1 //===- InstCombineLoadStoreAlloca.cpp -------------------------------------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the visit functions for load, store and alloca.
11 //
12 //===----------------------------------------------------------------------===//
14 #include "InstCombine.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/Analysis/Loads.h"
17 #include "llvm/IR/DataLayout.h"
18 #include "llvm/IR/IntrinsicInst.h"
19 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
20 #include "llvm/Transforms/Utils/Local.h"
21 using namespace llvm;
23 STATISTIC(NumDeadStore, "Number of dead stores eliminated");
24 STATISTIC(NumGlobalCopies, "Number of allocas copied from constant global");
26 /// pointsToConstantGlobal - Return true if V (possibly indirectly) points to
27 /// some part of a constant global variable. This intentionally only accepts
28 /// constant expressions because we can't rewrite arbitrary instructions.
29 static bool pointsToConstantGlobal(Value *V) {
30 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
31 return GV->isConstant();
32 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
33 if (CE->getOpcode() == Instruction::BitCast ||
34 CE->getOpcode() == Instruction::GetElementPtr)
35 return pointsToConstantGlobal(CE->getOperand(0));
36 return false;
37 }
39 /// isOnlyCopiedFromConstantGlobal - Recursively walk the uses of a (derived)
40 /// pointer to an alloca. Ignore any reads of the pointer, return false if we
41 /// see any stores or other unknown uses. If we see pointer arithmetic, keep
42 /// track of whether it moves the pointer (with IsOffset) but otherwise traverse
43 /// the uses. If we see a memcpy/memmove that targets an unoffseted pointer to
44 /// the alloca, and if the source pointer is a pointer to a constant global, we
45 /// can optimize this.
46 static bool
47 isOnlyCopiedFromConstantGlobal(Value *V, MemTransferInst *&TheCopy,
48 SmallVectorImpl<Instruction *> &ToDelete,
49 bool IsOffset = false) {
50 // We track lifetime intrinsics as we encounter them. If we decide to go
51 // ahead and replace the value with the global, this lets the caller quickly
52 // eliminate the markers.
54 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI!=E; ++UI) {
55 User *U = cast<Instruction>(*UI);
57 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
58 // Ignore non-volatile loads, they are always ok.
59 if (!LI->isSimple()) return false;
60 continue;
61 }
63 if (BitCastInst *BCI = dyn_cast<BitCastInst>(U)) {
64 // If uses of the bitcast are ok, we are ok.
65 if (!isOnlyCopiedFromConstantGlobal(BCI, TheCopy, ToDelete, IsOffset))
66 return false;
67 continue;
68 }
69 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
70 // If the GEP has all zero indices, it doesn't offset the pointer. If it
71 // doesn't, it does.
72 if (!isOnlyCopiedFromConstantGlobal(
73 GEP, TheCopy, ToDelete, IsOffset || !GEP->hasAllZeroIndices()))
74 return false;
75 continue;
76 }
78 if (CallSite CS = U) {
79 // If this is the function being called then we treat it like a load and
80 // ignore it.
81 if (CS.isCallee(UI))
82 continue;
84 // If this is a readonly/readnone call site, then we know it is just a
85 // load (but one that potentially returns the value itself), so we can
86 // ignore it if we know that the value isn't captured.
87 unsigned ArgNo = CS.getArgumentNo(UI);
88 if (CS.onlyReadsMemory() &&
89 (CS.getInstruction()->use_empty() || CS.doesNotCapture(ArgNo)))
90 continue;
92 // If this is being passed as a byval argument, the caller is making a
93 // copy, so it is only a read of the alloca.
94 if (CS.isByValArgument(ArgNo))
95 continue;
96 }
98 // Lifetime intrinsics can be handled by the caller.
99 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
100 if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
101 II->getIntrinsicID() == Intrinsic::lifetime_end) {
102 assert(II->use_empty() && "Lifetime markers have no result to use!");
103 ToDelete.push_back(II);
104 continue;
105 }
106 }
108 // If this is isn't our memcpy/memmove, reject it as something we can't
109 // handle.
110 MemTransferInst *MI = dyn_cast<MemTransferInst>(U);
111 if (MI == 0)
112 return false;
114 // If the transfer is using the alloca as a source of the transfer, then
115 // ignore it since it is a load (unless the transfer is volatile).
116 if (UI.getOperandNo() == 1) {
117 if (MI->isVolatile()) return false;
118 continue;
119 }
121 // If we already have seen a copy, reject the second one.
122 if (TheCopy) return false;
124 // If the pointer has been offset from the start of the alloca, we can't
125 // safely handle this.
126 if (IsOffset) return false;
128 // If the memintrinsic isn't using the alloca as the dest, reject it.
129 if (UI.getOperandNo() != 0) return false;
131 // If the source of the memcpy/move is not a constant global, reject it.
132 if (!pointsToConstantGlobal(MI->getSource()))
133 return false;
135 // Otherwise, the transform is safe. Remember the copy instruction.
136 TheCopy = MI;
137 }
138 return true;
139 }
141 /// isOnlyCopiedFromConstantGlobal - Return true if the specified alloca is only
142 /// modified by a copy from a constant global. If we can prove this, we can
143 /// replace any uses of the alloca with uses of the global directly.
144 static MemTransferInst *
145 isOnlyCopiedFromConstantGlobal(AllocaInst *AI,
146 SmallVectorImpl<Instruction *> &ToDelete) {
147 MemTransferInst *TheCopy = 0;
148 if (isOnlyCopiedFromConstantGlobal(AI, TheCopy, ToDelete))
149 return TheCopy;
150 return 0;
151 }
153 Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
154 // Ensure that the alloca array size argument has type intptr_t, so that
155 // any casting is exposed early.
156 if (TD) {
157 Type *IntPtrTy = TD->getIntPtrType(AI.getType());
158 if (AI.getArraySize()->getType() != IntPtrTy) {
159 Value *V = Builder->CreateIntCast(AI.getArraySize(),
160 IntPtrTy, false);
161 AI.setOperand(0, V);
162 return &AI;
163 }
164 }
166 // Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1
167 if (AI.isArrayAllocation()) { // Check C != 1
168 if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) {
169 Type *NewTy =
170 ArrayType::get(AI.getAllocatedType(), C->getZExtValue());
171 AllocaInst *New = Builder->CreateAlloca(NewTy, 0, AI.getName());
172 New->setAlignment(AI.getAlignment());
174 // Scan to the end of the allocation instructions, to skip over a block of
175 // allocas if possible...also skip interleaved debug info
176 //
177 BasicBlock::iterator It = New;
178 while (isa<AllocaInst>(*It) || isa<DbgInfoIntrinsic>(*It)) ++It;
180 // Now that I is pointing to the first non-allocation-inst in the block,
181 // insert our getelementptr instruction...
182 //
183 Type *IdxTy = TD
184 ? TD->getIntPtrType(AI.getType())
185 : Type::getInt64Ty(AI.getContext());
186 Value *NullIdx = Constant::getNullValue(IdxTy);
187 Value *Idx[2] = { NullIdx, NullIdx };
188 Instruction *GEP =
189 GetElementPtrInst::CreateInBounds(New, Idx, New->getName() + ".sub");
190 InsertNewInstBefore(GEP, *It);
192 // Now make everything use the getelementptr instead of the original
193 // allocation.
194 return ReplaceInstUsesWith(AI, GEP);
195 } else if (isa<UndefValue>(AI.getArraySize())) {
196 return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));
197 }
198 }
200 if (TD && AI.getAllocatedType()->isSized()) {
201 // If the alignment is 0 (unspecified), assign it the preferred alignment.
202 if (AI.getAlignment() == 0)
203 AI.setAlignment(TD->getPrefTypeAlignment(AI.getAllocatedType()));
205 // Move all alloca's of zero byte objects to the entry block and merge them
206 // together. Note that we only do this for alloca's, because malloc should
207 // allocate and return a unique pointer, even for a zero byte allocation.
208 if (TD->getTypeAllocSize(AI.getAllocatedType()) == 0) {
209 // For a zero sized alloca there is no point in doing an array allocation.
210 // This is helpful if the array size is a complicated expression not used
211 // elsewhere.
212 if (AI.isArrayAllocation()) {
213 AI.setOperand(0, ConstantInt::get(AI.getArraySize()->getType(), 1));
214 return &AI;
215 }
217 // Get the first instruction in the entry block.
218 BasicBlock &EntryBlock = AI.getParent()->getParent()->getEntryBlock();
219 Instruction *FirstInst = EntryBlock.getFirstNonPHIOrDbg();
220 if (FirstInst != &AI) {
221 // If the entry block doesn't start with a zero-size alloca then move
222 // this one to the start of the entry block. There is no problem with
223 // dominance as the array size was forced to a constant earlier already.
224 AllocaInst *EntryAI = dyn_cast<AllocaInst>(FirstInst);
225 if (!EntryAI || !EntryAI->getAllocatedType()->isSized() ||
226 TD->getTypeAllocSize(EntryAI->getAllocatedType()) != 0) {
227 AI.moveBefore(FirstInst);
228 return &AI;
229 }
231 // If the alignment of the entry block alloca is 0 (unspecified),
232 // assign it the preferred alignment.
233 if (EntryAI->getAlignment() == 0)
234 EntryAI->setAlignment(
235 TD->getPrefTypeAlignment(EntryAI->getAllocatedType()));
236 // Replace this zero-sized alloca with the one at the start of the entry
237 // block after ensuring that the address will be aligned enough for both
238 // types.
239 unsigned MaxAlign = std::max(EntryAI->getAlignment(),
240 AI.getAlignment());
241 EntryAI->setAlignment(MaxAlign);
242 if (AI.getType() != EntryAI->getType())
243 return new BitCastInst(EntryAI, AI.getType());
244 return ReplaceInstUsesWith(AI, EntryAI);
245 }
246 }
247 }
249 if (AI.getAlignment()) {
250 // Check to see if this allocation is only modified by a memcpy/memmove from
251 // a constant global whose alignment is equal to or exceeds that of the
252 // allocation. If this is the case, we can change all users to use
253 // the constant global instead. This is commonly produced by the CFE by
254 // constructs like "void foo() { int A[] = {1,2,3,4,5,6,7,8,9...}; }" if 'A'
255 // is only subsequently read.
256 SmallVector<Instruction *, 4> ToDelete;
257 if (MemTransferInst *Copy = isOnlyCopiedFromConstantGlobal(&AI, ToDelete)) {
258 unsigned SourceAlign = getOrEnforceKnownAlignment(Copy->getSource(),
259 AI.getAlignment(), TD);
260 if (AI.getAlignment() <= SourceAlign) {
261 DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n');
262 DEBUG(dbgs() << " memcpy = " << *Copy << '\n');
263 for (unsigned i = 0, e = ToDelete.size(); i != e; ++i)
264 EraseInstFromFunction(*ToDelete[i]);
265 Constant *TheSrc = cast<Constant>(Copy->getSource());
266 Instruction *NewI
267 = ReplaceInstUsesWith(AI, ConstantExpr::getBitCast(TheSrc,
268 AI.getType()));
269 EraseInstFromFunction(*Copy);
270 ++NumGlobalCopies;
271 return NewI;
272 }
273 }
274 }
276 // At last, use the generic allocation site handler to aggressively remove
277 // unused allocas.
278 return visitAllocSite(AI);
279 }
282 /// InstCombineLoadCast - Fold 'load (cast P)' -> cast (load P)' when possible.
283 static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI,
284 const DataLayout *TD) {
285 User *CI = cast<User>(LI.getOperand(0));
286 Value *CastOp = CI->getOperand(0);
288 PointerType *DestTy = cast<PointerType>(CI->getType());
289 Type *DestPTy = DestTy->getElementType();
290 if (PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType())) {
292 // If the address spaces don't match, don't eliminate the cast.
293 if (DestTy->getAddressSpace() != SrcTy->getAddressSpace())
294 return 0;
296 Type *SrcPTy = SrcTy->getElementType();
298 if (DestPTy->isIntegerTy() || DestPTy->isPointerTy() ||
299 DestPTy->isVectorTy()) {
300 // If the source is an array, the code below will not succeed. Check to
301 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for
302 // constants.
303 if (ArrayType *ASrcTy = dyn_cast<ArrayType>(SrcPTy))
304 if (Constant *CSrc = dyn_cast<Constant>(CastOp))
305 if (ASrcTy->getNumElements() != 0) {
306 Type *IdxTy = TD
307 ? TD->getIntPtrType(SrcTy)
308 : Type::getInt64Ty(SrcTy->getContext());
309 Value *Idx = Constant::getNullValue(IdxTy);
310 Value *Idxs[2] = { Idx, Idx };
311 CastOp = ConstantExpr::getGetElementPtr(CSrc, Idxs);
312 SrcTy = cast<PointerType>(CastOp->getType());
313 SrcPTy = SrcTy->getElementType();
314 }
316 if (IC.getDataLayout() &&
317 (SrcPTy->isIntegerTy() || SrcPTy->isPointerTy() ||
318 SrcPTy->isVectorTy()) &&
319 // Do not allow turning this into a load of an integer, which is then
320 // casted to a pointer, this pessimizes pointer analysis a lot.
321 (SrcPTy->isPtrOrPtrVectorTy() ==
322 LI.getType()->isPtrOrPtrVectorTy()) &&
323 IC.getDataLayout()->getTypeSizeInBits(SrcPTy) ==
324 IC.getDataLayout()->getTypeSizeInBits(DestPTy)) {
326 // Okay, we are casting from one integer or pointer type to another of
327 // the same size. Instead of casting the pointer before the load, cast
328 // the result of the loaded value.
329 LoadInst *NewLoad =
330 IC.Builder->CreateLoad(CastOp, LI.isVolatile(), CI->getName());
331 NewLoad->setAlignment(LI.getAlignment());
332 NewLoad->setAtomic(LI.getOrdering(), LI.getSynchScope());
333 // Now cast the result of the load.
334 return new BitCastInst(NewLoad, LI.getType());
335 }
336 }
337 }
338 return 0;
339 }
341 Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
342 Value *Op = LI.getOperand(0);
344 // Attempt to improve the alignment.
345 if (TD) {
346 unsigned KnownAlign =
347 getOrEnforceKnownAlignment(Op, TD->getPrefTypeAlignment(LI.getType()),TD);
348 unsigned LoadAlign = LI.getAlignment();
349 unsigned EffectiveLoadAlign = LoadAlign != 0 ? LoadAlign :
350 TD->getABITypeAlignment(LI.getType());
352 if (KnownAlign > EffectiveLoadAlign)
353 LI.setAlignment(KnownAlign);
354 else if (LoadAlign == 0)
355 LI.setAlignment(EffectiveLoadAlign);
356 }
358 // load (cast X) --> cast (load X) iff safe.
359 if (isa<CastInst>(Op))
360 if (Instruction *Res = InstCombineLoadCast(*this, LI, TD))
361 return Res;
363 // None of the following transforms are legal for volatile/atomic loads.
364 // FIXME: Some of it is okay for atomic loads; needs refactoring.
365 if (!LI.isSimple()) return 0;
367 // Do really simple store-to-load forwarding and load CSE, to catch cases
368 // where there are several consecutive memory accesses to the same location,
369 // separated by a few arithmetic operations.
370 BasicBlock::iterator BBI = &LI;
371 if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,6))
372 return ReplaceInstUsesWith(LI, AvailableVal);
374 // load(gep null, ...) -> unreachable
375 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
376 const Value *GEPI0 = GEPI->getOperand(0);
377 // TODO: Consider a target hook for valid address spaces for this xform.
378 if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){
379 // Insert a new store to null instruction before the load to indicate
380 // that this code is not reachable. We do this instead of inserting
381 // an unreachable instruction directly because we cannot modify the
382 // CFG.
383 new StoreInst(UndefValue::get(LI.getType()),
384 Constant::getNullValue(Op->getType()), &LI);
385 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
386 }
387 }
389 // load null/undef -> unreachable
390 // TODO: Consider a target hook for valid address spaces for this xform.
391 if (isa<UndefValue>(Op) ||
392 (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) {
393 // Insert a new store to null instruction before the load to indicate that
394 // this code is not reachable. We do this instead of inserting an
395 // unreachable instruction directly because we cannot modify the CFG.
396 new StoreInst(UndefValue::get(LI.getType()),
397 Constant::getNullValue(Op->getType()), &LI);
398 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
399 }
401 // Instcombine load (constantexpr_cast global) -> cast (load global)
402 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Op))
403 if (CE->isCast())
404 if (Instruction *Res = InstCombineLoadCast(*this, LI, TD))
405 return Res;
407 if (Op->hasOneUse()) {
408 // Change select and PHI nodes to select values instead of addresses: this
409 // helps alias analysis out a lot, allows many others simplifications, and
410 // exposes redundancy in the code.
411 //
412 // Note that we cannot do the transformation unless we know that the
413 // introduced loads cannot trap! Something like this is valid as long as
414 // the condition is always false: load (select bool %C, int* null, int* %G),
415 // but it would not be valid if we transformed it to load from null
416 // unconditionally.
417 //
418 if (SelectInst *SI = dyn_cast<SelectInst>(Op)) {
419 // load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2).
420 unsigned Align = LI.getAlignment();
421 if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, Align, TD) &&
422 isSafeToLoadUnconditionally(SI->getOperand(2), SI, Align, TD)) {
423 LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1),
424 SI->getOperand(1)->getName()+".val");
425 LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2),
426 SI->getOperand(2)->getName()+".val");
427 V1->setAlignment(Align);
428 V2->setAlignment(Align);
429 return SelectInst::Create(SI->getCondition(), V1, V2);
430 }
432 // load (select (cond, null, P)) -> load P
433 if (Constant *C = dyn_cast<Constant>(SI->getOperand(1)))
434 if (C->isNullValue()) {
435 LI.setOperand(0, SI->getOperand(2));
436 return &LI;
437 }
439 // load (select (cond, P, null)) -> load P
440 if (Constant *C = dyn_cast<Constant>(SI->getOperand(2)))
441 if (C->isNullValue()) {
442 LI.setOperand(0, SI->getOperand(1));
443 return &LI;
444 }
445 }
446 }
447 return 0;
448 }
450 /// InstCombineStoreToCast - Fold store V, (cast P) -> store (cast V), P
451 /// when possible. This makes it generally easy to do alias analysis and/or
452 /// SROA/mem2reg of the memory object.
453 static Instruction *InstCombineStoreToCast(InstCombiner &IC, StoreInst &SI) {
454 User *CI = cast<User>(SI.getOperand(1));
455 Value *CastOp = CI->getOperand(0);
457 Type *DestPTy = cast<PointerType>(CI->getType())->getElementType();
458 PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType());
459 if (SrcTy == 0) return 0;
461 Type *SrcPTy = SrcTy->getElementType();
463 if (!DestPTy->isIntegerTy() && !DestPTy->isPointerTy())
464 return 0;
466 /// NewGEPIndices - If SrcPTy is an aggregate type, we can emit a "noop gep"
467 /// to its first element. This allows us to handle things like:
468 /// store i32 xxx, (bitcast {foo*, float}* %P to i32*)
469 /// on 32-bit hosts.
470 SmallVector<Value*, 4> NewGEPIndices;
472 // If the source is an array, the code below will not succeed. Check to
473 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for
474 // constants.
475 if (SrcPTy->isArrayTy() || SrcPTy->isStructTy()) {
476 // Index through pointer.
477 Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext()));
478 NewGEPIndices.push_back(Zero);
480 while (1) {
481 if (StructType *STy = dyn_cast<StructType>(SrcPTy)) {
482 if (!STy->getNumElements()) /* Struct can be empty {} */
483 break;
484 NewGEPIndices.push_back(Zero);
485 SrcPTy = STy->getElementType(0);
486 } else if (ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) {
487 NewGEPIndices.push_back(Zero);
488 SrcPTy = ATy->getElementType();
489 } else {
490 break;
491 }
492 }
494 SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace());
495 }
497 if (!SrcPTy->isIntegerTy() && !SrcPTy->isPointerTy())
498 return 0;
500 // If the pointers point into different address spaces or if they point to
501 // values with different sizes, we can't do the transformation.
502 if (!IC.getDataLayout() ||
503 SrcTy->getAddressSpace() !=
504 cast<PointerType>(CI->getType())->getAddressSpace() ||
505 IC.getDataLayout()->getTypeSizeInBits(SrcPTy) !=
506 IC.getDataLayout()->getTypeSizeInBits(DestPTy))
507 return 0;
509 // Okay, we are casting from one integer or pointer type to another of
510 // the same size. Instead of casting the pointer before
511 // the store, cast the value to be stored.
512 Value *NewCast;
513 Value *SIOp0 = SI.getOperand(0);
514 Instruction::CastOps opcode = Instruction::BitCast;
515 Type* CastSrcTy = SIOp0->getType();
516 Type* CastDstTy = SrcPTy;
517 if (CastDstTy->isPointerTy()) {
518 if (CastSrcTy->isIntegerTy())
519 opcode = Instruction::IntToPtr;
520 } else if (CastDstTy->isIntegerTy()) {
521 if (SIOp0->getType()->isPointerTy())
522 opcode = Instruction::PtrToInt;
523 }
525 // SIOp0 is a pointer to aggregate and this is a store to the first field,
526 // emit a GEP to index into its first field.
527 if (!NewGEPIndices.empty())
528 CastOp = IC.Builder->CreateInBoundsGEP(CastOp, NewGEPIndices);
530 NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy,
531 SIOp0->getName()+".c");
532 SI.setOperand(0, NewCast);
533 SI.setOperand(1, CastOp);
534 return &SI;
535 }
537 /// equivalentAddressValues - Test if A and B will obviously have the same
538 /// value. This includes recognizing that %t0 and %t1 will have the same
539 /// value in code like this:
540 /// %t0 = getelementptr \@a, 0, 3
541 /// store i32 0, i32* %t0
542 /// %t1 = getelementptr \@a, 0, 3
543 /// %t2 = load i32* %t1
544 ///
545 static bool equivalentAddressValues(Value *A, Value *B) {
546 // Test if the values are trivially equivalent.
547 if (A == B) return true;
549 // Test if the values come form identical arithmetic instructions.
550 // This uses isIdenticalToWhenDefined instead of isIdenticalTo because
551 // its only used to compare two uses within the same basic block, which
552 // means that they'll always either have the same value or one of them
553 // will have an undefined value.
554 if (isa<BinaryOperator>(A) ||
555 isa<CastInst>(A) ||
556 isa<PHINode>(A) ||
557 isa<GetElementPtrInst>(A))
558 if (Instruction *BI = dyn_cast<Instruction>(B))
559 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
560 return true;
562 // Otherwise they may not be equivalent.
563 return false;
564 }
566 Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
567 Value *Val = SI.getOperand(0);
568 Value *Ptr = SI.getOperand(1);
570 // Attempt to improve the alignment.
571 if (TD) {
572 unsigned KnownAlign =
573 getOrEnforceKnownAlignment(Ptr, TD->getPrefTypeAlignment(Val->getType()),
574 TD);
575 unsigned StoreAlign = SI.getAlignment();
576 unsigned EffectiveStoreAlign = StoreAlign != 0 ? StoreAlign :
577 TD->getABITypeAlignment(Val->getType());
579 if (KnownAlign > EffectiveStoreAlign)
580 SI.setAlignment(KnownAlign);
581 else if (StoreAlign == 0)
582 SI.setAlignment(EffectiveStoreAlign);
583 }
585 // Don't hack volatile/atomic stores.
586 // FIXME: Some bits are legal for atomic stores; needs refactoring.
587 if (!SI.isSimple()) return 0;
589 // If the RHS is an alloca with a single use, zapify the store, making the
590 // alloca dead.
591 if (Ptr->hasOneUse()) {
592 if (isa<AllocaInst>(Ptr))
593 return EraseInstFromFunction(SI);
594 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) {
595 if (isa<AllocaInst>(GEP->getOperand(0))) {
596 if (GEP->getOperand(0)->hasOneUse())
597 return EraseInstFromFunction(SI);
598 }
599 }
600 }
602 // Do really simple DSE, to catch cases where there are several consecutive
603 // stores to the same location, separated by a few arithmetic operations. This
604 // situation often occurs with bitfield accesses.
605 BasicBlock::iterator BBI = &SI;
606 for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts;
607 --ScanInsts) {
608 --BBI;
609 // Don't count debug info directives, lest they affect codegen,
610 // and we skip pointer-to-pointer bitcasts, which are NOPs.
611 if (isa<DbgInfoIntrinsic>(BBI) ||
612 (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
613 ScanInsts++;
614 continue;
615 }
617 if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) {
618 // Prev store isn't volatile, and stores to the same location?
619 if (PrevSI->isSimple() && equivalentAddressValues(PrevSI->getOperand(1),
620 SI.getOperand(1))) {
621 ++NumDeadStore;
622 ++BBI;
623 EraseInstFromFunction(*PrevSI);
624 continue;
625 }
626 break;
627 }
629 // If this is a load, we have to stop. However, if the loaded value is from
630 // the pointer we're loading and is producing the pointer we're storing,
631 // then *this* store is dead (X = load P; store X -> P).
632 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
633 if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) &&
634 LI->isSimple())
635 return EraseInstFromFunction(SI);
637 // Otherwise, this is a load from some other location. Stores before it
638 // may not be dead.
639 break;
640 }
642 // Don't skip over loads or things that can modify memory.
643 if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory())
644 break;
645 }
647 // store X, null -> turns into 'unreachable' in SimplifyCFG
648 if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) {
649 if (!isa<UndefValue>(Val)) {
650 SI.setOperand(0, UndefValue::get(Val->getType()));
651 if (Instruction *U = dyn_cast<Instruction>(Val))
652 Worklist.Add(U); // Dropped a use.
653 }
654 return 0; // Do not modify these!
655 }
657 // store undef, Ptr -> noop
658 if (isa<UndefValue>(Val))
659 return EraseInstFromFunction(SI);
661 // If the pointer destination is a cast, see if we can fold the cast into the
662 // source instead.
663 if (isa<CastInst>(Ptr))
664 if (Instruction *Res = InstCombineStoreToCast(*this, SI))
665 return Res;
666 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
667 if (CE->isCast())
668 if (Instruction *Res = InstCombineStoreToCast(*this, SI))
669 return Res;
672 // If this store is the last instruction in the basic block (possibly
673 // excepting debug info instructions), and if the block ends with an
674 // unconditional branch, try to move it to the successor block.
675 BBI = &SI;
676 do {
677 ++BBI;
678 } while (isa<DbgInfoIntrinsic>(BBI) ||
679 (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy()));
680 if (BranchInst *BI = dyn_cast<BranchInst>(BBI))
681 if (BI->isUnconditional())
682 if (SimplifyStoreAtEndOfBlock(SI))
683 return 0; // xform done!
685 return 0;
686 }
688 /// SimplifyStoreAtEndOfBlock - Turn things like:
689 /// if () { *P = v1; } else { *P = v2 }
690 /// into a phi node with a store in the successor.
691 ///
692 /// Simplify things like:
693 /// *P = v1; if () { *P = v2; }
694 /// into a phi node with a store in the successor.
695 ///
696 bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) {
697 BasicBlock *StoreBB = SI.getParent();
699 // Check to see if the successor block has exactly two incoming edges. If
700 // so, see if the other predecessor contains a store to the same location.
701 // if so, insert a PHI node (if needed) and move the stores down.
702 BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0);
704 // Determine whether Dest has exactly two predecessors and, if so, compute
705 // the other predecessor.
706 pred_iterator PI = pred_begin(DestBB);
707 BasicBlock *P = *PI;
708 BasicBlock *OtherBB = 0;
710 if (P != StoreBB)
711 OtherBB = P;
713 if (++PI == pred_end(DestBB))
714 return false;
716 P = *PI;
717 if (P != StoreBB) {
718 if (OtherBB)
719 return false;
720 OtherBB = P;
721 }
722 if (++PI != pred_end(DestBB))
723 return false;
725 // Bail out if all the relevant blocks aren't distinct (this can happen,
726 // for example, if SI is in an infinite loop)
727 if (StoreBB == DestBB || OtherBB == DestBB)
728 return false;
730 // Verify that the other block ends in a branch and is not otherwise empty.
731 BasicBlock::iterator BBI = OtherBB->getTerminator();
732 BranchInst *OtherBr = dyn_cast<BranchInst>(BBI);
733 if (!OtherBr || BBI == OtherBB->begin())
734 return false;
736 // If the other block ends in an unconditional branch, check for the 'if then
737 // else' case. there is an instruction before the branch.
738 StoreInst *OtherStore = 0;
739 if (OtherBr->isUnconditional()) {
740 --BBI;
741 // Skip over debugging info.
742 while (isa<DbgInfoIntrinsic>(BBI) ||
743 (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
744 if (BBI==OtherBB->begin())
745 return false;
746 --BBI;
747 }
748 // If this isn't a store, isn't a store to the same location, or is not the
749 // right kind of store, bail out.
750 OtherStore = dyn_cast<StoreInst>(BBI);
751 if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) ||
752 !SI.isSameOperationAs(OtherStore))
753 return false;
754 } else {
755 // Otherwise, the other block ended with a conditional branch. If one of the
756 // destinations is StoreBB, then we have the if/then case.
757 if (OtherBr->getSuccessor(0) != StoreBB &&
758 OtherBr->getSuccessor(1) != StoreBB)
759 return false;
761 // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an
762 // if/then triangle. See if there is a store to the same ptr as SI that
763 // lives in OtherBB.
764 for (;; --BBI) {
765 // Check to see if we find the matching store.
766 if ((OtherStore = dyn_cast<StoreInst>(BBI))) {
767 if (OtherStore->getOperand(1) != SI.getOperand(1) ||
768 !SI.isSameOperationAs(OtherStore))
769 return false;
770 break;
771 }
772 // If we find something that may be using or overwriting the stored
773 // value, or if we run out of instructions, we can't do the xform.
774 if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() ||
775 BBI == OtherBB->begin())
776 return false;
777 }
779 // In order to eliminate the store in OtherBr, we have to
780 // make sure nothing reads or overwrites the stored value in
781 // StoreBB.
782 for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) {
783 // FIXME: This should really be AA driven.
784 if (I->mayReadFromMemory() || I->mayWriteToMemory())
785 return false;
786 }
787 }
789 // Insert a PHI node now if we need it.
790 Value *MergedVal = OtherStore->getOperand(0);
791 if (MergedVal != SI.getOperand(0)) {
792 PHINode *PN = PHINode::Create(MergedVal->getType(), 2, "storemerge");
793 PN->addIncoming(SI.getOperand(0), SI.getParent());
794 PN->addIncoming(OtherStore->getOperand(0), OtherBB);
795 MergedVal = InsertNewInstBefore(PN, DestBB->front());
796 }
798 // Advance to a place where it is safe to insert the new store and
799 // insert it.
800 BBI = DestBB->getFirstInsertionPt();
801 StoreInst *NewSI = new StoreInst(MergedVal, SI.getOperand(1),
802 SI.isVolatile(),
803 SI.getAlignment(),
804 SI.getOrdering(),
805 SI.getSynchScope());
806 InsertNewInstBefore(NewSI, *BBI);
807 NewSI->setDebugLoc(OtherStore->getDebugLoc());
809 // If the two stores had the same TBAA tag, preserve it.
810 if (MDNode *TBAATag = SI.getMetadata(LLVMContext::MD_tbaa))
811 if ((TBAATag = MDNode::getMostGenericTBAA(TBAATag,
812 OtherStore->getMetadata(LLVMContext::MD_tbaa))))
813 NewSI->setMetadata(LLVMContext::MD_tbaa, TBAATag);
816 // Nuke the old stores.
817 EraseInstFromFunction(SI);
818 EraseInstFromFunction(*OtherStore);
819 return true;
820 }