1 //===- InstCombineLoadStoreAlloca.cpp -------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the visit functions for load, store and alloca.
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"
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));
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.
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;
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))
69 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
70 // If the GEP has all zero indices, it doesn't offset the pointer. If it
72 if (!isOnlyCopiedFromConstantGlobal(
73 GEP, TheCopy, ToDelete, IsOffset || !GEP->hasAllZeroIndices()))
78 if (CallSite CS = U) {
79 // If this is the function being called then we treat it like a load and
84 // Inalloca arguments are clobbered by the call.
85 unsigned ArgNo = CS.getArgumentNo(UI);
86 if (CS.isInAllocaArgument(ArgNo))
89 // If this is a readonly/readnone call site, then we know it is just a
90 // load (but one that potentially returns the value itself), so we can
91 // ignore it if we know that the value isn't captured.
92 if (CS.onlyReadsMemory() &&
93 (CS.getInstruction()->use_empty() || CS.doesNotCapture(ArgNo)))
96 // If this is being passed as a byval argument, the caller is making a
97 // copy, so it is only a read of the alloca.
98 if (CS.isByValArgument(ArgNo))
102 // Lifetime intrinsics can be handled by the caller.
103 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
104 if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
105 II->getIntrinsicID() == Intrinsic::lifetime_end) {
106 assert(II->use_empty() && "Lifetime markers have no result to use!");
107 ToDelete.push_back(II);
112 // If this is isn't our memcpy/memmove, reject it as something we can't
114 MemTransferInst *MI = dyn_cast<MemTransferInst>(U);
118 // If the transfer is using the alloca as a source of the transfer, then
119 // ignore it since it is a load (unless the transfer is volatile).
120 if (UI.getOperandNo() == 1) {
121 if (MI->isVolatile()) return false;
125 // If we already have seen a copy, reject the second one.
126 if (TheCopy) return false;
128 // If the pointer has been offset from the start of the alloca, we can't
129 // safely handle this.
130 if (IsOffset) return false;
132 // If the memintrinsic isn't using the alloca as the dest, reject it.
133 if (UI.getOperandNo() != 0) return false;
135 // If the source of the memcpy/move is not a constant global, reject it.
136 if (!pointsToConstantGlobal(MI->getSource()))
139 // Otherwise, the transform is safe. Remember the copy instruction.
145 /// isOnlyCopiedFromConstantGlobal - Return true if the specified alloca is only
146 /// modified by a copy from a constant global. If we can prove this, we can
147 /// replace any uses of the alloca with uses of the global directly.
148 static MemTransferInst *
149 isOnlyCopiedFromConstantGlobal(AllocaInst *AI,
150 SmallVectorImpl<Instruction *> &ToDelete) {
151 MemTransferInst *TheCopy = 0;
152 if (isOnlyCopiedFromConstantGlobal(AI, TheCopy, ToDelete))
157 Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
158 // Ensure that the alloca array size argument has type intptr_t, so that
159 // any casting is exposed early.
161 Type *IntPtrTy = DL->getIntPtrType(AI.getType());
162 if (AI.getArraySize()->getType() != IntPtrTy) {
163 Value *V = Builder->CreateIntCast(AI.getArraySize(),
170 // Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1
171 if (AI.isArrayAllocation()) { // Check C != 1
172 if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) {
174 ArrayType::get(AI.getAllocatedType(), C->getZExtValue());
175 AllocaInst *New = Builder->CreateAlloca(NewTy, 0, AI.getName());
176 New->setAlignment(AI.getAlignment());
178 // Scan to the end of the allocation instructions, to skip over a block of
179 // allocas if possible...also skip interleaved debug info
181 BasicBlock::iterator It = New;
182 while (isa<AllocaInst>(*It) || isa<DbgInfoIntrinsic>(*It)) ++It;
184 // Now that I is pointing to the first non-allocation-inst in the block,
185 // insert our getelementptr instruction...
188 ? DL->getIntPtrType(AI.getType())
189 : Type::getInt64Ty(AI.getContext());
190 Value *NullIdx = Constant::getNullValue(IdxTy);
191 Value *Idx[2] = { NullIdx, NullIdx };
193 GetElementPtrInst::CreateInBounds(New, Idx, New->getName() + ".sub");
194 InsertNewInstBefore(GEP, *It);
196 // Now make everything use the getelementptr instead of the original
198 return ReplaceInstUsesWith(AI, GEP);
199 } else if (isa<UndefValue>(AI.getArraySize())) {
200 return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));
204 if (DL && AI.getAllocatedType()->isSized()) {
205 // If the alignment is 0 (unspecified), assign it the preferred alignment.
206 if (AI.getAlignment() == 0)
207 AI.setAlignment(DL->getPrefTypeAlignment(AI.getAllocatedType()));
209 // Move all alloca's of zero byte objects to the entry block and merge them
210 // together. Note that we only do this for alloca's, because malloc should
211 // allocate and return a unique pointer, even for a zero byte allocation.
212 if (DL->getTypeAllocSize(AI.getAllocatedType()) == 0) {
213 // For a zero sized alloca there is no point in doing an array allocation.
214 // This is helpful if the array size is a complicated expression not used
216 if (AI.isArrayAllocation()) {
217 AI.setOperand(0, ConstantInt::get(AI.getArraySize()->getType(), 1));
221 // Get the first instruction in the entry block.
222 BasicBlock &EntryBlock = AI.getParent()->getParent()->getEntryBlock();
223 Instruction *FirstInst = EntryBlock.getFirstNonPHIOrDbg();
224 if (FirstInst != &AI) {
225 // If the entry block doesn't start with a zero-size alloca then move
226 // this one to the start of the entry block. There is no problem with
227 // dominance as the array size was forced to a constant earlier already.
228 AllocaInst *EntryAI = dyn_cast<AllocaInst>(FirstInst);
229 if (!EntryAI || !EntryAI->getAllocatedType()->isSized() ||
230 DL->getTypeAllocSize(EntryAI->getAllocatedType()) != 0) {
231 AI.moveBefore(FirstInst);
235 // If the alignment of the entry block alloca is 0 (unspecified),
236 // assign it the preferred alignment.
237 if (EntryAI->getAlignment() == 0)
238 EntryAI->setAlignment(
239 DL->getPrefTypeAlignment(EntryAI->getAllocatedType()));
240 // Replace this zero-sized alloca with the one at the start of the entry
241 // block after ensuring that the address will be aligned enough for both
243 unsigned MaxAlign = std::max(EntryAI->getAlignment(),
245 EntryAI->setAlignment(MaxAlign);
246 if (AI.getType() != EntryAI->getType())
247 return new BitCastInst(EntryAI, AI.getType());
248 return ReplaceInstUsesWith(AI, EntryAI);
253 if (AI.getAlignment()) {
254 // Check to see if this allocation is only modified by a memcpy/memmove from
255 // a constant global whose alignment is equal to or exceeds that of the
256 // allocation. If this is the case, we can change all users to use
257 // the constant global instead. This is commonly produced by the CFE by
258 // constructs like "void foo() { int A[] = {1,2,3,4,5,6,7,8,9...}; }" if 'A'
259 // is only subsequently read.
260 SmallVector<Instruction *, 4> ToDelete;
261 if (MemTransferInst *Copy = isOnlyCopiedFromConstantGlobal(&AI, ToDelete)) {
262 unsigned SourceAlign = getOrEnforceKnownAlignment(Copy->getSource(),
263 AI.getAlignment(), DL);
264 if (AI.getAlignment() <= SourceAlign) {
265 DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n');
266 DEBUG(dbgs() << " memcpy = " << *Copy << '\n');
267 for (unsigned i = 0, e = ToDelete.size(); i != e; ++i)
268 EraseInstFromFunction(*ToDelete[i]);
269 Constant *TheSrc = cast<Constant>(Copy->getSource());
271 = ConstantExpr::getPointerBitCastOrAddrSpaceCast(TheSrc, AI.getType());
272 Instruction *NewI = ReplaceInstUsesWith(AI, Cast);
273 EraseInstFromFunction(*Copy);
280 // At last, use the generic allocation site handler to aggressively remove
282 return visitAllocSite(AI);
286 /// InstCombineLoadCast - Fold 'load (cast P)' -> cast (load P)' when possible.
287 static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI,
288 const DataLayout *DL) {
289 User *CI = cast<User>(LI.getOperand(0));
290 Value *CastOp = CI->getOperand(0);
292 PointerType *DestTy = cast<PointerType>(CI->getType());
293 Type *DestPTy = DestTy->getElementType();
294 if (PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType())) {
296 // If the address spaces don't match, don't eliminate the cast.
297 if (DestTy->getAddressSpace() != SrcTy->getAddressSpace())
300 Type *SrcPTy = SrcTy->getElementType();
302 if (DestPTy->isIntegerTy() || DestPTy->isPointerTy() ||
303 DestPTy->isVectorTy()) {
304 // If the source is an array, the code below will not succeed. Check to
305 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for
307 if (ArrayType *ASrcTy = dyn_cast<ArrayType>(SrcPTy))
308 if (Constant *CSrc = dyn_cast<Constant>(CastOp))
309 if (ASrcTy->getNumElements() != 0) {
311 ? DL->getIntPtrType(SrcTy)
312 : Type::getInt64Ty(SrcTy->getContext());
313 Value *Idx = Constant::getNullValue(IdxTy);
314 Value *Idxs[2] = { Idx, Idx };
315 CastOp = ConstantExpr::getGetElementPtr(CSrc, Idxs);
316 SrcTy = cast<PointerType>(CastOp->getType());
317 SrcPTy = SrcTy->getElementType();
320 if (IC.getDataLayout() &&
321 (SrcPTy->isIntegerTy() || SrcPTy->isPointerTy() ||
322 SrcPTy->isVectorTy()) &&
323 // Do not allow turning this into a load of an integer, which is then
324 // casted to a pointer, this pessimizes pointer analysis a lot.
325 (SrcPTy->isPtrOrPtrVectorTy() ==
326 LI.getType()->isPtrOrPtrVectorTy()) &&
327 IC.getDataLayout()->getTypeSizeInBits(SrcPTy) ==
328 IC.getDataLayout()->getTypeSizeInBits(DestPTy)) {
330 // Okay, we are casting from one integer or pointer type to another of
331 // the same size. Instead of casting the pointer before the load, cast
332 // the result of the loaded value.
334 IC.Builder->CreateLoad(CastOp, LI.isVolatile(), CI->getName());
335 NewLoad->setAlignment(LI.getAlignment());
336 NewLoad->setAtomic(LI.getOrdering(), LI.getSynchScope());
337 // Now cast the result of the load.
338 return new BitCastInst(NewLoad, LI.getType());
345 Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
346 Value *Op = LI.getOperand(0);
348 // Attempt to improve the alignment.
350 unsigned KnownAlign =
351 getOrEnforceKnownAlignment(Op, DL->getPrefTypeAlignment(LI.getType()),DL);
352 unsigned LoadAlign = LI.getAlignment();
353 unsigned EffectiveLoadAlign = LoadAlign != 0 ? LoadAlign :
354 DL->getABITypeAlignment(LI.getType());
356 if (KnownAlign > EffectiveLoadAlign)
357 LI.setAlignment(KnownAlign);
358 else if (LoadAlign == 0)
359 LI.setAlignment(EffectiveLoadAlign);
362 // load (cast X) --> cast (load X) iff safe.
363 if (isa<CastInst>(Op))
364 if (Instruction *Res = InstCombineLoadCast(*this, LI, DL))
367 // None of the following transforms are legal for volatile/atomic loads.
368 // FIXME: Some of it is okay for atomic loads; needs refactoring.
369 if (!LI.isSimple()) return 0;
371 // Do really simple store-to-load forwarding and load CSE, to catch cases
372 // where there are several consecutive memory accesses to the same location,
373 // separated by a few arithmetic operations.
374 BasicBlock::iterator BBI = &LI;
375 if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,6))
376 return ReplaceInstUsesWith(LI, AvailableVal);
378 // load(gep null, ...) -> unreachable
379 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
380 const Value *GEPI0 = GEPI->getOperand(0);
381 // TODO: Consider a target hook for valid address spaces for this xform.
382 if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){
383 // Insert a new store to null instruction before the load to indicate
384 // that this code is not reachable. We do this instead of inserting
385 // an unreachable instruction directly because we cannot modify the
387 new StoreInst(UndefValue::get(LI.getType()),
388 Constant::getNullValue(Op->getType()), &LI);
389 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
393 // load null/undef -> unreachable
394 // TODO: Consider a target hook for valid address spaces for this xform.
395 if (isa<UndefValue>(Op) ||
396 (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) {
397 // Insert a new store to null instruction before the load to indicate that
398 // this code is not reachable. We do this instead of inserting an
399 // unreachable instruction directly because we cannot modify the CFG.
400 new StoreInst(UndefValue::get(LI.getType()),
401 Constant::getNullValue(Op->getType()), &LI);
402 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
405 // Instcombine load (constantexpr_cast global) -> cast (load global)
406 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Op))
408 if (Instruction *Res = InstCombineLoadCast(*this, LI, DL))
411 if (Op->hasOneUse()) {
412 // Change select and PHI nodes to select values instead of addresses: this
413 // helps alias analysis out a lot, allows many others simplifications, and
414 // exposes redundancy in the code.
416 // Note that we cannot do the transformation unless we know that the
417 // introduced loads cannot trap! Something like this is valid as long as
418 // the condition is always false: load (select bool %C, int* null, int* %G),
419 // but it would not be valid if we transformed it to load from null
422 if (SelectInst *SI = dyn_cast<SelectInst>(Op)) {
423 // load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2).
424 unsigned Align = LI.getAlignment();
425 if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, Align, DL) &&
426 isSafeToLoadUnconditionally(SI->getOperand(2), SI, Align, DL)) {
427 LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1),
428 SI->getOperand(1)->getName()+".val");
429 LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2),
430 SI->getOperand(2)->getName()+".val");
431 V1->setAlignment(Align);
432 V2->setAlignment(Align);
433 return SelectInst::Create(SI->getCondition(), V1, V2);
436 // load (select (cond, null, P)) -> load P
437 if (Constant *C = dyn_cast<Constant>(SI->getOperand(1)))
438 if (C->isNullValue()) {
439 LI.setOperand(0, SI->getOperand(2));
443 // load (select (cond, P, null)) -> load P
444 if (Constant *C = dyn_cast<Constant>(SI->getOperand(2)))
445 if (C->isNullValue()) {
446 LI.setOperand(0, SI->getOperand(1));
454 /// InstCombineStoreToCast - Fold store V, (cast P) -> store (cast V), P
455 /// when possible. This makes it generally easy to do alias analysis and/or
456 /// SROA/mem2reg of the memory object.
457 static Instruction *InstCombineStoreToCast(InstCombiner &IC, StoreInst &SI) {
458 User *CI = cast<User>(SI.getOperand(1));
459 Value *CastOp = CI->getOperand(0);
461 Type *DestPTy = cast<PointerType>(CI->getType())->getElementType();
462 PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType());
463 if (SrcTy == 0) return 0;
465 Type *SrcPTy = SrcTy->getElementType();
467 if (!DestPTy->isIntegerTy() && !DestPTy->isPointerTy())
470 /// NewGEPIndices - If SrcPTy is an aggregate type, we can emit a "noop gep"
471 /// to its first element. This allows us to handle things like:
472 /// store i32 xxx, (bitcast {foo*, float}* %P to i32*)
474 SmallVector<Value*, 4> NewGEPIndices;
476 // If the source is an array, the code below will not succeed. Check to
477 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for
479 if (SrcPTy->isArrayTy() || SrcPTy->isStructTy()) {
480 // Index through pointer.
481 Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext()));
482 NewGEPIndices.push_back(Zero);
485 if (StructType *STy = dyn_cast<StructType>(SrcPTy)) {
486 if (!STy->getNumElements()) /* Struct can be empty {} */
488 NewGEPIndices.push_back(Zero);
489 SrcPTy = STy->getElementType(0);
490 } else if (ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) {
491 NewGEPIndices.push_back(Zero);
492 SrcPTy = ATy->getElementType();
498 SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace());
501 if (!SrcPTy->isIntegerTy() && !SrcPTy->isPointerTy())
504 // If the pointers point into different address spaces or if they point to
505 // values with different sizes, we can't do the transformation.
506 if (!IC.getDataLayout() ||
507 SrcTy->getAddressSpace() !=
508 cast<PointerType>(CI->getType())->getAddressSpace() ||
509 IC.getDataLayout()->getTypeSizeInBits(SrcPTy) !=
510 IC.getDataLayout()->getTypeSizeInBits(DestPTy))
513 // Okay, we are casting from one integer or pointer type to another of
514 // the same size. Instead of casting the pointer before
515 // the store, cast the value to be stored.
517 Value *SIOp0 = SI.getOperand(0);
518 Instruction::CastOps opcode = Instruction::BitCast;
519 Type* CastSrcTy = SIOp0->getType();
520 Type* CastDstTy = SrcPTy;
521 if (CastDstTy->isPointerTy()) {
522 if (CastSrcTy->isIntegerTy())
523 opcode = Instruction::IntToPtr;
524 } else if (CastDstTy->isIntegerTy()) {
525 if (SIOp0->getType()->isPointerTy())
526 opcode = Instruction::PtrToInt;
529 // SIOp0 is a pointer to aggregate and this is a store to the first field,
530 // emit a GEP to index into its first field.
531 if (!NewGEPIndices.empty())
532 CastOp = IC.Builder->CreateInBoundsGEP(CastOp, NewGEPIndices);
534 NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy,
535 SIOp0->getName()+".c");
536 SI.setOperand(0, NewCast);
537 SI.setOperand(1, CastOp);
541 /// equivalentAddressValues - Test if A and B will obviously have the same
542 /// value. This includes recognizing that %t0 and %t1 will have the same
543 /// value in code like this:
544 /// %t0 = getelementptr \@a, 0, 3
545 /// store i32 0, i32* %t0
546 /// %t1 = getelementptr \@a, 0, 3
547 /// %t2 = load i32* %t1
549 static bool equivalentAddressValues(Value *A, Value *B) {
550 // Test if the values are trivially equivalent.
551 if (A == B) return true;
553 // Test if the values come form identical arithmetic instructions.
554 // This uses isIdenticalToWhenDefined instead of isIdenticalTo because
555 // its only used to compare two uses within the same basic block, which
556 // means that they'll always either have the same value or one of them
557 // will have an undefined value.
558 if (isa<BinaryOperator>(A) ||
561 isa<GetElementPtrInst>(A))
562 if (Instruction *BI = dyn_cast<Instruction>(B))
563 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
566 // Otherwise they may not be equivalent.
570 Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
571 Value *Val = SI.getOperand(0);
572 Value *Ptr = SI.getOperand(1);
574 // Attempt to improve the alignment.
576 unsigned KnownAlign =
577 getOrEnforceKnownAlignment(Ptr, DL->getPrefTypeAlignment(Val->getType()),
579 unsigned StoreAlign = SI.getAlignment();
580 unsigned EffectiveStoreAlign = StoreAlign != 0 ? StoreAlign :
581 DL->getABITypeAlignment(Val->getType());
583 if (KnownAlign > EffectiveStoreAlign)
584 SI.setAlignment(KnownAlign);
585 else if (StoreAlign == 0)
586 SI.setAlignment(EffectiveStoreAlign);
589 // Don't hack volatile/atomic stores.
590 // FIXME: Some bits are legal for atomic stores; needs refactoring.
591 if (!SI.isSimple()) return 0;
593 // If the RHS is an alloca with a single use, zapify the store, making the
595 if (Ptr->hasOneUse()) {
596 if (isa<AllocaInst>(Ptr))
597 return EraseInstFromFunction(SI);
598 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) {
599 if (isa<AllocaInst>(GEP->getOperand(0))) {
600 if (GEP->getOperand(0)->hasOneUse())
601 return EraseInstFromFunction(SI);
606 // Do really simple DSE, to catch cases where there are several consecutive
607 // stores to the same location, separated by a few arithmetic operations. This
608 // situation often occurs with bitfield accesses.
609 BasicBlock::iterator BBI = &SI;
610 for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts;
613 // Don't count debug info directives, lest they affect codegen,
614 // and we skip pointer-to-pointer bitcasts, which are NOPs.
615 if (isa<DbgInfoIntrinsic>(BBI) ||
616 (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
621 if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) {
622 // Prev store isn't volatile, and stores to the same location?
623 if (PrevSI->isSimple() && equivalentAddressValues(PrevSI->getOperand(1),
627 EraseInstFromFunction(*PrevSI);
633 // If this is a load, we have to stop. However, if the loaded value is from
634 // the pointer we're loading and is producing the pointer we're storing,
635 // then *this* store is dead (X = load P; store X -> P).
636 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
637 if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) &&
639 return EraseInstFromFunction(SI);
641 // Otherwise, this is a load from some other location. Stores before it
646 // Don't skip over loads or things that can modify memory.
647 if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory())
651 // store X, null -> turns into 'unreachable' in SimplifyCFG
652 if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) {
653 if (!isa<UndefValue>(Val)) {
654 SI.setOperand(0, UndefValue::get(Val->getType()));
655 if (Instruction *U = dyn_cast<Instruction>(Val))
656 Worklist.Add(U); // Dropped a use.
658 return 0; // Do not modify these!
661 // store undef, Ptr -> noop
662 if (isa<UndefValue>(Val))
663 return EraseInstFromFunction(SI);
665 // If the pointer destination is a cast, see if we can fold the cast into the
667 if (isa<CastInst>(Ptr))
668 if (Instruction *Res = InstCombineStoreToCast(*this, SI))
670 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
672 if (Instruction *Res = InstCombineStoreToCast(*this, SI))
676 // If this store is the last instruction in the basic block (possibly
677 // excepting debug info instructions), and if the block ends with an
678 // unconditional branch, try to move it to the successor block.
682 } while (isa<DbgInfoIntrinsic>(BBI) ||
683 (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy()));
684 if (BranchInst *BI = dyn_cast<BranchInst>(BBI))
685 if (BI->isUnconditional())
686 if (SimplifyStoreAtEndOfBlock(SI))
687 return 0; // xform done!
692 /// SimplifyStoreAtEndOfBlock - Turn things like:
693 /// if () { *P = v1; } else { *P = v2 }
694 /// into a phi node with a store in the successor.
696 /// Simplify things like:
697 /// *P = v1; if () { *P = v2; }
698 /// into a phi node with a store in the successor.
700 bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) {
701 BasicBlock *StoreBB = SI.getParent();
703 // Check to see if the successor block has exactly two incoming edges. If
704 // so, see if the other predecessor contains a store to the same location.
705 // if so, insert a PHI node (if needed) and move the stores down.
706 BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0);
708 // Determine whether Dest has exactly two predecessors and, if so, compute
709 // the other predecessor.
710 pred_iterator PI = pred_begin(DestBB);
712 BasicBlock *OtherBB = 0;
717 if (++PI == pred_end(DestBB))
726 if (++PI != pred_end(DestBB))
729 // Bail out if all the relevant blocks aren't distinct (this can happen,
730 // for example, if SI is in an infinite loop)
731 if (StoreBB == DestBB || OtherBB == DestBB)
734 // Verify that the other block ends in a branch and is not otherwise empty.
735 BasicBlock::iterator BBI = OtherBB->getTerminator();
736 BranchInst *OtherBr = dyn_cast<BranchInst>(BBI);
737 if (!OtherBr || BBI == OtherBB->begin())
740 // If the other block ends in an unconditional branch, check for the 'if then
741 // else' case. there is an instruction before the branch.
742 StoreInst *OtherStore = 0;
743 if (OtherBr->isUnconditional()) {
745 // Skip over debugging info.
746 while (isa<DbgInfoIntrinsic>(BBI) ||
747 (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) {
748 if (BBI==OtherBB->begin())
752 // If this isn't a store, isn't a store to the same location, or is not the
753 // right kind of store, bail out.
754 OtherStore = dyn_cast<StoreInst>(BBI);
755 if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) ||
756 !SI.isSameOperationAs(OtherStore))
759 // Otherwise, the other block ended with a conditional branch. If one of the
760 // destinations is StoreBB, then we have the if/then case.
761 if (OtherBr->getSuccessor(0) != StoreBB &&
762 OtherBr->getSuccessor(1) != StoreBB)
765 // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an
766 // if/then triangle. See if there is a store to the same ptr as SI that
769 // Check to see if we find the matching store.
770 if ((OtherStore = dyn_cast<StoreInst>(BBI))) {
771 if (OtherStore->getOperand(1) != SI.getOperand(1) ||
772 !SI.isSameOperationAs(OtherStore))
776 // If we find something that may be using or overwriting the stored
777 // value, or if we run out of instructions, we can't do the xform.
778 if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() ||
779 BBI == OtherBB->begin())
783 // In order to eliminate the store in OtherBr, we have to
784 // make sure nothing reads or overwrites the stored value in
786 for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) {
787 // FIXME: This should really be AA driven.
788 if (I->mayReadFromMemory() || I->mayWriteToMemory())
793 // Insert a PHI node now if we need it.
794 Value *MergedVal = OtherStore->getOperand(0);
795 if (MergedVal != SI.getOperand(0)) {
796 PHINode *PN = PHINode::Create(MergedVal->getType(), 2, "storemerge");
797 PN->addIncoming(SI.getOperand(0), SI.getParent());
798 PN->addIncoming(OtherStore->getOperand(0), OtherBB);
799 MergedVal = InsertNewInstBefore(PN, DestBB->front());
802 // Advance to a place where it is safe to insert the new store and
804 BBI = DestBB->getFirstInsertionPt();
805 StoreInst *NewSI = new StoreInst(MergedVal, SI.getOperand(1),
810 InsertNewInstBefore(NewSI, *BBI);
811 NewSI->setDebugLoc(OtherStore->getDebugLoc());
813 // If the two stores had the same TBAA tag, preserve it.
814 if (MDNode *TBAATag = SI.getMetadata(LLVMContext::MD_tbaa))
815 if ((TBAATag = MDNode::getMostGenericTBAA(TBAATag,
816 OtherStore->getMetadata(LLVMContext::MD_tbaa))))
817 NewSI->setMetadata(LLVMContext::MD_tbaa, TBAATag);
820 // Nuke the old stores.
821 EraseInstFromFunction(SI);
822 EraseInstFromFunction(*OtherStore);