//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass forwards branches to unconditional branches to make them branch // directly to the target block. This pass often results in dead MBB's, which // it then removes. // // Note that this pass must be run after register allocation, it cannot handle // SSA form. // //===----------------------------------------------------------------------===// #include "BranchFolding.h" #include "llvm/ADT/STLExtras.h" #include "llvm/ADT/SmallSet.h" #include "llvm/ADT/Statistic.h" #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" #include "llvm/CodeGen/MachineFunctionPass.h" #include "llvm/CodeGen/MachineJumpTableInfo.h" #include "llvm/CodeGen/MachineModuleInfo.h" #include "llvm/CodeGen/MachineRegisterInfo.h" #include "llvm/CodeGen/Passes.h" #include "llvm/CodeGen/RegisterScavenging.h" #include "llvm/IR/Function.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/ErrorHandling.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Target/TargetInstrInfo.h" #include "llvm/Target/TargetRegisterInfo.h" #include "llvm/Target/TargetSubtargetInfo.h" #include using namespace llvm; #define DEBUG_TYPE "branchfolding" STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); STATISTIC(NumBranchOpts, "Number of branches optimized"); STATISTIC(NumTailMerge , "Number of block tails merged"); STATISTIC(NumHoist , "Number of times common instructions are hoisted"); static cl::opt FlagEnableTailMerge("enable-tail-merge", cl::init(cl::BOU_UNSET), cl::Hidden); // Throttle for huge numbers of predecessors (compile speed problems) static cl::opt TailMergeThreshold("tail-merge-threshold", cl::desc("Max number of predecessors to consider tail merging"), cl::init(150), cl::Hidden); // Heuristic for tail merging (and, inversely, tail duplication). // TODO: This should be replaced with a target query. static cl::opt TailMergeSize("tail-merge-size", cl::desc("Min number of instructions to consider tail merging"), cl::init(3), cl::Hidden); namespace { /// BranchFolderPass - Wrap branch folder in a machine function pass. class BranchFolderPass : public MachineFunctionPass { public: static char ID; explicit BranchFolderPass(): MachineFunctionPass(ID) {} bool runOnMachineFunction(MachineFunction &MF) override; void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addRequired(); AU.addRequired(); AU.addRequired(); MachineFunctionPass::getAnalysisUsage(AU); } }; } char BranchFolderPass::ID = 0; char &llvm::BranchFolderPassID = BranchFolderPass::ID; INITIALIZE_PASS(BranchFolderPass, "branch-folder", "Control Flow Optimizer", false, false) bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { if (skipOptnoneFunction(*MF.getFunction())) return false; TargetPassConfig *PassConfig = &getAnalysis(); // TailMerge can create jump into if branches that make CFG irreducible for // HW that requires structurized CFG. bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() && PassConfig->getEnableTailMerge(); BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, getAnalysis(), getAnalysis()); return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(), MF.getSubtarget().getRegisterInfo(), getAnalysisIfAvailable()); } BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist, const MachineBlockFrequencyInfo &FreqInfo, const MachineBranchProbabilityInfo &ProbInfo) : EnableHoistCommonCode(CommonHoist), MBBFreqInfo(FreqInfo), MBPI(ProbInfo) { switch (FlagEnableTailMerge) { case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; case cl::BOU_TRUE: EnableTailMerge = true; break; case cl::BOU_FALSE: EnableTailMerge = false; break; } } /// RemoveDeadBlock - Remove the specified dead machine basic block from the /// function, updating the CFG. void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { assert(MBB->pred_empty() && "MBB must be dead!"); DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); MachineFunction *MF = MBB->getParent(); // drop all successors. while (!MBB->succ_empty()) MBB->removeSuccessor(MBB->succ_end()-1); // Avoid matching if this pointer gets reused. TriedMerging.erase(MBB); // Remove the block. MF->erase(MBB); } /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def /// followed by terminators, and if the implicitly defined registers are not /// used by the terminators, remove those implicit_def's. e.g. /// BB1: /// r0 = implicit_def /// r1 = implicit_def /// br /// This block can be optimized away later if the implicit instructions are /// removed. bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { SmallSet ImpDefRegs; MachineBasicBlock::iterator I = MBB->begin(); while (I != MBB->end()) { if (!I->isImplicitDef()) break; unsigned Reg = I->getOperand(0).getReg(); for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true); SubRegs.isValid(); ++SubRegs) ImpDefRegs.insert(*SubRegs); ++I; } if (ImpDefRegs.empty()) return false; MachineBasicBlock::iterator FirstTerm = I; while (I != MBB->end()) { if (!TII->isUnpredicatedTerminator(I)) return false; // See if it uses any of the implicitly defined registers. for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { MachineOperand &MO = I->getOperand(i); if (!MO.isReg() || !MO.isUse()) continue; unsigned Reg = MO.getReg(); if (ImpDefRegs.count(Reg)) return false; } ++I; } I = MBB->begin(); while (I != FirstTerm) { MachineInstr *ImpDefMI = &*I; ++I; MBB->erase(ImpDefMI); } return true; } /// OptimizeFunction - Perhaps branch folding, tail merging and other /// CFG optimizations on the given function. bool BranchFolder::OptimizeFunction(MachineFunction &MF, const TargetInstrInfo *tii, const TargetRegisterInfo *tri, MachineModuleInfo *mmi) { if (!tii) return false; TriedMerging.clear(); TII = tii; TRI = tri; MMI = mmi; RS = nullptr; // Use a RegScavenger to help update liveness when required. MachineRegisterInfo &MRI = MF.getRegInfo(); if (MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF)) RS = new RegScavenger(); else MRI.invalidateLiveness(); // Fix CFG. The later algorithms expect it to be right. bool MadeChange = false; for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { MachineBasicBlock *MBB = I, *TBB = nullptr, *FBB = nullptr; SmallVector Cond; if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); MadeChange |= OptimizeImpDefsBlock(MBB); } bool MadeChangeThisIteration = true; while (MadeChangeThisIteration) { MadeChangeThisIteration = TailMergeBlocks(MF); MadeChangeThisIteration |= OptimizeBranches(MF); if (EnableHoistCommonCode) MadeChangeThisIteration |= HoistCommonCode(MF); MadeChange |= MadeChangeThisIteration; } // See if any jump tables have become dead as the code generator // did its thing. MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); if (!JTI) { delete RS; return MadeChange; } // Walk the function to find jump tables that are live. BitVector JTIsLive(JTI->getJumpTables().size()); for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); BB != E; ++BB) { for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { MachineOperand &Op = I->getOperand(op); if (!Op.isJTI()) continue; // Remember that this JT is live. JTIsLive.set(Op.getIndex()); } } // Finally, remove dead jump tables. This happens when the // indirect jump was unreachable (and thus deleted). for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) if (!JTIsLive.test(i)) { JTI->RemoveJumpTable(i); MadeChange = true; } delete RS; return MadeChange; } //===----------------------------------------------------------------------===// // Tail Merging of Blocks //===----------------------------------------------------------------------===// /// HashMachineInstr - Compute a hash value for MI and its operands. static unsigned HashMachineInstr(const MachineInstr *MI) { unsigned Hash = MI->getOpcode(); for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { const MachineOperand &Op = MI->getOperand(i); // Merge in bits from the operand if easy. unsigned OperandHash = 0; switch (Op.getType()) { case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; case MachineOperand::MO_MachineBasicBlock: OperandHash = Op.getMBB()->getNumber(); break; case MachineOperand::MO_FrameIndex: case MachineOperand::MO_ConstantPoolIndex: case MachineOperand::MO_JumpTableIndex: OperandHash = Op.getIndex(); break; case MachineOperand::MO_GlobalAddress: case MachineOperand::MO_ExternalSymbol: // Global address / external symbol are too hard, don't bother, but do // pull in the offset. OperandHash = Op.getOffset(); break; default: break; } Hash += ((OperandHash << 3) | Op.getType()) << (i&31); } return Hash; } /// HashEndOfMBB - Hash the last instruction in the MBB. static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { MachineBasicBlock::const_iterator I = MBB->end(); if (I == MBB->begin()) return 0; // Empty MBB. --I; // Skip debug info so it will not affect codegen. while (I->isDebugValue()) { if (I==MBB->begin()) return 0; // MBB empty except for debug info. --I; } return HashMachineInstr(I); } /// ComputeCommonTailLength - Given two machine basic blocks, compute the number /// of instructions they actually have in common together at their end. Return /// iterators for the first shared instruction in each block. static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2, MachineBasicBlock::iterator &I1, MachineBasicBlock::iterator &I2) { I1 = MBB1->end(); I2 = MBB2->end(); unsigned TailLen = 0; while (I1 != MBB1->begin() && I2 != MBB2->begin()) { --I1; --I2; // Skip debugging pseudos; necessary to avoid changing the code. while (I1->isDebugValue()) { if (I1==MBB1->begin()) { while (I2->isDebugValue()) { if (I2==MBB2->begin()) // I1==DBG at begin; I2==DBG at begin return TailLen; --I2; } ++I2; // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin return TailLen; } --I1; } // I1==first (untested) non-DBG preceding known match while (I2->isDebugValue()) { if (I2==MBB2->begin()) { ++I1; // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin return TailLen; } --I2; } // I1, I2==first (untested) non-DBGs preceding known match if (!I1->isIdenticalTo(I2) || // FIXME: This check is dubious. It's used to get around a problem where // people incorrectly expect inline asm directives to remain in the same // relative order. This is untenable because normal compiler // optimizations (like this one) may reorder and/or merge these // directives. I1->isInlineAsm()) { ++I1; ++I2; break; } ++TailLen; } // Back past possible debugging pseudos at beginning of block. This matters // when one block differs from the other only by whether debugging pseudos // are present at the beginning. (This way, the various checks later for // I1==MBB1->begin() work as expected.) if (I1 == MBB1->begin() && I2 != MBB2->begin()) { --I2; while (I2->isDebugValue()) { if (I2 == MBB2->begin()) return TailLen; --I2; } ++I2; } if (I2 == MBB2->begin() && I1 != MBB1->begin()) { --I1; while (I1->isDebugValue()) { if (I1 == MBB1->begin()) return TailLen; --I1; } ++I1; } return TailLen; } void BranchFolder::MaintainLiveIns(MachineBasicBlock *CurMBB, MachineBasicBlock *NewMBB) { if (RS) { RS->enterBasicBlock(CurMBB); if (!CurMBB->empty()) RS->forward(std::prev(CurMBB->end())); for (unsigned int i = 1, e = TRI->getNumRegs(); i != e; i++) if (RS->isRegUsed(i, false)) NewMBB->addLiveIn(i); } } /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything /// after it, replacing it with an unconditional branch to NewDest. void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, MachineBasicBlock *NewDest) { MachineBasicBlock *CurMBB = OldInst->getParent(); TII->ReplaceTailWithBranchTo(OldInst, NewDest); // For targets that use the register scavenger, we must maintain LiveIns. MaintainLiveIns(CurMBB, NewDest); ++NumTailMerge; } /// SplitMBBAt - Given a machine basic block and an iterator into it, split the /// MBB so that the part before the iterator falls into the part starting at the /// iterator. This returns the new MBB. MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, MachineBasicBlock::iterator BBI1, const BasicBlock *BB) { if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1)) return nullptr; MachineFunction &MF = *CurMBB.getParent(); // Create the fall-through block. MachineFunction::iterator MBBI = &CurMBB; MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(BB); CurMBB.getParent()->insert(++MBBI, NewMBB); // Move all the successors of this block to the specified block. NewMBB->transferSuccessors(&CurMBB); // Add an edge from CurMBB to NewMBB for the fall-through. CurMBB.addSuccessor(NewMBB); // Splice the code over. NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); // NewMBB inherits CurMBB's block frequency. MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB)); // For targets that use the register scavenger, we must maintain LiveIns. MaintainLiveIns(&CurMBB, NewMBB); return NewMBB; } /// EstimateRuntime - Make a rough estimate for how long it will take to run /// the specified code. static unsigned EstimateRuntime(MachineBasicBlock::iterator I, MachineBasicBlock::iterator E) { unsigned Time = 0; for (; I != E; ++I) { if (I->isDebugValue()) continue; if (I->isCall()) Time += 10; else if (I->mayLoad() || I->mayStore()) Time += 2; else ++Time; } return Time; } // CurMBB needs to add an unconditional branch to SuccMBB (we removed these // branches temporarily for tail merging). In the case where CurMBB ends // with a conditional branch to the next block, optimize by reversing the // test and conditionally branching to SuccMBB instead. static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, const TargetInstrInfo *TII) { MachineFunction *MF = CurMBB->getParent(); MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB)); MachineBasicBlock *TBB = nullptr, *FBB = nullptr; SmallVector Cond; DebugLoc dl; // FIXME: this is nowhere if (I != MF->end() && !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { MachineBasicBlock *NextBB = I; if (TBB == NextBB && !Cond.empty() && !FBB) { if (!TII->ReverseBranchCondition(Cond)) { TII->RemoveBranch(*CurMBB); TII->InsertBranch(*CurMBB, SuccBB, nullptr, Cond, dl); return; } } } TII->InsertBranch(*CurMBB, SuccBB, nullptr, SmallVector(), dl); } bool BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { if (getHash() < o.getHash()) return true; if (getHash() > o.getHash()) return false; if (getBlock()->getNumber() < o.getBlock()->getNumber()) return true; if (getBlock()->getNumber() > o.getBlock()->getNumber()) return false; // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing // an object with itself. #ifndef _GLIBCXX_DEBUG llvm_unreachable("Predecessor appears twice"); #else return false; #endif } BlockFrequency BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const { auto I = MergedBBFreq.find(MBB); if (I != MergedBBFreq.end()) return I->second; return MBFI.getBlockFreq(MBB); } void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB, BlockFrequency F) { MergedBBFreq[MBB] = F; } /// CountTerminators - Count the number of terminators in the given /// block and set I to the position of the first non-terminator, if there /// is one, or MBB->end() otherwise. static unsigned CountTerminators(MachineBasicBlock *MBB, MachineBasicBlock::iterator &I) { I = MBB->end(); unsigned NumTerms = 0; for (;;) { if (I == MBB->begin()) { I = MBB->end(); break; } --I; if (!I->isTerminator()) break; ++NumTerms; } return NumTerms; } /// ProfitableToMerge - Check if two machine basic blocks have a common tail /// and decide if it would be profitable to merge those tails. Return the /// length of the common tail and iterators to the first common instruction /// in each block. static bool ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2, unsigned minCommonTailLength, unsigned &CommonTailLen, MachineBasicBlock::iterator &I1, MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB, MachineBasicBlock *PredBB) { CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); if (CommonTailLen == 0) return false; DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber() << " and BB#" << MBB2->getNumber() << " is " << CommonTailLen << '\n'); // It's almost always profitable to merge any number of non-terminator // instructions with the block that falls through into the common successor. if (MBB1 == PredBB || MBB2 == PredBB) { MachineBasicBlock::iterator I; unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); if (CommonTailLen > NumTerms) return true; } // If one of the blocks can be completely merged and happens to be in // a position where the other could fall through into it, merge any number // of instructions, because it can be done without a branch. // TODO: If the blocks are not adjacent, move one of them so that they are? if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin()) return true; if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) return true; // If both blocks have an unconditional branch temporarily stripped out, // count that as an additional common instruction for the following // heuristics. unsigned EffectiveTailLen = CommonTailLen; if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && !MBB1->back().isBarrier() && !MBB2->back().isBarrier()) ++EffectiveTailLen; // Check if the common tail is long enough to be worthwhile. if (EffectiveTailLen >= minCommonTailLength) return true; // If we are optimizing for code size, 2 instructions in common is enough if // we don't have to split a block. At worst we will be introducing 1 new // branch instruction, which is likely to be smaller than the 2 // instructions that would be deleted in the merge. MachineFunction *MF = MBB1->getParent(); if (EffectiveTailLen >= 2 && MF->getFunction()->getAttributes(). hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize) && (I1 == MBB1->begin() || I2 == MBB2->begin())) return true; return false; } /// ComputeSameTails - Look through all the blocks in MergePotentials that have /// hash CurHash (guaranteed to match the last element). Build the vector /// SameTails of all those that have the (same) largest number of instructions /// in common of any pair of these blocks. SameTails entries contain an /// iterator into MergePotentials (from which the MachineBasicBlock can be /// found) and a MachineBasicBlock::iterator into that MBB indicating the /// instruction where the matching code sequence begins. /// Order of elements in SameTails is the reverse of the order in which /// those blocks appear in MergePotentials (where they are not necessarily /// consecutive). unsigned BranchFolder::ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength, MachineBasicBlock *SuccBB, MachineBasicBlock *PredBB) { unsigned maxCommonTailLength = 0U; SameTails.clear(); MachineBasicBlock::iterator TrialBBI1, TrialBBI2; MPIterator HighestMPIter = std::prev(MergePotentials.end()); for (MPIterator CurMPIter = std::prev(MergePotentials.end()), B = MergePotentials.begin(); CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) { for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) { unsigned CommonTailLen; if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), minCommonTailLength, CommonTailLen, TrialBBI1, TrialBBI2, SuccBB, PredBB)) { if (CommonTailLen > maxCommonTailLength) { SameTails.clear(); maxCommonTailLength = CommonTailLen; HighestMPIter = CurMPIter; SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); } if (HighestMPIter == CurMPIter && CommonTailLen == maxCommonTailLength) SameTails.push_back(SameTailElt(I, TrialBBI2)); } if (I == B) break; } } return maxCommonTailLength; } /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from /// MergePotentials, restoring branches at ends of blocks as appropriate. void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock *SuccBB, MachineBasicBlock *PredBB) { MPIterator CurMPIter, B; for (CurMPIter = std::prev(MergePotentials.end()), B = MergePotentials.begin(); CurMPIter->getHash() == CurHash; --CurMPIter) { // Put the unconditional branch back, if we need one. MachineBasicBlock *CurMBB = CurMPIter->getBlock(); if (SuccBB && CurMBB != PredBB) FixTail(CurMBB, SuccBB, TII); if (CurMPIter == B) break; } if (CurMPIter->getHash() != CurHash) CurMPIter++; MergePotentials.erase(CurMPIter, MergePotentials.end()); } /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist /// only of the common tail. Create a block that does by splitting one. bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, MachineBasicBlock *SuccBB, unsigned maxCommonTailLength, unsigned &commonTailIndex) { commonTailIndex = 0; unsigned TimeEstimate = ~0U; for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { // Use PredBB if possible; that doesn't require a new branch. if (SameTails[i].getBlock() == PredBB) { commonTailIndex = i; break; } // Otherwise, make a (fairly bogus) choice based on estimate of // how long it will take the various blocks to execute. unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), SameTails[i].getTailStartPos()); if (t <= TimeEstimate) { TimeEstimate = t; commonTailIndex = i; } } MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].getTailStartPos(); MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); // If the common tail includes any debug info we will take it pretty // randomly from one of the inputs. Might be better to remove it? DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size " << maxCommonTailLength); // If the split block unconditionally falls-thru to SuccBB, it will be // merged. In control flow terms it should then take SuccBB's name. e.g. If // SuccBB is an inner loop, the common tail is still part of the inner loop. const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ? SuccBB->getBasicBlock() : MBB->getBasicBlock(); MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB); if (!newMBB) { DEBUG(dbgs() << "... failed!"); return false; } SameTails[commonTailIndex].setBlock(newMBB); SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); // If we split PredBB, newMBB is the new predecessor. if (PredBB == MBB) PredBB = newMBB; return true; } // See if any of the blocks in MergePotentials (which all have a common single // successor, or all have no successor) can be tail-merged. If there is a // successor, any blocks in MergePotentials that are not tail-merged and // are not immediately before Succ must have an unconditional branch to // Succ added (but the predecessor/successor lists need no adjustment). // The lone predecessor of Succ that falls through into Succ, // if any, is given in PredBB. bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, MachineBasicBlock *PredBB) { bool MadeChange = false; // Except for the special cases below, tail-merge if there are at least // this many instructions in common. unsigned minCommonTailLength = TailMergeSize; DEBUG(dbgs() << "\nTryTailMergeBlocks: "; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber() << (i == e-1 ? "" : ", "); dbgs() << "\n"; if (SuccBB) { dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n'; if (PredBB) dbgs() << " which has fall-through from BB#" << PredBB->getNumber() << "\n"; } dbgs() << "Looking for common tails of at least " << minCommonTailLength << " instruction" << (minCommonTailLength == 1 ? "" : "s") << '\n'; ); // Sort by hash value so that blocks with identical end sequences sort // together. std::stable_sort(MergePotentials.begin(), MergePotentials.end()); // Walk through equivalence sets looking for actual exact matches. while (MergePotentials.size() > 1) { unsigned CurHash = MergePotentials.back().getHash(); // Build SameTails, identifying the set of blocks with this hash code // and with the maximum number of instructions in common. unsigned maxCommonTailLength = ComputeSameTails(CurHash, minCommonTailLength, SuccBB, PredBB); // If we didn't find any pair that has at least minCommonTailLength // instructions in common, remove all blocks with this hash code and retry. if (SameTails.empty()) { RemoveBlocksWithHash(CurHash, SuccBB, PredBB); continue; } // If one of the blocks is the entire common tail (and not the entry // block, which we can't jump to), we can treat all blocks with this same // tail at once. Use PredBB if that is one of the possibilities, as that // will not introduce any extra branches. MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> getParent()->begin(); unsigned commonTailIndex = SameTails.size(); // If there are two blocks, check to see if one can be made to fall through // into the other. if (SameTails.size() == 2 && SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && SameTails[1].tailIsWholeBlock()) commonTailIndex = 1; else if (SameTails.size() == 2 && SameTails[1].getBlock()->isLayoutSuccessor( SameTails[0].getBlock()) && SameTails[0].tailIsWholeBlock()) commonTailIndex = 0; else { // Otherwise just pick one, favoring the fall-through predecessor if // there is one. for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { MachineBasicBlock *MBB = SameTails[i].getBlock(); if (MBB == EntryBB && SameTails[i].tailIsWholeBlock()) continue; if (MBB == PredBB) { commonTailIndex = i; break; } if (SameTails[i].tailIsWholeBlock()) commonTailIndex = i; } } if (commonTailIndex == SameTails.size() || (SameTails[commonTailIndex].getBlock() == PredBB && !SameTails[commonTailIndex].tailIsWholeBlock())) { // None of the blocks consist entirely of the common tail. // Split a block so that one does. if (!CreateCommonTailOnlyBlock(PredBB, SuccBB, maxCommonTailLength, commonTailIndex)) { RemoveBlocksWithHash(CurHash, SuccBB, PredBB); continue; } } MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); // Recompute commont tail MBB's edge weights and block frequency. setCommonTailEdgeWeights(*MBB); // MBB is common tail. Adjust all other BB's to jump to this one. // Traversal must be forwards so erases work. DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber() << " for "); for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { if (commonTailIndex == i) continue; DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber() << (i == e-1 ? "" : ", ")); // Hack the end off BB i, making it jump to BB commonTailIndex instead. ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); // BB i is no longer a predecessor of SuccBB; remove it from the worklist. MergePotentials.erase(SameTails[i].getMPIter()); } DEBUG(dbgs() << "\n"); // We leave commonTailIndex in the worklist in case there are other blocks // that match it with a smaller number of instructions. MadeChange = true; } return MadeChange; } bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { bool MadeChange = false; if (!EnableTailMerge) return MadeChange; // First find blocks with no successors. MergePotentials.clear(); for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E && MergePotentials.size() < TailMergeThreshold; ++I) { if (TriedMerging.count(I)) continue; if (I->succ_empty()) MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I)); } // If this is a large problem, avoid visiting the same basic blocks // multiple times. if (MergePotentials.size() == TailMergeThreshold) for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) TriedMerging.insert(MergePotentials[i].getBlock()); // See if we can do any tail merging on those. if (MergePotentials.size() >= 2) MadeChange |= TryTailMergeBlocks(nullptr, nullptr); // Look at blocks (IBB) with multiple predecessors (PBB). // We change each predecessor to a canonical form, by // (1) temporarily removing any unconditional branch from the predecessor // to IBB, and // (2) alter conditional branches so they branch to the other block // not IBB; this may require adding back an unconditional branch to IBB // later, where there wasn't one coming in. E.g. // Bcc IBB // fallthrough to QBB // here becomes // Bncc QBB // with a conceptual B to IBB after that, which never actually exists. // With those changes, we see whether the predecessors' tails match, // and merge them if so. We change things out of canonical form and // back to the way they were later in the process. (OptimizeBranches // would undo some of this, but we can't use it, because we'd get into // a compile-time infinite loop repeatedly doing and undoing the same // transformations.) for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end(); I != E; ++I) { if (I->pred_size() < 2) continue; SmallPtrSet UniquePreds; MachineBasicBlock *IBB = I; MachineBasicBlock *PredBB = std::prev(I); MergePotentials.clear(); for (MachineBasicBlock::pred_iterator P = I->pred_begin(), E2 = I->pred_end(); P != E2 && MergePotentials.size() < TailMergeThreshold; ++P) { MachineBasicBlock *PBB = *P; if (TriedMerging.count(PBB)) continue; // Skip blocks that loop to themselves, can't tail merge these. if (PBB == IBB) continue; // Visit each predecessor only once. if (!UniquePreds.insert(PBB).second) continue; // Skip blocks which may jump to a landing pad. Can't tail merge these. if (PBB->getLandingPadSuccessor()) continue; MachineBasicBlock *TBB = nullptr, *FBB = nullptr; SmallVector Cond; if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { // Failing case: IBB is the target of a cbr, and we cannot reverse the // branch. SmallVector NewCond(Cond); if (!Cond.empty() && TBB == IBB) { if (TII->ReverseBranchCondition(NewCond)) continue; // This is the QBB case described above if (!FBB) FBB = std::next(MachineFunction::iterator(PBB)); } // Failing case: the only way IBB can be reached from PBB is via // exception handling. Happens for landing pads. Would be nice to have // a bit in the edge so we didn't have to do all this. if (IBB->isLandingPad()) { MachineFunction::iterator IP = PBB; IP++; MachineBasicBlock *PredNextBB = nullptr; if (IP != MF.end()) PredNextBB = IP; if (!TBB) { if (IBB != PredNextBB) // fallthrough continue; } else if (FBB) { if (TBB != IBB && FBB != IBB) // cbr then ubr continue; } else if (Cond.empty()) { if (TBB != IBB) // ubr continue; } else { if (TBB != IBB && IBB != PredNextBB) // cbr continue; } } // Remove the unconditional branch at the end, if any. if (TBB && (Cond.empty() || FBB)) { DebugLoc dl; // FIXME: this is nowhere TII->RemoveBranch(*PBB); if (!Cond.empty()) // reinsert conditional branch only, for now TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr, NewCond, dl); } MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P)); } } // If this is a large problem, avoid visiting the same basic blocks multiple // times. if (MergePotentials.size() == TailMergeThreshold) for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) TriedMerging.insert(MergePotentials[i].getBlock()); if (MergePotentials.size() >= 2) MadeChange |= TryTailMergeBlocks(IBB, PredBB); // Reinsert an unconditional branch if needed. The 1 below can occur as a // result of removing blocks in TryTailMergeBlocks. PredBB = std::prev(I); // this may have been changed in TryTailMergeBlocks if (MergePotentials.size() == 1 && MergePotentials.begin()->getBlock() != PredBB) FixTail(MergePotentials.begin()->getBlock(), IBB, TII); } return MadeChange; } void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) { SmallVector EdgeFreqLs(TailMBB.succ_size()); BlockFrequency AccumulatedMBBFreq; // Aggregate edge frequency of successor edge j: // edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)), // where bb is a basic block that is in SameTails. for (const auto &Src : SameTails) { const MachineBasicBlock *SrcMBB = Src.getBlock(); BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB); AccumulatedMBBFreq += BlockFreq; // It is not necessary to recompute edge weights if TailBB has less than two // successors. if (TailMBB.succ_size() <= 1) continue; auto EdgeFreq = EdgeFreqLs.begin(); for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end(); SuccI != SuccE; ++SuccI, ++EdgeFreq) *EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI); } MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq); if (TailMBB.succ_size() <= 1) return; auto MaxEdgeFreq = *std::max_element(EdgeFreqLs.begin(), EdgeFreqLs.end()); uint64_t Scale = MaxEdgeFreq.getFrequency() / UINT32_MAX + 1; auto EdgeFreq = EdgeFreqLs.begin(); for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end(); SuccI != SuccE; ++SuccI, ++EdgeFreq) TailMBB.setSuccWeight(SuccI, EdgeFreq->getFrequency() / Scale); } //===----------------------------------------------------------------------===// // Branch Optimization //===----------------------------------------------------------------------===// bool BranchFolder::OptimizeBranches(MachineFunction &MF) { bool MadeChange = false; // Make sure blocks are numbered in order MF.RenumberBlocks(); for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end(); I != E; ) { MachineBasicBlock *MBB = I++; MadeChange |= OptimizeBlock(MBB); // If it is dead, remove it. if (MBB->pred_empty()) { RemoveDeadBlock(MBB); MadeChange = true; ++NumDeadBlocks; } } return MadeChange; } // Blocks should be considered empty if they contain only debug info; // else the debug info would affect codegen. static bool IsEmptyBlock(MachineBasicBlock *MBB) { if (MBB->empty()) return true; for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) { if (!MBBI->isDebugValue()) return false; } return true; } // Blocks with only debug info and branches should be considered the same // as blocks with only branches. static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) { MachineBasicBlock::iterator MBBI, MBBE; for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) { if (!MBBI->isDebugValue()) break; } return (MBBI->isBranch()); } /// IsBetterFallthrough - Return true if it would be clearly better to /// fall-through to MBB1 than to fall through into MBB2. This has to return /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will /// result in infinite loops. static bool IsBetterFallthrough(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2) { // Right now, we use a simple heuristic. If MBB2 ends with a call, and // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to // optimize branches that branch to either a return block or an assert block // into a fallthrough to the return. if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false; // If there is a clear successor ordering we make sure that one block // will fall through to the next if (MBB1->isSuccessor(MBB2)) return true; if (MBB2->isSuccessor(MBB1)) return false; // Neither block consists entirely of debug info (per IsEmptyBlock check), // so we needn't test for falling off the beginning here. MachineBasicBlock::iterator MBB1I = --MBB1->end(); while (MBB1I->isDebugValue()) --MBB1I; MachineBasicBlock::iterator MBB2I = --MBB2->end(); while (MBB2I->isDebugValue()) --MBB2I; return MBB2I->isCall() && !MBB1I->isCall(); } /// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch /// instructions on the block. Always use the DebugLoc of the first /// branching instruction found unless its absent, in which case use the /// DebugLoc of the second if present. static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) { MachineBasicBlock::iterator I = MBB.end(); if (I == MBB.begin()) return DebugLoc(); --I; while (I->isDebugValue() && I != MBB.begin()) --I; if (I->isBranch()) return I->getDebugLoc(); return DebugLoc(); } /// OptimizeBlock - Analyze and optimize control flow related to the specified /// block. This is never called on the entry block. bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { bool MadeChange = false; MachineFunction &MF = *MBB->getParent(); ReoptimizeBlock: MachineFunction::iterator FallThrough = MBB; ++FallThrough; // If this block is empty, make everyone use its fall-through, not the block // explicitly. Landing pads should not do this since the landing-pad table // points to this block. Blocks with their addresses taken shouldn't be // optimized away. if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { // Dead block? Leave for cleanup later. if (MBB->pred_empty()) return MadeChange; if (FallThrough == MF.end()) { // TODO: Simplify preds to not branch here if possible! } else { // Rewrite all predecessors of the old block to go to the fallthrough // instead. while (!MBB->pred_empty()) { MachineBasicBlock *Pred = *(MBB->pred_end()-1); Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); } // If MBB was the target of a jump table, update jump tables to go to the // fallthrough instead. if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) MJTI->ReplaceMBBInJumpTables(MBB, FallThrough); MadeChange = true; } return MadeChange; } // Check to see if we can simplify the terminator of the block before this // one. MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB)); MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr; SmallVector PriorCond; bool PriorUnAnalyzable = TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); if (!PriorUnAnalyzable) { // If the CFG for the prior block has extra edges, remove them. MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, !PriorCond.empty()); // If the previous branch is conditional and both conditions go to the same // destination, remove the branch, replacing it with an unconditional one or // a fall-through. if (PriorTBB && PriorTBB == PriorFBB) { DebugLoc dl = getBranchDebugLoc(PrevBB); TII->RemoveBranch(PrevBB); PriorCond.clear(); if (PriorTBB != MBB) TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl); MadeChange = true; ++NumBranchOpts; goto ReoptimizeBlock; } // If the previous block unconditionally falls through to this block and // this block has no other predecessors, move the contents of this block // into the prior block. This doesn't usually happen when SimplifyCFG // has been used, but it can happen if tail merging splits a fall-through // predecessor of a block. // This has to check PrevBB->succ_size() because EH edges are ignored by // AnalyzeBranch. if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && PrevBB.succ_size() == 1 && !MBB->hasAddressTaken() && !MBB->isLandingPad()) { DEBUG(dbgs() << "\nMerging into block: " << PrevBB << "From MBB: " << *MBB); // Remove redundant DBG_VALUEs first. if (PrevBB.begin() != PrevBB.end()) { MachineBasicBlock::iterator PrevBBIter = PrevBB.end(); --PrevBBIter; MachineBasicBlock::iterator MBBIter = MBB->begin(); // Check if DBG_VALUE at the end of PrevBB is identical to the // DBG_VALUE at the beginning of MBB. while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end() && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) { if (!MBBIter->isIdenticalTo(PrevBBIter)) break; MachineInstr *DuplicateDbg = MBBIter; ++MBBIter; -- PrevBBIter; DuplicateDbg->eraseFromParent(); } } PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); PrevBB.removeSuccessor(PrevBB.succ_begin()); assert(PrevBB.succ_empty()); PrevBB.transferSuccessors(MBB); MadeChange = true; return MadeChange; } // If the previous branch *only* branches to *this* block (conditional or // not) remove the branch. if (PriorTBB == MBB && !PriorFBB) { TII->RemoveBranch(PrevBB); MadeChange = true; ++NumBranchOpts; goto ReoptimizeBlock; } // If the prior block branches somewhere else on the condition and here if // the condition is false, remove the uncond second branch. if (PriorFBB == MBB) { DebugLoc dl = getBranchDebugLoc(PrevBB); TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl); MadeChange = true; ++NumBranchOpts; goto ReoptimizeBlock; } // If the prior block branches here on true and somewhere else on false, and // if the branch condition is reversible, reverse the branch to create a // fall-through. if (PriorTBB == MBB) { SmallVector NewPriorCond(PriorCond); if (!TII->ReverseBranchCondition(NewPriorCond)) { DebugLoc dl = getBranchDebugLoc(PrevBB); TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl); MadeChange = true; ++NumBranchOpts; goto ReoptimizeBlock; } } // If this block has no successors (e.g. it is a return block or ends with // a call to a no-return function like abort or __cxa_throw) and if the pred // falls through into this block, and if it would otherwise fall through // into the block after this, move this block to the end of the function. // // We consider it more likely that execution will stay in the function (e.g. // due to loops) than it is to exit it. This asserts in loops etc, moving // the assert condition out of the loop body. if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB && MachineFunction::iterator(PriorTBB) == FallThrough && !MBB->canFallThrough()) { bool DoTransform = true; // We have to be careful that the succs of PredBB aren't both no-successor // blocks. If neither have successors and if PredBB is the second from // last block in the function, we'd just keep swapping the two blocks for // last. Only do the swap if one is clearly better to fall through than // the other. if (FallThrough == --MF.end() && !IsBetterFallthrough(PriorTBB, MBB)) DoTransform = false; if (DoTransform) { // Reverse the branch so we will fall through on the previous true cond. SmallVector NewPriorCond(PriorCond); if (!TII->ReverseBranchCondition(NewPriorCond)) { DEBUG(dbgs() << "\nMoving MBB: " << *MBB << "To make fallthrough to: " << *PriorTBB << "\n"); DebugLoc dl = getBranchDebugLoc(PrevBB); TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl); // Move this block to the end of the function. MBB->moveAfter(--MF.end()); MadeChange = true; ++NumBranchOpts; return MadeChange; } } } } // Analyze the branch in the current block. MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr; SmallVector CurCond; bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); if (!CurUnAnalyzable) { // If the CFG for the prior block has extra edges, remove them. MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); // If this is a two-way branch, and the FBB branches to this block, reverse // the condition so the single-basic-block loop is faster. Instead of: // Loop: xxx; jcc Out; jmp Loop // we want: // Loop: xxx; jncc Loop; jmp Out if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { SmallVector NewCond(CurCond); if (!TII->ReverseBranchCondition(NewCond)) { DebugLoc dl = getBranchDebugLoc(*MBB); TII->RemoveBranch(*MBB); TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl); MadeChange = true; ++NumBranchOpts; goto ReoptimizeBlock; } } // If this branch is the only thing in its block, see if we can forward // other blocks across it. if (CurTBB && CurCond.empty() && !CurFBB && IsBranchOnlyBlock(MBB) && CurTBB != MBB && !MBB->hasAddressTaken()) { DebugLoc dl = getBranchDebugLoc(*MBB); // This block may contain just an unconditional branch. Because there can // be 'non-branch terminators' in the block, try removing the branch and // then seeing if the block is empty. TII->RemoveBranch(*MBB); // If the only things remaining in the block are debug info, remove these // as well, so this will behave the same as an empty block in non-debug // mode. if (!MBB->empty()) { bool NonDebugInfoFound = false; for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E; ++I) { if (!I->isDebugValue()) { NonDebugInfoFound = true; break; } } if (!NonDebugInfoFound) // Make the block empty, losing the debug info (we could probably // improve this in some cases.) MBB->erase(MBB->begin(), MBB->end()); } // If this block is just an unconditional branch to CurTBB, we can // usually completely eliminate the block. The only case we cannot // completely eliminate the block is when the block before this one // falls through into MBB and we can't understand the prior block's branch // condition. if (MBB->empty()) { bool PredHasNoFallThrough = !PrevBB.canFallThrough(); if (PredHasNoFallThrough || !PriorUnAnalyzable || !PrevBB.isSuccessor(MBB)) { // If the prior block falls through into us, turn it into an // explicit branch to us to make updates simpler. if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && PriorTBB != MBB && PriorFBB != MBB) { if (!PriorTBB) { assert(PriorCond.empty() && !PriorFBB && "Bad branch analysis"); PriorTBB = MBB; } else { assert(!PriorFBB && "Machine CFG out of date!"); PriorFBB = MBB; } DebugLoc pdl = getBranchDebugLoc(PrevBB); TII->RemoveBranch(PrevBB); TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl); } // Iterate through all the predecessors, revectoring each in-turn. size_t PI = 0; bool DidChange = false; bool HasBranchToSelf = false; while(PI != MBB->pred_size()) { MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); if (PMBB == MBB) { // If this block has an uncond branch to itself, leave it. ++PI; HasBranchToSelf = true; } else { DidChange = true; PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); // If this change resulted in PMBB ending in a conditional // branch where both conditions go to the same destination, // change this to an unconditional branch (and fix the CFG). MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr; SmallVector NewCurCond; bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, NewCurFBB, NewCurCond, true); if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { DebugLoc pdl = getBranchDebugLoc(*PMBB); TII->RemoveBranch(*PMBB); NewCurCond.clear(); TII->InsertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl); MadeChange = true; ++NumBranchOpts; PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false); } } } // Change any jumptables to go to the new MBB. if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) MJTI->ReplaceMBBInJumpTables(MBB, CurTBB); if (DidChange) { ++NumBranchOpts; MadeChange = true; if (!HasBranchToSelf) return MadeChange; } } } // Add the branch back if the block is more than just an uncond branch. TII->InsertBranch(*MBB, CurTBB, nullptr, CurCond, dl); } } // If the prior block doesn't fall through into this block, and if this // block doesn't fall through into some other block, see if we can find a // place to move this block where a fall-through will happen. if (!PrevBB.canFallThrough()) { // Now we know that there was no fall-through into this block, check to // see if it has a fall-through into its successor. bool CurFallsThru = MBB->canFallThrough(); if (!MBB->isLandingPad()) { // Check all the predecessors of this block. If one of them has no fall // throughs, move this block right after it. for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), E = MBB->pred_end(); PI != E; ++PI) { // Analyze the branch at the end of the pred. MachineBasicBlock *PredBB = *PI; MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr; SmallVector PredCond; if (PredBB != MBB && !PredBB->canFallThrough() && !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) && (!CurFallsThru || !CurTBB || !CurFBB) && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { // If the current block doesn't fall through, just move it. // If the current block can fall through and does not end with a // conditional branch, we need to append an unconditional jump to // the (current) next block. To avoid a possible compile-time // infinite loop, move blocks only backward in this case. // Also, if there are already 2 branches here, we cannot add a third; // this means we have the case // Bcc next // B elsewhere // next: if (CurFallsThru) { MachineBasicBlock *NextBB = std::next(MachineFunction::iterator(MBB)); CurCond.clear(); TII->InsertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc()); } MBB->moveAfter(PredBB); MadeChange = true; goto ReoptimizeBlock; } } } if (!CurFallsThru) { // Check all successors to see if we can move this block before it. for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), E = MBB->succ_end(); SI != E; ++SI) { // Analyze the branch at the end of the block before the succ. MachineBasicBlock *SuccBB = *SI; MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; // If this block doesn't already fall-through to that successor, and if // the succ doesn't already have a block that can fall through into it, // and if the successor isn't an EH destination, we can arrange for the // fallthrough to happen. if (SuccBB != MBB && &*SuccPrev != MBB && !SuccPrev->canFallThrough() && !CurUnAnalyzable && !SuccBB->isLandingPad()) { MBB->moveBefore(SuccBB); MadeChange = true; goto ReoptimizeBlock; } } // Okay, there is no really great place to put this block. If, however, // the block before this one would be a fall-through if this block were // removed, move this block to the end of the function. MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr; SmallVector PrevCond; if (FallThrough != MF.end() && !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && PrevBB.isSuccessor(FallThrough)) { MBB->moveAfter(--MF.end()); MadeChange = true; return MadeChange; } } } return MadeChange; } //===----------------------------------------------------------------------===// // Hoist Common Code //===----------------------------------------------------------------------===// /// HoistCommonCode - Hoist common instruction sequences at the start of basic /// blocks to their common predecessor. bool BranchFolder::HoistCommonCode(MachineFunction &MF) { bool MadeChange = false; for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) { MachineBasicBlock *MBB = I++; MadeChange |= HoistCommonCodeInSuccs(MBB); } return MadeChange; } /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given /// its 'true' successor. static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB, MachineBasicBlock *TrueBB) { for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(), E = BB->succ_end(); SI != E; ++SI) { MachineBasicBlock *SuccBB = *SI; if (SuccBB != TrueBB) return SuccBB; } return nullptr; } /// findHoistingInsertPosAndDeps - Find the location to move common instructions /// in successors to. The location is usually just before the terminator, /// however if the terminator is a conditional branch and its previous /// instruction is the flag setting instruction, the previous instruction is /// the preferred location. This function also gathers uses and defs of the /// instructions from the insertion point to the end of the block. The data is /// used by HoistCommonCodeInSuccs to ensure safety. static MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB, const TargetInstrInfo *TII, const TargetRegisterInfo *TRI, SmallSet &Uses, SmallSet &Defs) { MachineBasicBlock::iterator Loc = MBB->getFirstTerminator(); if (!TII->isUnpredicatedTerminator(Loc)) return MBB->end(); for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) { const MachineOperand &MO = Loc->getOperand(i); if (!MO.isReg()) continue; unsigned Reg = MO.getReg(); if (!Reg) continue; if (MO.isUse()) { for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) Uses.insert(*AI); } else { if (!MO.isDead()) // Don't try to hoist code in the rare case the terminator defines a // register that is later used. return MBB->end(); // If the terminator defines a register, make sure we don't hoist // the instruction whose def might be clobbered by the terminator. for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) Defs.insert(*AI); } } if (Uses.empty()) return Loc; if (Loc == MBB->begin()) return MBB->end(); // The terminator is probably a conditional branch, try not to separate the // branch from condition setting instruction. MachineBasicBlock::iterator PI = Loc; --PI; while (PI != MBB->begin() && PI->isDebugValue()) --PI; bool IsDef = false; for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) { const MachineOperand &MO = PI->getOperand(i); // If PI has a regmask operand, it is probably a call. Separate away. if (MO.isRegMask()) return Loc; if (!MO.isReg() || MO.isUse()) continue; unsigned Reg = MO.getReg(); if (!Reg) continue; if (Uses.count(Reg)) IsDef = true; } if (!IsDef) // The condition setting instruction is not just before the conditional // branch. return Loc; // Be conservative, don't insert instruction above something that may have // side-effects. And since it's potentially bad to separate flag setting // instruction from the conditional branch, just abort the optimization // completely. // Also avoid moving code above predicated instruction since it's hard to // reason about register liveness with predicated instruction. bool DontMoveAcrossStore = true; if (!PI->isSafeToMove(TII, nullptr, DontMoveAcrossStore) || TII->isPredicated(PI)) return MBB->end(); // Find out what registers are live. Note this routine is ignoring other live // registers which are only used by instructions in successor blocks. for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) { const MachineOperand &MO = PI->getOperand(i); if (!MO.isReg()) continue; unsigned Reg = MO.getReg(); if (!Reg) continue; if (MO.isUse()) { for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) Uses.insert(*AI); } else { if (Uses.erase(Reg)) { for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs) Uses.erase(*SubRegs); // Use sub-registers to be conservative } for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) Defs.insert(*AI); } } return PI; } /// HoistCommonCodeInSuccs - If the successors of MBB has common instruction /// sequence at the start of the function, move the instructions before MBB /// terminator if it's legal. bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) { MachineBasicBlock *TBB = nullptr, *FBB = nullptr; SmallVector Cond; if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty()) return false; if (!FBB) FBB = findFalseBlock(MBB, TBB); if (!FBB) // Malformed bcc? True and false blocks are the same? return false; // Restrict the optimization to cases where MBB is the only predecessor, // it is an obvious win. if (TBB->pred_size() > 1 || FBB->pred_size() > 1) return false; // Find a suitable position to hoist the common instructions to. Also figure // out which registers are used or defined by instructions from the insertion // point to the end of the block. SmallSet Uses, Defs; MachineBasicBlock::iterator Loc = findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs); if (Loc == MBB->end()) return false; bool HasDups = false; SmallVector LocalDefs; SmallSet LocalDefsSet; MachineBasicBlock::iterator TIB = TBB->begin(); MachineBasicBlock::iterator FIB = FBB->begin(); MachineBasicBlock::iterator TIE = TBB->end(); MachineBasicBlock::iterator FIE = FBB->end(); while (TIB != TIE && FIB != FIE) { // Skip dbg_value instructions. These do not count. if (TIB->isDebugValue()) { while (TIB != TIE && TIB->isDebugValue()) ++TIB; if (TIB == TIE) break; } if (FIB->isDebugValue()) { while (FIB != FIE && FIB->isDebugValue()) ++FIB; if (FIB == FIE) break; } if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead)) break; if (TII->isPredicated(TIB)) // Hard to reason about register liveness with predicated instruction. break; bool IsSafe = true; for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) { MachineOperand &MO = TIB->getOperand(i); // Don't attempt to hoist instructions with register masks. if (MO.isRegMask()) { IsSafe = false; break; } if (!MO.isReg()) continue; unsigned Reg = MO.getReg(); if (!Reg) continue; if (MO.isDef()) { if (Uses.count(Reg)) { // Avoid clobbering a register that's used by the instruction at // the point of insertion. IsSafe = false; break; } if (Defs.count(Reg) && !MO.isDead()) { // Don't hoist the instruction if the def would be clobber by the // instruction at the point insertion. FIXME: This is overly // conservative. It should be possible to hoist the instructions // in BB2 in the following example: // BB1: // r1, eflag = op1 r2, r3 // brcc eflag // // BB2: // r1 = op2, ... // = op3, r1 IsSafe = false; break; } } else if (!LocalDefsSet.count(Reg)) { if (Defs.count(Reg)) { // Use is defined by the instruction at the point of insertion. IsSafe = false; break; } if (MO.isKill() && Uses.count(Reg)) // Kills a register that's read by the instruction at the point of // insertion. Remove the kill marker. MO.setIsKill(false); } } if (!IsSafe) break; bool DontMoveAcrossStore = true; if (!TIB->isSafeToMove(TII, nullptr, DontMoveAcrossStore)) break; // Remove kills from LocalDefsSet, these registers had short live ranges. for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) { MachineOperand &MO = TIB->getOperand(i); if (!MO.isReg() || !MO.isUse() || !MO.isKill()) continue; unsigned Reg = MO.getReg(); if (!Reg || !LocalDefsSet.count(Reg)) continue; for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) LocalDefsSet.erase(*AI); } // Track local defs so we can update liveins. for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) { MachineOperand &MO = TIB->getOperand(i); if (!MO.isReg() || !MO.isDef() || MO.isDead()) continue; unsigned Reg = MO.getReg(); if (!Reg) continue; LocalDefs.push_back(Reg); for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) LocalDefsSet.insert(*AI); } HasDups = true; ++TIB; ++FIB; } if (!HasDups) return false; MBB->splice(Loc, TBB, TBB->begin(), TIB); FBB->erase(FBB->begin(), FIB); // Update livein's. for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) { unsigned Def = LocalDefs[i]; if (LocalDefsSet.count(Def)) { TBB->addLiveIn(Def); FBB->addLiveIn(Def); } } ++NumHoist; return true; }