that the same value flows in from both // blocks. SmallVector PHIs; if (!PBI || PBI->isUnconditional() || (BI->isConditional() && !SafeToMergeTerminators(BI, PBI)) || (!BI->isConditional() && !isProfitableToFoldUnconditional(BI, PBI, Cond, PHIs))) continue; // Determine if the two branches share a common destination. Instruction::BinaryOps Opc = Instruction::BinaryOpsEnd; bool InvertPredCond = false; if (BI->isConditional()) { if (PBI->getSuccessor(0) == TrueDest) Opc = Instruction::Or; else if (PBI->getSuccessor(1) == FalseDest) Opc = Instruction::And; else if (PBI->getSuccessor(0) == FalseDest) Opc = Instruction::And, InvertPredCond = true; else if (PBI->getSuccessor(1) == TrueDest) Opc = Instruction::Or, InvertPredCond = true; else continue; } else { if (PBI->getSuccessor(0) != TrueDest && PBI->getSuccessor(1) != TrueDest) continue; } // Ensure that any values used in the bonus instruction are also used // by the terminator of the predecessor. This means that those values // must already have been resolved, so we won't be inhibiting the // out-of-order core by speculating them earlier. We also allow // instructions that are used by the terminator's condition because it // exposes more merging opportunities. bool UsedByBranch = (BonusInst && BonusInst->hasOneUse() && BonusInst->user_back() == Cond); if (BonusInst && !UsedByBranch) { // Collect the values used by the bonus inst SmallPtrSet UsedValues; for (Instruction::op_iterator OI = BonusInst->op_begin(), OE = BonusInst->op_end(); OI != OE; ++OI) { Value *V = *OI; if (!isa(V) && !isa(V)) UsedValues.insert(V); } SmallVector, 4> Worklist; Worklist.push_back(std::make_pair(PBI->getOperand(0), 0)); // Walk up to four levels back up the use-def chain of the predecessor's // terminator to see if all those values were used. The choice of four // levels is arbitrary, to provide a compile-time-cost bound. while (!Worklist.empty()) { std::pair Pair = Worklist.back(); Worklist.pop_back(); if (Pair.second >= 4) continue; UsedValues.erase(Pair.first); if (UsedValues.empty()) break; if (Instruction *I = dyn_cast(Pair.first)) { for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end(); OI != OE; ++OI) Worklist.push_back(std::make_pair(OI->get(), Pair.second+1)); } } if (!UsedValues.empty()) return false; } DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB); IRBuilder<> Builder(PBI); // If we need to invert the condition in the pred block to match, do so now. if (InvertPredCond) { Value *NewCond = PBI->getCondition(); if (NewCond->hasOneUse() && isa(NewCond)) { CmpInst *CI = cast(NewCond); CI->setPredicate(CI->getInversePredicate()); } else { NewCond = Builder.CreateNot(NewCond, PBI->getCondition()->getName()+".not"); } PBI->setCondition(NewCond); PBI->swapSuccessors(); } // If we have a bonus inst, clone it into the predecessor block. Instruction *NewBonus = nullptr; if (BonusInst) { NewBonus = BonusInst->clone(); // If we moved a load, we cannot any longer claim any knowledge about // its potential value. The previous information might have been valid // only given the branch precondition. // For an analogous reason, we must also drop all the metadata whose // semantics we don't understand. NewBonus->dropUnknownMetadata(LLVMContext::MD_dbg); PredBlock->getInstList().insert(PBI, NewBonus); NewBonus->takeName(BonusInst); BonusInst->setName(BonusInst->getName()+".old"); } // Clone Cond into the predecessor basic block, and or/and the // two conditions together. Instruction *New = Cond->clone(); if (BonusInst) New->replaceUsesOfWith(BonusInst, NewBonus); PredBlock->getInstList().insert(PBI, New); New->takeName(Cond); Cond->setName(New->getName()+".old"); if (BI->isConditional()) { Instruction *NewCond = cast(Builder.CreateBinOp(Opc, PBI->getCondition(), New, "or.cond")); PBI->setCondition(NewCond); uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight; bool PredHasWeights = ExtractBranchMetadata(PBI, PredTrueWeight, PredFalseWeight); bool SuccHasWeights = ExtractBranchMetadata(BI, SuccTrueWeight, SuccFalseWeight); SmallVector NewWeights; if (PBI->getSuccessor(0) == BB) { if (PredHasWeights && SuccHasWeights) { // PBI: br i1 %x, BB, FalseDest // BI: br i1 %y, TrueDest, FalseDest //TrueWeight is TrueWeight for PBI * TrueWeight for BI. NewWeights.push_back(PredTrueWeight * SuccTrueWeight); //FalseWeight is FalseWeight for PBI * TotalWeight for BI + // TrueWeight for PBI * FalseWeight for BI. // We assume that total weights of a BranchInst can fit into 32 bits. // Therefore, we will not have overflow using 64-bit arithmetic. NewWeights.push_back(PredFalseWeight * (SuccFalseWeight + SuccTrueWeight) + PredTrueWeight * SuccFalseWeight); } AddPredecessorToBlock(TrueDest, PredBlock, BB); PBI->setSuccessor(0, TrueDest); } if (PBI->getSuccessor(1) == BB) { if (PredHasWeights && SuccHasWeights) { // PBI: br i1 %x, TrueDest, BB // BI: br i1 %y, TrueDest, FalseDest //TrueWeight is TrueWeight for PBI * TotalWeight for BI + // FalseWeight for PBI * TrueWeight for BI. NewWeights.push_back(PredTrueWeight * (SuccFalseWeight + SuccTrueWeight) + PredFalseWeight * SuccTrueWeight); //FalseWeight is FalseWeight for PBI * FalseWeight for BI. NewWeights.push_back(PredFalseWeight * SuccFalseWeight); } AddPredecessorToBlock(FalseDest, PredBlock, BB); PBI->setSuccessor(1, FalseDest); } if (NewWeights.size() == 2) { // Halve the weights if any of them cannot fit in an uint32_t FitWeights(NewWeights); SmallVector MDWeights(NewWeights.begin(),NewWeights.end()); PBI->setMetadata(LLVMContext::MD_prof, MDBuilder(BI->getContext()). createBranchWeights(MDWeights)); } else PBI->setMetadata(LLVMContext::MD_prof, nullptr); } else { // Update PHI nodes in the common successors. for (unsigned i = 0, e = PHIs.size(); i != e; ++i) { ConstantInt *PBI_C = cast( PHIs[i]->getIncomingValueForBlock(PBI->getParent())); assert(PBI_C->getType()->isIntegerTy(1)); Instruction *MergedCond = nullptr; if (PBI->getSuccessor(0) == TrueDest) { // Create (PBI_Cond and PBI_C) or (!PBI_Cond and BI_Value) // PBI_C is true: PBI_Cond or (!PBI_Cond and BI_Value) // is false: !PBI_Cond and BI_Value Instruction *NotCond = cast(Builder.CreateNot(PBI->getCondition(), "not.cond")); MergedCond = cast(Builder.CreateBinOp(Instruction::And, NotCond, New, "and.cond")); if (PBI_C->isOne()) MergedCond = cast(Builder.CreateBinOp(Instruction::Or, PBI->getCondition(), MergedCond, "or.cond")); } else { // Create (PBI_Cond and BI_Value) or (!PBI_Cond and PBI_C) // PBI_C is true: (PBI_Cond and BI_Value) or (!PBI_Cond) // is false: PBI_Cond and BI_Value MergedCond = cast(Builder.CreateBinOp(Instruction::And, PBI->getCondition(), New, "and.cond")); if (PBI_C->isOne()) { Instruction *NotCond = cast(Builder.CreateNot(PBI->getCondition(), "not.cond")); MergedCond = cast(Builder.CreateBinOp(Instruction::Or, NotCond, MergedCond, "or.cond")); } } // Update PHI Node. PHIs[i]->setIncomingValue(PHIs[i]->getBasicBlockIndex(PBI->getParent()), MergedCond); } // Change PBI from Conditional to Unconditional. BranchInst *New_PBI = BranchInst::Create(TrueDest, PBI); EraseTerminatorInstAndDCECond(PBI); PBI = New_PBI; } // TODO: If BB is reachable from all paths through PredBlock, then we // could replace PBI's branch probabilities with BI's. // Copy any debug value intrinsics into the end of PredBlock. for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) if (isa(*I)) I->clone()->insertBefore(PBI); return true; } return false; } /// SimplifyCondBranchToCondBranch - If we have a conditional branch as a /// predecessor of another block, this function tries to simplify it. We know /// that PBI and BI are both conditional branches, and BI is in one of the /// successor blocks of PBI - PBI branches to BI. static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) { assert(PBI->isConditional() && BI->isConditional()); BasicBlock *BB = BI->getParent(); // If this block ends with a branch instruction, and if there is a // predecessor that ends on a branch of the same condition, make // this conditional branch redundant. if (PBI->getCondition() == BI->getCondition() && PBI->getSuccessor(0) != PBI->getSuccessor(1)) { // Okay, the outcome of this conditional branch is statically // knowable. If this block had a single pred, handle specially. if (BB->getSinglePredecessor()) { // Turn this into a branch on constant. bool CondIsTrue = PBI->getSuccessor(0) == BB; BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue)); return true; // Nuke the branch on constant. } // Otherwise, if there are multiple predecessors, insert a PHI that merges // in the constant and simplify the block result. Subsequent passes of // simplifycfg will thread the block. if (BlockIsSimpleEnoughToThreadThrough(BB)) { pred_iterator PB = pred_begin(BB), PE = pred_end(BB); PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()), std::distance(PB, PE), BI->getCondition()->getName() + ".pr", BB->begin()); // Okay, we're going to insert the PHI node. Since PBI is not the only // predecessor, compute the PHI'd conditional value for all of the preds. // Any predecessor where the condition is not computable we keep symbolic. for (pred_iterator PI = PB; PI != PE; ++PI) { BasicBlock *P = *PI; if ((PBI = dyn_cast(P->getTerminator())) && PBI != BI && PBI->isConditional() && PBI->getCondition() == BI->getCondition() && PBI->getSuccessor(0) != PBI->getSuccessor(1)) { bool CondIsTrue = PBI->getSuccessor(0) == BB; NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue), P); } else { NewPN->addIncoming(BI->getCondition(), P); } } BI->setCondition(NewPN); return true; } } // If this is a conditional branch in an empty block, and if any // predecessors are a conditional branch to one of our destinations, // fold the conditions into logical ops and one cond br. BasicBlock::iterator BBI = BB->begin(); // Ignore dbg intrinsics. while (isa(BBI)) ++BBI; if (&*BBI != BI) return false; if (ConstantExpr *CE = dyn_cast(BI->getCondition())) if (CE->canTrap()) return false; int PBIOp, BIOp; if (PBI->getSuccessor(0) == BI->getSuccessor(0)) PBIOp = BIOp = 0; else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) PBIOp = 0, BIOp = 1; else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) PBIOp = 1, BIOp = 0; else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) PBIOp = BIOp = 1; else return false; // Check to make sure that the other destination of this branch // isn't BB itself. If so, this is an infinite loop that will // keep getting unwound. if (PBI->getSuccessor(PBIOp) == BB) return false; // Do not perform this transformation if it would require // insertion of a large number of select instructions. For targets // without predication/cmovs, this is a big pessimization. // Also do not perform this transformation if any phi node in the common // destination block can trap when reached by BB or PBB (PR17073). In that // case, it would be unsafe to hoist the operation into a select instruction. BasicBlock *CommonDest = PBI->getSuccessor(PBIOp); unsigned NumPhis = 0; for (BasicBlock::iterator II = CommonDest->begin(); isa(II); ++II, ++NumPhis) { if (NumPhis > 2) // Disable this xform. return false; PHINode *PN = cast(II); Value *BIV = PN->getIncomingValueForBlock(BB); if (ConstantExpr *CE = dyn_cast(BIV)) if (CE->canTrap()) return false; unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent()); Value *PBIV = PN->getIncomingValue(PBBIdx); if (ConstantExpr *CE = dyn_cast(PBIV)) if (CE->canTrap()) return false; } // Finally, if everything is ok, fold the branches to logical ops. BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1); DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent() << "AND: " << *BI->getParent()); // If OtherDest *is* BB, then BB is a basic block with a single conditional // branch in it, where one edge (OtherDest) goes back to itself but the other // exits. We don't *know* that the program avoids the infinite loop // (even though that seems likely). If we do this xform naively, we'll end up // recursively unpeeling the loop. Since we know that (after the xform is // done) that the block *is* infinite if reached, we just make it an obviously // infinite loop with no cond branch. if (OtherDest == BB) { // Insert it at the end of the function, because it's either code, // or it won't matter if it's hot. :) BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(), "infloop", BB->getParent()); BranchInst::Create(InfLoopBlock, InfLoopBlock); OtherDest = InfLoopBlock; } DEBUG(dbgs() << *PBI->getParent()->getParent()); // BI may have other predecessors. Because of this, we leave // it alone, but modify PBI. // Make sure we get to CommonDest on True&True directions. Value *PBICond = PBI->getCondition(); IRBuilder Builder(PBI); if (PBIOp) PBICond = Builder.CreateNot(PBICond, PBICond->getName()+".not"); Value *BICond = BI->getCondition(); if (BIOp) BICond = Builder.CreateNot(BICond, BICond->getName()+".not"); // Merge the conditions. Value *Cond = Builder.CreateOr(PBICond, BICond, "brmerge"); // Modify PBI to branch on the new condition to the new dests. PBI->setCondition(Cond); PBI->setSuccessor(0, CommonDest); PBI->setSuccessor(1, OtherDest); // Update branch weight for PBI. uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight; bool PredHasWeights = ExtractBranchMetadata(PBI, PredTrueWeight, PredFalseWeight); bool SuccHasWeights = ExtractBranchMetadata(BI, SuccTrueWeight, SuccFalseWeight); if (PredHasWeights && SuccHasWeights) { uint64_t PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight; uint64_t PredOther = PBIOp ?PredTrueWeight : PredFalseWeight; uint64_t SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight; uint64_t SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight; // The weight to CommonDest should be PredCommon * SuccTotal + // PredOther * SuccCommon. // The weight to OtherDest should be PredOther * SuccOther. SmallVector NewWeights; NewWeights.push_back(PredCommon * (SuccCommon + SuccOther) + PredOther * SuccCommon); NewWeights.push_back(PredOther * SuccOther); // Halve the weights if any of them cannot fit in an uint32_t FitWeights(NewWeights); SmallVector MDWeights(NewWeights.begin(),NewWeights.end()); PBI->setMetadata(LLVMContext::MD_prof, MDBuilder(BI->getContext()). createBranchWeights(MDWeights)); } // OtherDest may have phi nodes. If so, add an entry from PBI's // block that are identical to the entries for BI's block. AddPredecessorToBlock(OtherDest, PBI->getParent(), BB); // We know that the CommonDest already had an edge from PBI to // it. If it has PHIs though, the PHIs may have different // entries for BB and PBI's BB. If so, insert a select to make // them agree. PHINode *PN; for (BasicBlock::iterator II = CommonDest->begin(); (PN = dyn_cast(II)); ++II) { Value *BIV = PN->getIncomingValueForBlock(BB); unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent()); Value *PBIV = PN->getIncomingValue(PBBIdx); if (BIV != PBIV) { // Insert a select in PBI to pick the right value. Value *NV = cast (Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName()+".mux")); PN->setIncomingValue(PBBIdx, NV); } } DEBUG(dbgs() << "INTO: " << *PBI->getParent()); DEBUG(dbgs() << *PBI->getParent()->getParent()); // This basic block is probably dead. We know it has at least // one fewer predecessor. return true; } // SimplifyTerminatorOnSelect - Simplifies a terminator by replacing it with a // branch to TrueBB if Cond is true or to FalseBB if Cond is false. // Takes care of updating the successors and removing the old terminator. // Also makes sure not to introduce new successors by assuming that edges to // non-successor TrueBBs and FalseBBs aren't reachable. static bool SimplifyTerminatorOnSelect(TerminatorInst *OldTerm, Value *Cond, BasicBlock *TrueBB, BasicBlock *FalseBB, uint32_t TrueWeight, uint32_t FalseWeight){ // Remove any superfluous successor edges from the CFG. // First, figure out which successors to preserve. // If TrueBB and FalseBB are equal, only try to preserve one copy of that // successor. BasicBlock *KeepEdge1 = TrueBB; BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : nullptr; // Then remove the rest. for (unsigned I = 0, E = OldTerm->getNumSuccessors(); I != E; ++I) { BasicBlock *Succ = OldTerm->getSuccessor(I); // Make sure only to keep exactly one copy of each edge. if (Succ == KeepEdge1) KeepEdge1 = nullptr; else if (Succ == KeepEdge2) KeepEdge2 = nullptr; else Succ->removePredecessor(OldTerm->getParent()); } IRBuilder<> Builder(OldTerm); Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc()); // Insert an appropriate new terminator. if (!KeepEdge1 && !KeepEdge2) { if (TrueBB == FalseBB) // We were only looking for one successor, and it was present. // Create an unconditional branch to it. Builder.CreateBr(TrueBB); else { // We found both of the successors we were looking for. // Create a conditional branch sharing the condition of the select. BranchInst *NewBI = Builder.CreateCondBr(Cond, TrueBB, FalseBB); if (TrueWeight != FalseWeight) NewBI->setMetadata(LLVMContext::MD_prof, MDBuilder(OldTerm->getContext()). createBranchWeights(TrueWeight, FalseWeight)); } } else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) { // Neither of the selected blocks were successors, so this // terminator must be unreachable. new UnreachableInst(OldTerm->getContext(), OldTerm); } else { // One of the selected values was a successor, but the other wasn't. // Insert an unconditional branch to the one that was found; // the edge to the one that wasn't must be unreachable. if (!KeepEdge1) // Only TrueBB was found. Builder.CreateBr(TrueBB); else // Only FalseBB was found. Builder.CreateBr(FalseBB); } EraseTerminatorInstAndDCECond(OldTerm); return true; } // SimplifySwitchOnSelect - Replaces // (switch (select cond, X, Y)) on constant X, Y // with a branch - conditional if X and Y lead to distinct BBs, // unconditional otherwise. static bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select) { // Check for constant integer values in the select. ConstantInt *TrueVal = dyn_cast(Select->getTrueValue()); ConstantInt *FalseVal = dyn_cast(Select->getFalseValue()); if (!TrueVal || !FalseVal) return false; // Find the relevant condition and destinations. Value *Condition = Select->getCondition(); BasicBlock *TrueBB = SI->findCaseValue(TrueVal).getCaseSuccessor(); BasicBlock *FalseBB = SI->findCaseValue(FalseVal).getCaseSuccessor(); // Get weight for TrueBB and FalseBB. uint32_t TrueWeight = 0, FalseWeight = 0; SmallVector Weights; bool HasWeights = HasBranchWeights(SI); if (HasWeights) { GetBranchWeights(SI, Weights); if (Weights.size() == 1 + SI->getNumCases()) { TrueWeight = (uint32_t)Weights[SI->findCaseValue(TrueVal). getSuccessorIndex()]; FalseWeight = (uint32_t)Weights[SI->findCaseValue(FalseVal). getSuccessorIndex()]; } } // Perform the actual simplification. return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB, TrueWeight, FalseWeight); } // SimplifyIndirectBrOnSelect - Replaces // (indirectbr (select cond, blockaddress(@fn, BlockA), // blockaddress(@fn, BlockB))) // with // (br cond, BlockA, BlockB). static bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI) { // Check that both operands of the select are block addresses. BlockAddress *TBA = dyn_cast(SI->getTrueValue()); BlockAddress *FBA = dyn_cast(SI->getFalseValue()); if (!TBA || !FBA) return false; // Extract the actual blocks. BasicBlock *TrueBB = TBA->getBasicBlock(); BasicBlock *FalseBB = FBA->getBasicBlock(); // Perform the actual simplification. return SimplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB, 0, 0); } /// TryToSimplifyUncondBranchWithICmpInIt - This is called when we find an icmp /// instruction (a seteq/setne with a constant) as the only instruction in a /// block that ends with an uncond branch. We are looking for a very specific /// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In /// this case, we merge the first two "or's of icmp" into a switch, but then the /// default value goes to an uncond block with a seteq in it, we get something /// like: /// /// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ] /// DEFAULT: /// %tmp = icmp eq i8 %A, 92 /// br label %end /// end: /// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ] /// /// We prefer to split the edge to 'end' so that there is a true/false entry to /// the PHI, merging the third icmp into the switch. static bool TryToSimplifyUncondBranchWithICmpInIt( ICmpInst *ICI, IRBuilder<> &Builder, const TargetTransformInfo &TTI, const DataLayout *DL) { BasicBlock *BB = ICI->getParent(); // If the block has any PHIs in it or the icmp has multiple uses, it is too // complex. if (isa(BB->begin()) || !ICI->hasOneUse()) return false; Value *V = ICI->getOperand(0); ConstantInt *Cst = cast(ICI->getOperand(1)); // The pattern we're looking for is where our only predecessor is a switch on // 'V' and this block is the default case for the switch. In this case we can // fold the compared value into the switch to simplify things. BasicBlock *Pred = BB->getSinglePredecessor(); if (!Pred || !isa(Pred->getTerminator())) return false; SwitchInst *SI = cast(Pred->getTerminator()); if (SI->getCondition() != V) return false; // If BB is reachable on a non-default case, then we simply know the value of // V in this block. Substitute it and constant fold the icmp instruction // away. if (SI->getDefaultDest() != BB) { ConstantInt *VVal = SI->findCaseDest(BB); assert(VVal && "Should have a unique destination value"); ICI->setOperand(0, VVal); if (Value *V = SimplifyInstruction(ICI, DL)) { ICI->replaceAllUsesWith(V); ICI->eraseFromParent(); } // BB is now empty, so it is likely to simplify away. return SimplifyCFG(BB, TTI, DL) | true; } // Ok, the block is reachable from the default dest. If the constant we're // comparing exists in one of the other edges, then we can constant fold ICI // and zap it. if (SI->findCaseValue(Cst) != SI->case_default()) { Value *V; if (ICI->getPredicate() == ICmpInst::ICMP_EQ) V = ConstantInt::getFalse(BB->getContext()); else V = ConstantInt::getTrue(BB->getContext()); ICI->replaceAllUsesWith(V); ICI->eraseFromParent(); // BB is now empty, so it is likely to simplify away. return SimplifyCFG(BB, TTI, DL) | true; } // The use of the icmp has to be in the 'end' block, by the only PHI node in // the block. BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0); PHINode *PHIUse = dyn_cast(ICI->user_back()); if (PHIUse == nullptr || PHIUse != &SuccBlock->front() || isa(++BasicBlock::iterator(PHIUse))) return false; // If the icmp is a SETEQ, then the default dest gets false, the new edge gets // true in the PHI. Constant *DefaultCst = ConstantInt::getTrue(BB->getContext()); Constant *NewCst = ConstantInt::getFalse(BB->getContext()); if (ICI->getPredicate() == ICmpInst::ICMP_EQ) std::swap(DefaultCst, NewCst); // Replace ICI (which is used by the PHI for the default value) with true or // false depending on if it is EQ or NE. ICI->replaceAllUsesWith(DefaultCst); ICI->eraseFromParent(); // Okay, the switch goes to this block on a default value. Add an edge from // the switch to the merge point on the compared value. BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB); SmallVector Weights; bool HasWeights = HasBranchWeights(SI); if (HasWeights) { GetBranchWeights(SI, Weights); if (Weights.size() == 1 + SI->getNumCases()) { // Split weight for default case to case for "Cst". Weights[0] = (Weights[0]+1) >> 1; Weights.push_back(Weights[0]); SmallVector MDWeights(Weights.begin(), Weights.end()); SI->setMetadata(LLVMContext::MD_prof, MDBuilder(SI->getContext()). createBranchWeights(MDWeights)); } } SI->addCase(Cst, NewBB); // NewBB branches to the phi block, add the uncond branch and the phi entry. Builder.SetInsertPoint(NewBB); Builder.SetCurrentDebugLocation(SI->getDebugLoc()); Builder.CreateBr(SuccBlock); PHIUse->addIncoming(NewCst, NewBB); return true; } /// SimplifyBranchOnICmpChain - The specified branch is a conditional branch. /// Check to see if it is branching on an or/and chain of icmp instructions, and /// fold it into a switch instruction if so. static bool SimplifyBranchOnICmpChain(BranchInst *BI, const DataLayout *DL, IRBuilder<> &Builder) { Instruction *Cond = dyn_cast(BI->getCondition()); if (!Cond) return false; // Change br (X == 0 | X == 1), T, F into a switch instruction. // If this is a bunch of seteq's or'd together, or if it's a bunch of // 'setne's and'ed together, collect them. Value *CompVal = nullptr; std::vector Values; bool TrueWhenEqual = true; Value *ExtraCase = nullptr; unsigned UsedICmps = 0; if (Cond->getOpcode() == Instruction::Or) { CompVal = GatherConstantCompares(Cond, Values, ExtraCase, DL, true, UsedICmps); } else if (Cond->getOpcode() == Instruction::And) { CompVal = GatherConstantCompares(Cond, Values, ExtraCase, DL, false, UsedICmps); TrueWhenEqual = false; } // If we didn't have a multiply compared value, fail. if (!CompVal) return false; // Avoid turning single icmps into a switch. if (UsedICmps <= 1) return false; // There might be duplicate constants in the list, which the switch // instruction can't handle, remove them now. array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate); Values.erase(std::unique(Values.begin(), Values.end()), Values.end()); // If Extra was used, we require at least two switch values to do the // transformation. A switch with one value is just an cond branch. if (ExtraCase && Values.size() < 2) return false; // TODO: Preserve branch weight metadata, similarly to how // FoldValueComparisonIntoPredecessors preserves it. // Figure out which block is which destination. BasicBlock *DefaultBB = BI->getSuccessor(1); BasicBlock *EdgeBB = BI->getSuccessor(0); if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB); BasicBlock *BB = BI->getParent(); DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size() << " cases into SWITCH. BB is:\n" << *BB); // If there are any extra values that couldn't be folded into the switch // then we evaluate them with an explicit branch first. Split the block // right before the condbr to handle it. if (ExtraCase) { BasicBlock *NewBB = BB->splitBasicBlock(BI, "switch.early.test"); // Remove the uncond branch added to the old block. TerminatorInst *OldTI = BB->getTerminator(); Builder.SetInsertPoint(OldTI); if (TrueWhenEqual) Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB); else Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB); OldTI->eraseFromParent(); // If there are PHI nodes in EdgeBB, then we need to add a new entry to them // for the edge we just added. AddPredecessorToBlock(EdgeBB, BB, NewBB); DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase << "\nEXTRABB = " << *BB); BB = NewBB; } Builder.SetInsertPoint(BI); // Convert pointer to int before we switch. if (CompVal->getType()->isPointerTy()) { assert(DL && "Cannot switch on pointer without DataLayout"); CompVal = Builder.CreatePtrToInt(CompVal, DL->getIntPtrType(CompVal->getType()), "magicptr"); } // Create the new switch instruction now. SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size()); // Add all of the 'cases' to the switch instruction. for (unsigned i = 0, e = Values.size(); i != e; ++i) New->addCase(Values[i], EdgeBB); // We added edges from PI to the EdgeBB. As such, if there were any // PHI nodes in EdgeBB, they need entries to be added corresponding to // the number of edges added. for (BasicBlock::iterator BBI = EdgeBB->begin(); isa(BBI); ++BBI) { PHINode *PN = cast(BBI); Value *InVal = PN->getIncomingValueForBlock(BB); for (unsigned i = 0, e = Values.size()-1; i != e; ++i) PN->addIncoming(InVal, BB); } // Erase the old branch instruction. EraseTerminatorInstAndDCECond(BI); DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n'); return true; } bool SimplifyCFGOpt::SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder) { // If this is a trivial landing pad that just continues unwinding the caught // exception then zap the landing pad, turning its invokes into calls. BasicBlock *BB = RI->getParent(); LandingPadInst *LPInst = dyn_cast(BB->getFirstNonPHI()); if (RI->getValue() != LPInst) // Not a landing pad, or the resume is not unwinding the exception that // caused control to branch here. return false; // Check that there are no other instructions except for debug intrinsics. BasicBlock::iterator I = LPInst, E = RI; while (++I != E) if (!isa(I)) return false; // Turn all invokes that unwind here into calls and delete the basic block. bool InvokeRequiresTableEntry = false; bool Changed = false; for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) { InvokeInst *II = cast((*PI++)->getTerminator()); if (II->hasFnAttr(Attribute::UWTable)) { // Don't remove an `invoke' instruction if the ABI requires an entry into // the table. InvokeRequiresTableEntry = true; continue; } SmallVector Args(II->op_begin(), II->op_end() - 3); // Insert a call instruction before the invoke. CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II); Call->takeName(II); Call->setCallingConv(II->getCallingConv()); Call->setAttributes(II->getAttributes()); Call->setDebugLoc(II->getDebugLoc()); // Anything that used the value produced by the invoke instruction now uses // the value produced by the call instruction. Note that we do this even // for void functions and calls with no uses so that the callgraph edge is // updated. II->replaceAllUsesWith(Call); BB->removePredecessor(II->getParent()); // Insert a branch to the normal destination right before the invoke. BranchInst::Create(II->getNormalDest(), II); // Finally, delete the invoke instruction! II->eraseFromParent(); Changed = true; } if (!InvokeRequiresTableEntry) // The landingpad is now unreachable. Zap it. BB->eraseFromParent(); return Changed; } bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) { BasicBlock *BB = RI->getParent(); if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false; // Find predecessors that end with branches. SmallVector UncondBranchPreds; SmallVector CondBranchPreds; for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { BasicBlock *P = *PI; TerminatorInst *PTI = P->getTerminator(); if (BranchInst *BI = dyn_cast(PTI)) { if (BI->isUnconditional()) UncondBranchPreds.push_back(P); else CondBranchPreds.push_back(BI); } } // If we found some, do the transformation! if (!UncondBranchPreds.empty() && DupRet) { while (!UncondBranchPreds.empty()) { BasicBlock *Pred = UncondBranchPreds.pop_back_val(); DEBUG(dbgs() << "FOLDING: " << *BB << "INTO UNCOND BRANCH PRED: " << *Pred); (void)FoldReturnIntoUncondBranch(RI, BB, Pred); } // If we eliminated all predecessors of the block, delete the block now. if (pred_begin(BB) == pred_end(BB)) // We know there are no successors, so just nuke the block. BB->eraseFromParent(); return true; } // Check out all of the conditional branches going to this return // instruction. If any of them just select between returns, change the // branch itself into a select/return pair. while (!CondBranchPreds.empty()) { BranchInst *BI = CondBranchPreds.pop_back_val(); // Check to see if the non-BB successor is also a return block. if (isa(BI->getSuccessor(0)->getTerminator()) && isa(BI->getSuccessor(1)->getTerminator()) && SimplifyCondBranchToTwoReturns(BI, Builder)) return true; } return false; } bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) { BasicBlock *BB = UI->getParent(); bool Changed = false; // If there are any instructions immediately before the unreachable that can // be removed, do so. while (UI != BB->begin()) { BasicBlock::iterator BBI = UI; --BBI; // Do not delete instructions that can have side effects which might cause // the unreachable to not be reachable; specifically, calls and volatile // operations may have this effect. if (isa(BBI) && !isa(BBI)) break; if (BBI->mayHaveSideEffects()) { if (StoreInst *SI = dyn_cast(BBI)) { if (SI->isVolatile()) break; } else if (LoadInst *LI = dyn_cast(BBI)) { if (LI->isVolatile()) break; } else if (AtomicRMWInst *RMWI = dyn_cast(BBI)) { if (RMWI->isVolatile()) break; } else if (AtomicCmpXchgInst *CXI = dyn_cast(BBI)) { if (CXI->isVolatile()) break; } else if (!isa(BBI) && !isa(BBI) && !isa(BBI)) { break; } // Note that deleting LandingPad's here is in fact okay, although it // involves a bit of subtle reasoning. If this inst is a LandingPad, // all the predecessors of this block will be the unwind edges of Invokes, // and we can therefore guarantee this block will be erased. } // Delete this instruction (any uses are guaranteed to be dead) if (!BBI->use_empty()) BBI->replaceAllUsesWith(UndefValue::get(BBI->getType())); BBI->eraseFromParent(); Changed = true; } // If the unreachable instruction is the first in the block, take a gander // at all of the predecessors of this instruction, and simplify them. if (&BB->front() != UI) return Changed; SmallVector Preds(pred_begin(BB), pred_end(BB)); for (unsigned i = 0, e = Preds.size(); i != e; ++i) { TerminatorInst *TI = Preds[i]->getTerminator(); IRBuilder<> Builder(TI); if (BranchInst *BI = dyn_cast(TI)) { if (BI->isUnconditional()) { if (BI->getSuccessor(0) == BB) { new UnreachableInst(TI->getContext(), TI); TI->eraseFromParent(); Changed = true; } } else { if (BI->getSuccessor(0) == BB) { Builder.CreateBr(BI->getSuccessor(1)); EraseTerminatorInstAndDCECond(BI); } else if (BI->getSuccessor(1) == BB) { Builder.CreateBr(BI->getSuccessor(0)); EraseTerminatorInstAndDCECond(BI); Changed = true; } } } else if (SwitchInst *SI = dyn_cast(TI)) { for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i) if (i.getCaseSuccessor() == BB) { BB->removePredecessor(SI->getParent()); SI->removeCase(i); --i; --e; Changed = true; } // If the default value is unreachable, figure out the most popular // destination and make it the default. if (SI->getDefaultDest() == BB) { std::map > Popularity; for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i) { std::pair &entry = Popularity[i.getCaseSuccessor()]; if (entry.first == 0) { entry.first = 1; entry.second = i.getCaseIndex(); } else { entry.first++; } } // Find the most popular block. unsigned MaxPop = 0; unsigned MaxIndex = 0; BasicBlock *MaxBlock = nullptr; for (std::map >::iterator I = Popularity.begin(), E = Popularity.end(); I != E; ++I) { if (I->second.first > MaxPop || (I->second.first == MaxPop && MaxIndex > I->second.second)) { MaxPop = I->second.first; MaxIndex = I->second.second; MaxBlock = I->first; } } if (MaxBlock) { // Make this the new default, allowing us to delete any explicit // edges to it. SI->setDefaultDest(MaxBlock); Changed = true; // If MaxBlock has phinodes in it, remove MaxPop-1 entries from // it. if (isa(MaxBlock->begin())) for (unsigned i = 0; i != MaxPop-1; ++i) MaxBlock->removePredecessor(SI->getParent()); for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i) if (i.getCaseSuccessor() == MaxBlock) { SI->removeCase(i); --i; --e; } } } } else if (InvokeInst *II = dyn_cast(TI)) { if (II->getUnwindDest() == BB) { // Convert the invoke to a call instruction. This would be a good // place to note that the call does not throw though. BranchInst *BI = Builder.CreateBr(II->getNormalDest()); II->removeFromParent(); // Take out of symbol table // Insert the call now... SmallVector Args(II->op_begin(), II->op_end()-3); Builder.SetInsertPoint(BI); CallInst *CI = Builder.CreateCall(II->getCalledValue(), Args, II->getName()); CI->setCallingConv(II->getCallingConv()); CI->setAttributes(II->getAttributes()); // If the invoke produced a value, the call does now instead. II->replaceAllUsesWith(CI); delete II; Changed = true; } } } // If this block is now dead, remove it. if (pred_begin(BB) == pred_end(BB) && BB != &BB->getParent()->getEntryBlock()) { // We know there are no successors, so just nuke the block. BB->eraseFromParent(); return true; } return Changed; } /// TurnSwitchRangeIntoICmp - Turns a switch with that contains only a /// integer range comparison into a sub, an icmp and a branch. static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) { assert(SI->getNumCases() > 1 && "Degenerate switch?"); // Make sure all cases point to the same destination and gather the values. SmallVector Cases; SwitchInst::CaseIt I = SI->case_begin(); Cases.push_back(I.getCaseValue()); SwitchInst::CaseIt PrevI = I++; for (SwitchInst::CaseIt E = SI->case_end(); I != E; PrevI = I++) { if (PrevI.getCaseSuccessor() != I.getCaseSuccessor()) return false; Cases.push_back(I.getCaseValue()); } assert(Cases.size() == SI->getNumCases() && "Not all cases gathered"); // Sort the case values, then check if they form a range we can transform. array_pod_sort(Cases.begin(), Cases.end(), ConstantIntSortPredicate); for (unsigned I = 1, E = Cases.size(); I != E; ++I) { if (Cases[I-1]->getValue() != Cases[I]->getValue()+1) return false; } Constant *Offset = ConstantExpr::getNeg(Cases.back()); Constant *NumCases = ConstantInt::get(Offset->getType(), SI->getNumCases()); Value *Sub = SI->getCondition(); if (!Offset->isNullValue()) Sub = Builder.CreateAdd(Sub, Offset, Sub->getName()+".off"); Value *Cmp; // If NumCases overflowed, then all possible values jump to the successor. if (NumCases->isNullValue() && SI->getNumCases() != 0) Cmp = ConstantInt::getTrue(SI->getContext()); else Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch"); BranchInst *NewBI = Builder.CreateCondBr( Cmp, SI->case_begin().getCaseSuccessor(), SI->getDefaultDest()); // Update weight for the newly-created conditional branch. SmallVector Weights; bool HasWeights = HasBranchWeights(SI); if (HasWeights) { GetBranchWeights(SI, Weights); if (Weights.size() == 1 + SI->getNumCases()) { // Combine all weights for the cases to be the true weight of NewBI. // We assume that the sum of all weights for a Terminator can fit into 32 // bits. uint32_t NewTrueWeight = 0; for (unsigned I = 1, E = Weights.size(); I != E; ++I) NewTrueWeight += (uint32_t)Weights[I]; NewBI->setMetadata(LLVMContext::MD_prof, MDBuilder(SI->getContext()). createBranchWeights(NewTrueWeight, (uint32_t)Weights[0])); } } // Prune obsolete incoming values off the successor's PHI nodes. for (BasicBlock::iterator BBI = SI->case_begin().getCaseSuccessor()->begin(); isa(BBI); ++BBI) { for (unsigned I = 0, E = SI->getNumCases()-1; I != E; ++I) cast(BBI)->removeIncomingValue(SI->getParent()); } SI->eraseFromParent(); return true; } /// EliminateDeadSwitchCases - Compute masked bits for the condition of a switch /// and use it to remove dead cases. static bool EliminateDeadSwitchCases(SwitchInst *SI) { Value *Cond = SI->getCondition(); unsigned Bits = Cond->getType()->getIntegerBitWidth(); APInt KnownZero(Bits, 0), KnownOne(Bits, 0); computeKnownBits(Cond, KnownZero, KnownOne); // Gather dead cases. SmallVector DeadCases; for (SwitchInst::CaseIt I = SI->case_begin(), E = SI->case_end(); I != E; ++I) { if ((I.getCaseValue()->getValue() & KnownZero) != 0 || (I.getCaseValue()->getValue() & KnownOne) != KnownOne) { DeadCases.push_back(I.getCaseValue()); DEBUG(dbgs() << "SimplifyCFG: switch case '" << I.getCaseValue() << "' is dead.\n"); } } SmallVector Weights; bool HasWeight = HasBranchWeights(SI); if (HasWeight) { GetBranchWeights(SI, Weights); HasWeight = (Weights.size() == 1 + SI->getNumCases()); } // Remove dead cases from the switch. for (unsigned I = 0, E = DeadCases.size(); I != E; ++I) { SwitchInst::CaseIt Case = SI->findCaseValue(DeadCases[I]); assert(Case != SI->case_default() && "Case was not found. Probably mistake in DeadCases forming."); if (HasWeight) { std::swap(Weights[Case.getCaseIndex()+1], Weights.back()); Weights.pop_back(); } // Prune unused values from PHI nodes. Case.getCaseSuccessor()->removePredecessor(SI->getParent()); SI->removeCase(Case); } if (HasWeight && Weights.size() >= 2) { SmallVector MDWeights(Weights.begin(), Weights.end()); SI->setMetadata(LLVMContext::MD_prof, MDBuilder(SI->getParent()->getContext()). createBranchWeights(MDWeights)); } return !DeadCases.empty(); } /// FindPHIForConditionForwarding - If BB would be eligible for simplification /// by TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated /// by an unconditional branch), look at the phi node for BB in the successor /// block and see if the incoming value is equal to CaseValue. If so, return /// the phi node, and set PhiIndex to BB's index in the phi node. static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue, BasicBlock *BB, int *PhiIndex) { if (BB->getFirstNonPHIOrDbg() != BB->getTerminator()) return nullptr; // BB must be empty to be a candidate for simplification. if (!BB->getSinglePredecessor()) return nullptr; // BB must be dominated by the switch. BranchInst *Branch = dyn_cast(BB->getTerminator()); if (!Branch || !Branch->isUnconditional()) return nullptr; // Terminator must be unconditional branch. BasicBlock *Succ = Branch->getSuccessor(0); BasicBlock::iterator I = Succ->begin(); while (PHINode *PHI = dyn_cast(I++)) { int Idx = PHI->getBasicBlockIndex(BB); assert(Idx >= 0 && "PHI has no entry for predecessor?"); Value *InValue = PHI->getIncomingValue(Idx); if (InValue != CaseValue) continue; *PhiIndex = Idx; return PHI; } return nullptr; } /// ForwardSwitchConditionToPHI - Try to forward the condition of a switch /// instruction to a phi node dominated by the switch, if that would mean that /// some of the destination blocks of the switch can be folded away. /// Returns true if a change is made. static bool ForwardSwitchConditionToPHI(SwitchInst *SI) { typedef DenseMap > ForwardingNodesMap; ForwardingNodesMap ForwardingNodes; for (SwitchInst::CaseIt I = SI->case_begin(), E = SI->case_end(); I != E; ++I) { ConstantInt *CaseValue = I.getCaseValue(); BasicBlock *CaseDest = I.getCaseSuccessor(); int PhiIndex; PHINode *PHI = FindPHIForConditionForwarding(CaseValue, CaseDest, &PhiIndex); if (!PHI) continue; ForwardingNodes[PHI].push_back(PhiIndex); } bool Changed = false; for (ForwardingNodesMap::iterator I = ForwardingNodes.begin(), E = ForwardingNodes.end(); I != E; ++I) { PHINode *Phi = I->first; SmallVectorImpl &Indexes = I->second; if (Indexes.size() < 2) continue; for (size_t I = 0, E = Indexes.size(); I != E; ++I) Phi->setIncomingValue(Indexes[I], SI->getCondition()); Changed = true; } return Changed; } /// ValidLookupTableConstant - Return true if the backend will be able to handle /// initializing an array of constants like C. static bool ValidLookupTableConstant(Constant *C) { if (C->isThreadDependent()) return false; if (C->isDLLImportDependent()) return false; if (ConstantExpr *CE = dyn_cast(C)) return CE->isGEPWithNoNotionalOverIndexing(); return isa(C) || isa(C) || isa(C) || isa(C) || isa(C); } /// LookupConstant - If V is a Constant, return it. Otherwise, try to look up /// its constant value in ConstantPool, returning 0 if it's not there. static Constant *LookupConstant(Value *V, const SmallDenseMap& ConstantPool) { if (Constant *C = dyn_cast(V)) return C; return ConstantPool.lookup(V); } /// ConstantFold - Try to fold instruction I into a constant. This works for /// simple instructions such as binary operations where both operands are /// constant or can be replaced by constants from the ConstantPool. Returns the /// resulting constant on success, 0 otherwise. static Constant * ConstantFold(Instruction *I, const SmallDenseMap &ConstantPool, const DataLayout *DL) { if (SelectInst *Select = dyn_cast(I)) { Constant *A = LookupConstant(Select->getCondition(), ConstantPool); if (!A) return nullptr; if (A->isAllOnesValue()) return LookupConstant(Select->getTrueValue(), ConstantPool); if (A->isNullValue()) return LookupConstant(Select->getFalseValue(), ConstantPool); return nullptr; } SmallVector COps; for (unsigned N = 0, E = I->getNumOperands(); N != E; ++N) { if (Constant *A = LookupConstant(I->getOperand(N), ConstantPool)) COps.push_back(A); else return nullptr; } if (CmpInst *Cmp = dyn_cast(I)) return ConstantFoldCompareInstOperands(Cmp->getPredicate(), COps[0], COps[1], DL); return ConstantFoldInstOperands(I->getOpcode(), I->getType(), COps, DL); } /// GetCaseResults - Try to determine the resulting constant values in phi nodes /// at the common destination basic block, *CommonDest, for one of the case /// destionations CaseDest corresponding to value CaseVal (0 for the default /// case), of a switch instruction SI. static bool GetCaseResults(SwitchInst *SI, ConstantInt *CaseVal, BasicBlock *CaseDest, BasicBlock **CommonDest, SmallVectorImpl > &Res, const DataLayout *DL) { // The block from which we enter the common destination. BasicBlock *Pred = SI->getParent(); // If CaseDest is empty except for some side-effect free instructions through // which we can constant-propagate the CaseVal, continue to its successor. SmallDenseMap ConstantPool; ConstantPool.insert(std::make_pair(SI->getCondition(), CaseVal)); for (BasicBlock::iterator I = CaseDest->begin(), E = CaseDest->end(); I != E; ++I) { if (TerminatorInst *T = dyn_cast(I)) { // If the terminator is a simple branch, continue to the next block. if (T->getNumSuccessors() != 1) return false; Pred = CaseDest; CaseDest = T->getSuccessor(0); } else if (isa(I)) { // Skip debug intrinsic. continue; } else if (Constant *C = ConstantFold(I, ConstantPool, DL)) { // Instruction is side-effect free and constant. ConstantPool.insert(std::make_pair(I, C)); } else { break; } } // If we did not have a CommonDest before, use the current one. if (!*CommonDest) *CommonDest = CaseDest; // If the destination isn't the common one, abort. if (CaseDest != *CommonDest) return false; // Get the values for this case from phi nodes in the destination block. BasicBlock::iterator I = (*CommonDest)->begin(); while (PHINode *PHI = dyn_cast(I++)) { int Idx = PHI->getBasicBlockIndex(Pred); if (Idx == -1) continue; Constant *ConstVal = LookupConstant(PHI->getIncomingValue(Idx), ConstantPool); if (!ConstVal) return false; // Note: If the constant comes from constant-propagating the case value // through the CaseDest basic block, it will be safe to remove the // instructions in that block. They cannot be used (except in the phi nodes // we visit) outside CaseDest, because that block does not dominate its // successor. If it did, we would not be in this phi node. // Be conservative about which kinds of constants we support. if (!ValidLookupTableConstant(ConstVal)) return false; Res.push_back(std::make_pair(PHI, ConstVal)); } return Res.size() > 0; } namespace { /// SwitchLookupTable - This class represents a lookup table that can be used /// to replace a switch. class SwitchLookupTable { public: /// SwitchLookupTable - Create a lookup table to use as a switch replacement /// with the contents of Values, using DefaultValue to fill any holes in the /// table. SwitchLookupTable(Module &M, uint64_t TableSize, ConstantInt *Offset, const SmallVectorImpl >& Values, Constant *DefaultValue, const DataLayout *DL); /// BuildLookup - Build instructions with Builder to retrieve the value at /// the position given by Index in the lookup table. Value *BuildLookup(Value *Index, IRBuilder<> &Builder); /// WouldFitInRegister - Return true if a table with TableSize elements of /// type ElementType would fit in a target-legal register. static bool WouldFitInRegister(const DataLayout *DL, uint64_t TableSize, const Type *ElementType); private: // Depending on the contents of the table, it can be represented in // different ways. enum { // For tables where each element contains the same value, we just have to // store that single value and return it for each lookup. SingleValueKind, // For small tables with integer elements, we can pack them into a bitmap // that fits into a target-legal register. Values are retrieved by // shift and mask operations. BitMapKind, // The table is stored as an array of values. Values are retrieved by load // instructions from the table. ArrayKind } Kind; // For SingleValueKind, this is the single value. Constant *SingleValue; // For BitMapKind, this is the bitmap. ConstantInt *BitMap; IntegerType *BitMapElementTy; // For ArrayKind, this is the array. GlobalVariable *Array; }; } SwitchLookupTable::SwitchLookupTable(Module &M, uint64_t TableSize, ConstantInt *Offset, const SmallVectorImpl >& Values, Constant *DefaultValue, const DataLayout *DL) : SingleValue(nullptr), BitMap(nullptr), BitMapElementTy(nullptr), Array(nullptr) { assert(Values.size() && "Can't build lookup table without values!"); assert(TableSize >= Values.size() && "Can't fit values in table!"); // If all values in the table are equal, this is that value. SingleValue = Values.begin()->second; Type *ValueType = Values.begin()->second->getType(); // Build up the table contents. SmallVector TableContents(TableSize); for (size_t I = 0, E = Values.size(); I != E; ++I) { ConstantInt *CaseVal = Values[I].first; Constant *CaseRes = Values[I].second; assert(CaseRes->getType() == ValueType); uint64_t Idx = (CaseVal->getValue() - Offset->getValue()) .getLimitedValue(); TableContents[Idx] = CaseRes; if (CaseRes != SingleValue) SingleValue = nullptr; } // Fill in any holes in the table with the default result. if (Values.size() < TableSize) { assert(DefaultValue && "Need a default value to fill the lookup table holes."); assert(DefaultValue->getType() == ValueType); for (uint64_t I = 0; I < TableSize; ++I) { if (!TableContents[I]) TableContents[I] = DefaultValue; } if (DefaultValue != SingleValue) SingleValue = nullptr; } // If each element in the table contains the same value, we only need to store // that single value. if (SingleValue) { Kind = SingleValueKind; return; } // If the type is integer and the table fits in a register, build a bitmap. if (WouldFitInRegister(DL, TableSize, ValueType)) { IntegerType *IT = cast(ValueType); APInt TableInt(TableSize * IT->getBitWidth(), 0); for (uint64_t I = TableSize; I > 0; --I) { TableInt <<= IT->getBitWidth(); // Insert values into the bitmap. Undef values are set to zero. if (!isa(TableContents[I - 1])) { ConstantInt *Val = cast(TableContents[I - 1]); TableInt |= Val->getValue().zext(TableInt.getBitWidth()); } } BitMap = ConstantInt::get(M.getContext(), TableInt); BitMapElementTy = IT; Kind = BitMapKind; ++NumBitMaps; return; } // Store the table in an array. ArrayType *ArrayTy = ArrayType::get(ValueType, TableSize); Constant *Initializer = ConstantArray::get(ArrayTy, TableContents); Array = new GlobalVariable(M, ArrayTy, /*constant=*/ true, GlobalVariable::PrivateLinkage, Initializer, "switch.table"); Array->setUnnamedAddr(true); Kind = ArrayKind; } Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder) { switch (Kind) { case SingleValueKind: return SingleValue; case BitMapKind: { // Type of the bitmap (e.g. i59). IntegerType *MapTy = BitMap->getType(); // Cast Index to the same type as the bitmap. // Note: The Index is <= the number of elements in the table, so // truncating it to the width of the bitmask is safe. Value *ShiftAmt = Builder.CreateZExtOrTrunc(Index, MapTy, "switch.cast"); // Multiply the shift amount by the element width. ShiftAmt = Builder.CreateMul(ShiftAmt, ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()), "switch.shiftamt"); // Shift down. Value *DownShifted = Builder.CreateLShr(BitMap, ShiftAmt, "switch.downshift"); // Mask off. return Builder.CreateTrunc(DownShifted, BitMapElementTy, "switch.masked"); } case ArrayKind: { Value *GEPIndices[] = { Builder.getInt32(0), Index }; Value *GEP = Builder.CreateInBoundsGEP(Array, GEPIndices, "switch.gep"); return Builder.CreateLoad(GEP, "switch.load"); } } llvm_unreachable("Unknown lookup table kind!"); } bool SwitchLookupTable::WouldFitInRegister(const DataLayout *DL, uint64_t TableSize, const Type *ElementType) { if (!DL) return false; const IntegerType *IT = dyn_cast(ElementType); if (!IT) return false; // FIXME: If the type is wider than it needs to be, e.g. i8 but all values // are <= 15, we could try to narrow the type. // Avoid overflow, fitsInLegalInteger uses unsigned int for the width. if (TableSize >= UINT_MAX/IT->getBitWidth()) return false; return DL->fitsInLegalInteger(TableSize * IT->getBitWidth()); } /// ShouldBuildLookupTable - Determine whether a lookup table should be built /// for this switch, based on the number of cases, size of the table and the /// types of the results. static bool ShouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize, const TargetTransformInfo &TTI, const DataLayout *DL, const SmallDenseMap& ResultTypes) { if (SI->getNumCases() > TableSize || TableSize >= UINT64_MAX / 10) return false; // TableSize overflowed, or mul below might overflow. bool AllTablesFitInRegister = true; bool HasIllegalType = false; for (SmallDenseMap::const_iterator I = ResultTypes.begin(), E = ResultTypes.end(); I != E; ++I) { Type *Ty = I->second; // Saturate this flag to true. HasIllegalType = HasIllegalType || !TTI.isTypeLegal(Ty); // Saturate this flag to false. AllTablesFitInRegister = AllTablesFitInRegister && SwitchLookupTable::WouldFitInRegister(DL, TableSize, Ty); // If both flags saturate, we're done. NOTE: This *only* works with // saturating flags, and all flags have to saturate first due to the // non-deterministic behavior of iterating over a dense map. if (HasIllegalType && !AllTablesFitInRegister) break; } // If each table would fit in a register, we should build it anyway. if (AllTablesFitInRegister) return true; // Don't build a table that doesn't fit in-register if it has illegal types. if (HasIllegalType) return false; // The table density should be at least 40%. This is the same criterion as for // jump tables, see SelectionDAGBuilder::handleJTSwitchCase. // FIXME: Find the best cut-off. return SI->getNumCases() * 10 >= TableSize * 4; } /// SwitchToLookupTable - If the switch is only used to initialize one or more /// phi nodes in a common successor block with different constant values, /// replace the switch with lookup tables. static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder, const TargetTransformInfo &TTI, const DataLayout* DL) { assert(SI->getNumCases() > 1 && "Degenerate switch?"); // Only build lookup table when we have a target that supports it. if (!TTI.shouldBuildLookupTables()) return false; // FIXME: If the switch is too sparse for a lookup table, perhaps we could // split off a dense part and build a lookup table for that. // FIXME: This creates arrays of GEPs to constant strings, which means each // GEP needs a runtime relocation in PIC code. We should just build one big // string and lookup indices into that. // Ignore switches with less than three cases. Lookup tables will not make them // faster, so we don't analyze them. if (SI->getNumCases() < 3) return false; // Figure out the corresponding result for each case value and phi node in the // common destination, as well as the the min and max case values. assert(SI->case_begin() != SI->case_end()); SwitchInst::CaseIt CI = SI->case_begin(); ConstantInt *MinCaseVal = CI.getCaseValue(); ConstantInt *MaxCaseVal = CI.getCaseValue(); BasicBlock *CommonDest = nullptr; typedef SmallVector, 4> ResultListTy; SmallDenseMap ResultLists; SmallDenseMap DefaultResults; SmallDenseMap ResultTypes; SmallVector PHIs; for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) { ConstantInt *CaseVal = CI.getCaseValue(); if (CaseVal->getValue().slt(MinCaseVal->getValue())) MinCaseVal = CaseVal; if (CaseVal->getValue().sgt(MaxCaseVal->getValue())) MaxCaseVal = CaseVal; // Resulting value at phi nodes for this case value. typedef SmallVector, 4> ResultsTy; ResultsTy Results; if (!GetCaseResults(SI, CaseVal, CI.getCaseSuccessor(), &CommonDest, Results, DL)) return false; // Append the result from this case to the list for each phi. for (ResultsTy::iterator I = Results.begin(), E = Results.end(); I!=E; ++I) { if (!ResultLists.count(I->first)) PHIs.push_back(I->first); ResultLists[I->first].push_back(std::make_pair(CaseVal, I->second)); } } // Keep track of the result types. for (size_t I = 0, E = PHIs.size(); I != E; ++I) { PHINode *PHI = PHIs[I]; ResultTypes[PHI] = ResultLists[PHI][0].second->getType(); } uint64_t NumResults = ResultLists[PHIs[0]].size(); APInt RangeSpread = MaxCaseVal->getValue() - MinCaseVal->getValue(); uint64_t TableSize = RangeSpread.getLimitedValue() + 1; bool TableHasHoles = (NumResults < TableSize); // If the table has holes, we need a constant result for the default case // or a bitmask that fits in a register. SmallVector, 4> DefaultResultsList; bool HasDefaultResults = false; if (TableHasHoles) { HasDefaultResults = GetCaseResults(SI, nullptr, SI->getDefaultDest(), &CommonDest, DefaultResultsList, DL); } bool NeedMask = (TableHasHoles && !HasDefaultResults); if (NeedMask) { // As an extra penalty for the validity test we require more cases. if (SI->getNumCases() < 4) // FIXME: Find best threshold value (benchmark). return false; if (!(DL && DL->fitsInLegalInteger(TableSize))) return false; } for (size_t I = 0, E = DefaultResultsList.size(); I != E; ++I) { PHINode *PHI = DefaultResultsList[I].first; Constant *Result = DefaultResultsList[I].second; DefaultResults[PHI] = Result; } if (!ShouldBuildLookupTable(SI, TableSize, TTI, DL, ResultTypes)) return false; // Create the BB that does the lookups. Module &Mod = *CommonDest->getParent()->getParent(); BasicBlock *LookupBB = BasicBlock::Create(Mod.getContext(), "switch.lookup", CommonDest->getParent(), CommonDest); // Compute the table index value. Builder.SetInsertPoint(SI); Value *TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal, "switch.tableidx"); // Compute the maximum table size representable by the integer type we are // switching upon. unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits(); uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize; assert(MaxTableSize >= TableSize && "It is impossible for a switch to have more entries than the max " "representable value of its input integer type's size."); // If we have a fully covered lookup table, unconditionally branch to the // lookup table BB. Otherwise, check if the condition value is within the case // range. If it is so, branch to the new BB. Otherwise branch to SI's default // destination. const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize; if (GeneratingCoveredLookupTable) { Builder.CreateBr(LookupBB); SI->getDefaultDest()->removePredecessor(SI->getParent()); } else { Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get( MinCaseVal->getType(), TableSize)); Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest()); } // Populate the BB that does the lookups. Builder.SetInsertPoint(LookupBB); if (NeedMask) { // Before doing the lookup we do the hole check. // The LookupBB is therefore re-purposed to do the hole check // and we create a new LookupBB. BasicBlock *MaskBB = LookupBB; MaskBB->setName("switch.hole_check"); LookupBB = BasicBlock::Create(Mod.getContext(), "switch.lookup", CommonDest->getParent(), CommonDest); // Build bitmask; fill in a 1 bit for every case. APInt MaskInt(TableSize, 0); APInt One(TableSize, 1); const ResultListTy &ResultList = ResultLists[PHIs[0]]; for (size_t I = 0, E = ResultList.size(); I != E; ++I) { uint64_t Idx = (ResultList[I].first->getValue() - MinCaseVal->getValue()).getLimitedValue(); MaskInt |= One << Idx; } ConstantInt *TableMask = ConstantInt::get(Mod.getContext(), MaskInt); // Get the TableIndex'th bit of the bitmask. // If this bit is 0 (meaning hole) jump to the default destination, // else continue with table lookup. IntegerType *MapTy = TableMask->getType(); Value *MaskIndex = Builder.CreateZExtOrTrunc(TableIndex, MapTy, "switch.maskindex"); Value *Shifted = Builder.CreateLShr(TableMask, MaskIndex, "switch.shifted"); Value *LoBit = Builder.CreateTrunc(Shifted, Type::getInt1Ty(Mod.getContext()), "switch.lobit"); Builder.CreateCondBr(LoBit, LookupBB, SI->getDefaultDest()); Builder.SetInsertPoint(LookupBB); AddPredecessorToBlock(SI->getDefaultDest(), MaskBB, SI->getParent()); } bool ReturnedEarly = false; for (size_t I = 0, E = PHIs.size(); I != E; ++I) { PHINode *PHI = PHIs[I]; // If using a bitmask, use any value to fill the lookup table holes. Constant *DV = NeedMask ? ResultLists[PHI][0].second : DefaultResults[PHI]; SwitchLookupTable Table(Mod, TableSize, MinCaseVal, ResultLists[PHI], DV, DL); Value *Result = Table.BuildLookup(TableIndex, Builder); // If the result is used to return immediately from the function, we want to // do that right here. if (PHI->hasOneUse() && isa(*PHI->user_begin()) && PHI->user_back() == CommonDest->getFirstNonPHIOrDbg()) { Builder.CreateRet(Result); ReturnedEarly = true; break; } PHI->addIncoming(Result, LookupBB); } if (!ReturnedEarly) Builder.CreateBr(CommonDest); // Remove the switch. for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) { BasicBlock *Succ = SI->getSuccessor(i); if (Succ == SI->getDefaultDest()) continue; Succ->removePredecessor(SI->getParent()); } SI->eraseFromParent(); ++NumLookupTables; if (NeedMask) ++NumLookupTablesHoles; return true; } bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) { BasicBlock *BB = SI->getParent(); if (isValueEqualityComparison(SI)) { // If we only have one predecessor, and if it is a branch on this value, // see if that predecessor totally determines the outcome of this switch. if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder)) return SimplifyCFG(BB, TTI, DL) | true; Value *Cond = SI->getCondition(); if (SelectInst *Select = dyn_cast(Cond)) if (SimplifySwitchOnSelect(SI, Select)) return SimplifyCFG(BB, TTI, DL) | true; // If the block only contains the switch, see if we can fold the block // away into any preds. BasicBlock::iterator BBI = BB->begin(); // Ignore dbg intrinsics. while (isa(BBI)) ++BBI; if (SI == &*BBI) if (FoldValueComparisonIntoPredecessors(SI, Builder)) return SimplifyCFG(BB, TTI, DL) | true; } // Try to transform the switch into an icmp and a branch. if (TurnSwitchRangeIntoICmp(SI, Builder)) return SimplifyCFG(BB, TTI, DL) | true; // Remove unreachable cases. if (EliminateDeadSwitchCases(SI)) return SimplifyCFG(BB, TTI, DL) | true; if (ForwardSwitchConditionToPHI(SI)) return SimplifyCFG(BB, TTI, DL) | true; if (SwitchToLookupTable(SI, Builder, TTI, DL)) return SimplifyCFG(BB, TTI, DL) | true; return false; } bool SimplifyCFGOpt::SimplifyIndirectBr(IndirectBrInst *IBI) { BasicBlock *BB = IBI->getParent(); bool Changed = false; // Eliminate redundant destinations. SmallPtrSet Succs; for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) { BasicBlock *Dest = IBI->getDestination(i); if (!Dest->hasAddressTaken() || !Succs.insert(Dest)) { Dest->removePredecessor(BB); IBI->removeDestination(i); --i; --e; Changed = true; } } if (IBI->getNumDestinations() == 0) { // If the indirectbr has no successors, change it to unreachable. new UnreachableInst(IBI->getContext(), IBI); EraseTerminatorInstAndDCECond(IBI); return true; } if (IBI->getNumDestinations() == 1) { // If the indirectbr has one successor, change it to a direct branch. BranchInst::Create(IBI->getDestination(0), IBI); EraseTerminatorInstAndDCECond(IBI); return true; } if (SelectInst *SI = dyn_cast(IBI->getAddress())) { if (SimplifyIndirectBrOnSelect(IBI, SI)) return SimplifyCFG(BB, TTI, DL) | true; } return Changed; } bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder){ BasicBlock *BB = BI->getParent(); if (SinkCommon && SinkThenElseCodeToEnd(BI)) return true; // If the Terminator is the only non-phi instruction, simplify the block. BasicBlock::iterator I = BB->getFirstNonPHIOrDbgOrLifetime(); if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() && TryToSimplifyUncondBranchFromEmptyBlock(BB)) return true; // If the only instruction in the block is a seteq/setne comparison // against a constant, try to simplify the block. if (ICmpInst *ICI = dyn_cast(I)) if (ICI->isEquality() && isa(ICI->getOperand(1))) { for (++I; isa(I); ++I) ; if (I->isTerminator() && TryToSimplifyUncondBranchWithICmpInIt(ICI, Builder, TTI, DL)) return true; } // If this basic block is ONLY a compare and a branch, and if a predecessor // branches to us and our successor, fold the comparison into the // predecessor and use logical operations to update the incoming value // for PHI nodes in common successor. if (FoldBranchToCommonDest(BI, DL)) return SimplifyCFG(BB, TTI, DL) | true; return false; } bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) { BasicBlock *BB = BI->getParent(); // Conditional branch if (isValueEqualityComparison(BI)) { // If we only have one predecessor, and if it is a branch on this value, // see if that predecessor totally determines the outcome of this // switch. if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder)) return SimplifyCFG(BB, TTI, DL) | true; // This block must be empty, except for the setcond inst, if it exists. // Ignore dbg intrinsics. BasicBlock::iterator I = BB->begin(); // Ignore dbg intrinsics. while (isa(I)) ++I; if (&*I == BI) { if (FoldValueComparisonIntoPredecessors(BI, Builder)) return SimplifyCFG(BB, TTI, DL) | true; } else if (&*I == cast(BI->getCondition())){ ++I; // Ignore dbg intrinsics. while (isa(I)) ++I; if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder)) return SimplifyCFG(BB, TTI, DL) | true; } } // Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction. if (SimplifyBranchOnICmpChain(BI, DL, Builder)) return true; // If this basic block is ONLY a compare and a branch, and if a predecessor // branches to us and one of our successors, fold the comparison into the // predecessor and use logical operations to pick the right destination. if (FoldBranchToCommonDest(BI, DL)) return SimplifyCFG(BB, TTI, DL) | true; // We have a conditional branch to two blocks that are only reachable // from BI. We know that the condbr dominates the two blocks, so see if // there is any identical code in the "then" and "else" blocks. If so, we // can hoist it up to the branching block. if (BI->getSuccessor(0)->getSinglePredecessor()) { if (BI->getSuccessor(1)->getSinglePredecessor()) { if (HoistThenElseCodeToIf(BI, DL)) return SimplifyCFG(BB, TTI, DL) | true; } else { // If Successor #1 has multiple preds, we may be able to conditionally // execute Successor #0 if it branches to Successor #1. TerminatorInst *Succ0TI = BI->getSuccessor(0)->getTerminator(); if (Succ0TI->getNumSuccessors() == 1 && Succ0TI->getSuccessor(0) == BI->getSuccessor(1)) if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0), DL)) return SimplifyCFG(BB, TTI, DL) | true; } } else if (BI->getSuccessor(1)->getSinglePredecessor()) { // If Successor #0 has multiple preds, we may be able to conditionally // execute Successor #1 if it branches to Successor #0. TerminatorInst *Succ1TI = BI->getSuccessor(1)->getTerminator(); if (Succ1TI->getNumSuccessors() == 1 && Succ1TI->getSuccessor(0) == BI->getSuccessor(0)) if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1), DL)) return SimplifyCFG(BB, TTI, DL) | true; } // If this is a branch on a phi node in the current block, thread control // through this block if any PHI node entries are constants. if (PHINode *PN = dyn_cast(BI->getCondition())) if (PN->getParent() == BI->getParent()) if (FoldCondBranchOnPHI(BI, DL)) return SimplifyCFG(BB, TTI, DL) | true; // Scan predecessor blocks for conditional branches. for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) if (BranchInst *PBI = dyn_cast((*PI)->getTerminator())) if (PBI != BI && PBI->isConditional()) if (SimplifyCondBranchToCondBranch(PBI, BI)) return SimplifyCFG(BB, TTI, DL) | true; return false; } /// Check if passing a value to an instruction will cause undefined behavior. static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) { Constant *C = dyn_cast(V); if (!C) return false; if (I->use_empty()) return false; if (C->isNullValue()) { // Only look at the first use, avoid hurting compile time with long uselists User *Use = *I->user_begin(); // Now make sure that there are no instructions in between that can alter // control flow (eg. calls) for (BasicBlock::iterator i = ++BasicBlock::iterator(I); &*i != Use; ++i) if (i == I->getParent()->end() || i->mayHaveSideEffects()) return false; // Look through GEPs. A load from a GEP derived from NULL is still undefined if (GetElementPtrInst *GEP = dyn_cast(Use)) if (GEP->getPointerOperand() == I) return passingValueIsAlwaysUndefined(V, GEP); // Look through bitcasts. if (BitCastInst *BC = dyn_cast(Use)) return passingValueIsAlwaysUndefined(V, BC); // Load from null is undefined. if (LoadInst *LI = dyn_cast(Use)) if (!LI->isVolatile()) return LI->getPointerAddressSpace() == 0; // Store to null is undefined. if (StoreInst *SI = dyn_cast(Use)) if (!SI->isVolatile()) return SI->getPointerAddressSpace() == 0 && SI->getPointerOperand() == I; } return false; } /// If BB has an incoming value that will always trigger undefined behavior /// (eg. null pointer dereference), remove the branch leading here. static bool removeUndefIntroducingPredecessor(BasicBlock *BB) { for (BasicBlock::iterator i = BB->begin(); PHINode *PHI = dyn_cast(i); ++i) for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i) if (passingValueIsAlwaysUndefined(PHI->getIncomingValue(i), PHI)) { TerminatorInst *T = PHI->getIncomingBlock(i)->getTerminator(); IRBuilder<> Builder(T); if (BranchInst *BI = dyn_cast(T)) { BB->removePredecessor(PHI->getIncomingBlock(i)); // Turn uncoditional branches into unreachables and remove the dead // destination from conditional branches. if (BI->isUnconditional()) Builder.CreateUnreachable(); else Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1) : BI->getSuccessor(0)); BI->eraseFromParent(); return true; } // TODO: SwitchInst. } return false; } bool SimplifyCFGOpt::run(BasicBlock *BB) { bool Changed = false; assert(BB && BB->getParent() && "Block not embedded in function!"); assert(BB->getTerminator() && "Degenerate basic block encountered!"); // Remove basic blocks that have no predecessors (except the entry block)... // or that just have themself as a predecessor. These are unreachable. if ((pred_begin(BB) == pred_end(BB) && BB != &BB->getParent()->getEntryBlock()) || BB->getSinglePredecessor() == BB) { DEBUG(dbgs() << "Removing BB: \n" << *BB); DeleteDeadBlock(BB); return true; } // Check to see if we can constant propagate this terminator instruction // away... Changed |= ConstantFoldTerminator(BB, true); // Check for and eliminate duplicate PHI nodes in this block. Changed |= EliminateDuplicatePHINodes(BB); // Check for and remove branches that will always cause undefined behavior. Changed |= removeUndefIntroducingPredecessor(BB); // Merge basic blocks into their predecessor if there is only one distinct // pred, and if there is only one distinct successor of the predecessor, and // if there are no PHI nodes. // if (MergeBlockIntoPredecessor(BB)) return true; IRBuilder<> Builder(BB); // If there is a trivial two-entry PHI node in this basic block, and we can // eliminate it, do so now. if (PHINode *PN = dyn_cast(BB->begin())) if (PN->getNumIncomingValues() == 2) Changed |= FoldTwoEntryPHINode(PN, DL); Builder.SetInsertPoint(BB->getTerminator()); if (BranchInst *BI = dyn_cast(BB->getTerminator())) { if (BI->isUnconditional()) { if (SimplifyUncondBranch(BI, Builder)) return true; } else { if (SimplifyCondBranch(BI, Builder)) return true; } } else if (ReturnInst *RI = dyn_cast(BB->getTerminator())) { if (SimplifyReturn(RI, Builder)) return true; } else if (ResumeInst *RI = dyn_cast(BB->getTerminator())) { if (SimplifyResume(RI, Builder)) return true; } else if (SwitchInst *SI = dyn_cast(BB->getTerminator())) { if (SimplifySwitch(SI, Builder)) return true; } else if (UnreachableInst *UI = dyn_cast(BB->getTerminator())) { if (SimplifyUnreachable(UI)) return true; } else if (IndirectBrInst *IBI = dyn_cast(BB->getTerminator())) { if (SimplifyIndirectBr(IBI)) return true; } return Changed; } /// SimplifyCFG - This function is used to do simplification of a CFG. For /// example, it adjusts branches to branches to eliminate the extra hop, it /// eliminates unreachable basic blocks, and does other "peephole" optimization /// of the CFG. It returns true if a modification was made. /// bool llvm::SimplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI, const DataLayout *DL) { return SimplifyCFGOpt(TTI, DL).run(BB); }