1 //===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
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 a trivial dead store elimination that only considers
11 // basic-block local redundant stores.
13 // FIXME: This should eventually be extended to be a post-dominator tree
14 // traversal. Doing so would be pretty trivial.
16 //===----------------------------------------------------------------------===//
18 #define DEBUG_TYPE "dse"
19 #include "llvm/Transforms/Scalar.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Function.h"
22 #include "llvm/GlobalVariable.h"
23 #include "llvm/Instructions.h"
24 #include "llvm/IntrinsicInst.h"
25 #include "llvm/Pass.h"
26 #include "llvm/Analysis/AliasAnalysis.h"
27 #include "llvm/Analysis/CaptureTracking.h"
28 #include "llvm/Analysis/Dominators.h"
29 #include "llvm/Analysis/MemoryBuiltins.h"
30 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
31 #include "llvm/Analysis/ValueTracking.h"
32 #include "llvm/Target/TargetData.h"
33 #include "llvm/Transforms/Utils/Local.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/ADT/SetVector.h"
36 #include "llvm/ADT/Statistic.h"
37 #include "llvm/ADT/STLExtras.h"
40 STATISTIC(NumFastStores, "Number of stores deleted");
41 STATISTIC(NumFastOther , "Number of other instrs removed");
44 struct DSE : public FunctionPass {
46 MemoryDependenceAnalysis *MD;
49 static char ID; // Pass identification, replacement for typeid
50 DSE() : FunctionPass(ID), AA(0), MD(0), DT(0) {
51 initializeDSEPass(*PassRegistry::getPassRegistry());
54 virtual bool runOnFunction(Function &F) {
55 AA = &getAnalysis<AliasAnalysis>();
56 MD = &getAnalysis<MemoryDependenceAnalysis>();
57 DT = &getAnalysis<DominatorTree>();
60 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
61 // Only check non-dead blocks. Dead blocks may have strange pointer
62 // cycles that will confuse alias analysis.
63 if (DT->isReachableFromEntry(I))
64 Changed |= runOnBasicBlock(*I);
66 AA = 0; MD = 0; DT = 0;
70 bool runOnBasicBlock(BasicBlock &BB);
71 bool HandleFree(CallInst *F);
72 bool handleEndBlock(BasicBlock &BB);
73 void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
74 SmallSetVector<Value*, 16> &DeadStackObjects);
76 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
78 AU.addRequired<DominatorTree>();
79 AU.addRequired<AliasAnalysis>();
80 AU.addRequired<MemoryDependenceAnalysis>();
81 AU.addPreserved<AliasAnalysis>();
82 AU.addPreserved<DominatorTree>();
83 AU.addPreserved<MemoryDependenceAnalysis>();
89 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
90 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
91 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
92 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
93 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
95 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
97 //===----------------------------------------------------------------------===//
99 //===----------------------------------------------------------------------===//
101 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
102 /// and zero out all the operands of this instruction. If any of them become
103 /// dead, delete them and the computation tree that feeds them.
105 /// If ValueSet is non-null, remove any deleted instructions from it as well.
107 static void DeleteDeadInstruction(Instruction *I,
108 MemoryDependenceAnalysis &MD,
109 const TargetLibraryInfo *TLI,
110 SmallSetVector<Value*, 16> *ValueSet = 0) {
111 SmallVector<Instruction*, 32> NowDeadInsts;
113 NowDeadInsts.push_back(I);
116 // Before we touch this instruction, remove it from memdep!
118 Instruction *DeadInst = NowDeadInsts.pop_back_val();
121 // This instruction is dead, zap it, in stages. Start by removing it from
122 // MemDep, which needs to know the operands and needs it to be in the
124 MD.removeInstruction(DeadInst);
126 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
127 Value *Op = DeadInst->getOperand(op);
128 DeadInst->setOperand(op, 0);
130 // If this operand just became dead, add it to the NowDeadInsts list.
131 if (!Op->use_empty()) continue;
133 if (Instruction *OpI = dyn_cast<Instruction>(Op))
134 if (isInstructionTriviallyDead(OpI, TLI))
135 NowDeadInsts.push_back(OpI);
138 DeadInst->eraseFromParent();
140 if (ValueSet) ValueSet->remove(DeadInst);
141 } while (!NowDeadInsts.empty());
145 /// hasMemoryWrite - Does this instruction write some memory? This only returns
146 /// true for things that we can analyze with other helpers below.
147 static bool hasMemoryWrite(Instruction *I) {
148 if (isa<StoreInst>(I))
150 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
151 switch (II->getIntrinsicID()) {
154 case Intrinsic::memset:
155 case Intrinsic::memmove:
156 case Intrinsic::memcpy:
157 case Intrinsic::init_trampoline:
158 case Intrinsic::lifetime_end:
165 /// getLocForWrite - Return a Location stored to by the specified instruction.
166 /// If isRemovable returns true, this function and getLocForRead completely
167 /// describe the memory operations for this instruction.
168 static AliasAnalysis::Location
169 getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
170 if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
171 return AA.getLocation(SI);
173 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
174 // memcpy/memmove/memset.
175 AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
176 // If we don't have target data around, an unknown size in Location means
177 // that we should use the size of the pointee type. This isn't valid for
178 // memset/memcpy, which writes more than an i8.
179 if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0)
180 return AliasAnalysis::Location();
184 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
185 if (II == 0) return AliasAnalysis::Location();
187 switch (II->getIntrinsicID()) {
188 default: return AliasAnalysis::Location(); // Unhandled intrinsic.
189 case Intrinsic::init_trampoline:
190 // If we don't have target data around, an unknown size in Location means
191 // that we should use the size of the pointee type. This isn't valid for
192 // init.trampoline, which writes more than an i8.
193 if (AA.getTargetData() == 0) return AliasAnalysis::Location();
195 // FIXME: We don't know the size of the trampoline, so we can't really
197 return AliasAnalysis::Location(II->getArgOperand(0));
198 case Intrinsic::lifetime_end: {
199 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
200 return AliasAnalysis::Location(II->getArgOperand(1), Len);
205 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
206 /// instruction if any.
207 static AliasAnalysis::Location
208 getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
209 assert(hasMemoryWrite(Inst) && "Unknown instruction case");
211 // The only instructions that both read and write are the mem transfer
212 // instructions (memcpy/memmove).
213 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
214 return AA.getLocationForSource(MTI);
215 return AliasAnalysis::Location();
219 /// isRemovable - If the value of this instruction and the memory it writes to
220 /// is unused, may we delete this instruction?
221 static bool isRemovable(Instruction *I) {
222 // Don't remove volatile/atomic stores.
223 if (StoreInst *SI = dyn_cast<StoreInst>(I))
224 return SI->isUnordered();
226 IntrinsicInst *II = cast<IntrinsicInst>(I);
227 switch (II->getIntrinsicID()) {
228 default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
229 case Intrinsic::lifetime_end:
230 // Never remove dead lifetime_end's, e.g. because it is followed by a
233 case Intrinsic::init_trampoline:
234 // Always safe to remove init_trampoline.
237 case Intrinsic::memset:
238 case Intrinsic::memmove:
239 case Intrinsic::memcpy:
240 // Don't remove volatile memory intrinsics.
241 return !cast<MemIntrinsic>(II)->isVolatile();
246 /// isShortenable - Returns true if this instruction can be safely shortened in
248 static bool isShortenable(Instruction *I) {
249 // Don't shorten stores for now
250 if (isa<StoreInst>(I))
253 IntrinsicInst *II = cast<IntrinsicInst>(I);
254 switch (II->getIntrinsicID()) {
255 default: return false;
256 case Intrinsic::memset:
257 case Intrinsic::memcpy:
258 // Do shorten memory intrinsics.
263 /// getStoredPointerOperand - Return the pointer that is being written to.
264 static Value *getStoredPointerOperand(Instruction *I) {
265 if (StoreInst *SI = dyn_cast<StoreInst>(I))
266 return SI->getPointerOperand();
267 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
268 return MI->getDest();
270 IntrinsicInst *II = cast<IntrinsicInst>(I);
271 switch (II->getIntrinsicID()) {
272 default: llvm_unreachable("Unexpected intrinsic!");
273 case Intrinsic::init_trampoline:
274 return II->getArgOperand(0);
278 static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
280 if (getObjectSize(V, Size, AA.getTargetData(), AA.getTargetLibraryInfo()))
282 return AliasAnalysis::UnknownSize;
294 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
295 /// completely overwrites a store to the 'Earlier' location.
296 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
297 /// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
298 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
299 const AliasAnalysis::Location &Earlier,
303 const Value *P1 = Earlier.Ptr->stripPointerCasts();
304 const Value *P2 = Later.Ptr->stripPointerCasts();
306 // If the start pointers are the same, we just have to compare sizes to see if
307 // the later store was larger than the earlier store.
309 // If we don't know the sizes of either access, then we can't do a
311 if (Later.Size == AliasAnalysis::UnknownSize ||
312 Earlier.Size == AliasAnalysis::UnknownSize) {
313 // If we have no TargetData information around, then the size of the store
314 // is inferrable from the pointee type. If they are the same type, then
315 // we know that the store is safe.
316 if (AA.getTargetData() == 0 &&
317 Later.Ptr->getType() == Earlier.Ptr->getType())
318 return OverwriteComplete;
320 return OverwriteUnknown;
323 // Make sure that the Later size is >= the Earlier size.
324 if (Later.Size >= Earlier.Size)
325 return OverwriteComplete;
328 // Otherwise, we have to have size information, and the later store has to be
329 // larger than the earlier one.
330 if (Later.Size == AliasAnalysis::UnknownSize ||
331 Earlier.Size == AliasAnalysis::UnknownSize ||
332 AA.getTargetData() == 0)
333 return OverwriteUnknown;
335 // Check to see if the later store is to the entire object (either a global,
336 // an alloca, or a byval argument). If so, then it clearly overwrites any
337 // other store to the same object.
338 const TargetData &TD = *AA.getTargetData();
340 const Value *UO1 = GetUnderlyingObject(P1, &TD),
341 *UO2 = GetUnderlyingObject(P2, &TD);
343 // If we can't resolve the same pointers to the same object, then we can't
344 // analyze them at all.
346 return OverwriteUnknown;
348 // If the "Later" store is to a recognizable object, get its size.
349 uint64_t ObjectSize = getPointerSize(UO2, AA);
350 if (ObjectSize != AliasAnalysis::UnknownSize)
351 if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
352 return OverwriteComplete;
354 // Okay, we have stores to two completely different pointers. Try to
355 // decompose the pointer into a "base + constant_offset" form. If the base
356 // pointers are equal, then we can reason about the two stores.
359 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
360 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
362 // If the base pointers still differ, we have two completely different stores.
364 return OverwriteUnknown;
366 // The later store completely overlaps the earlier store if:
368 // 1. Both start at the same offset and the later one's size is greater than
369 // or equal to the earlier one's, or
374 // 2. The earlier store has an offset greater than the later offset, but which
375 // still lies completely within the later store.
378 // |----- later ------|
380 // We have to be careful here as *Off is signed while *.Size is unsigned.
381 if (EarlierOff >= LaterOff &&
382 Later.Size >= Earlier.Size &&
383 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
384 return OverwriteComplete;
386 // The other interesting case is if the later store overwrites the end of
392 // In this case we may want to trim the size of earlier to avoid generating
393 // writes to addresses which will definitely be overwritten later
394 if (LaterOff > EarlierOff &&
395 LaterOff < int64_t(EarlierOff + Earlier.Size) &&
396 int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
399 // Otherwise, they don't completely overlap.
400 return OverwriteUnknown;
403 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
404 /// memory region into an identical pointer) then it doesn't actually make its
405 /// input dead in the traditional sense. Consider this case:
410 /// In this case, the second store to A does not make the first store to A dead.
411 /// The usual situation isn't an explicit A<-A store like this (which can be
412 /// trivially removed) but a case where two pointers may alias.
414 /// This function detects when it is unsafe to remove a dependent instruction
415 /// because the DSE inducing instruction may be a self-read.
416 static bool isPossibleSelfRead(Instruction *Inst,
417 const AliasAnalysis::Location &InstStoreLoc,
418 Instruction *DepWrite, AliasAnalysis &AA) {
419 // Self reads can only happen for instructions that read memory. Get the
421 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
422 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
424 // If the read and written loc obviously don't alias, it isn't a read.
425 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
427 // Okay, 'Inst' may copy over itself. However, we can still remove a the
428 // DepWrite instruction if we can prove that it reads from the same location
429 // as Inst. This handles useful cases like:
432 // Here we don't know if A/B may alias, but we do know that B/B are must
433 // aliases, so removing the first memcpy is safe (assuming it writes <= #
434 // bytes as the second one.
435 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
437 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
440 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
441 // then it can't be considered dead.
446 //===----------------------------------------------------------------------===//
448 //===----------------------------------------------------------------------===//
450 bool DSE::runOnBasicBlock(BasicBlock &BB) {
451 bool MadeChange = false;
453 // Do a top-down walk on the BB.
454 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
455 Instruction *Inst = BBI++;
457 // Handle 'free' calls specially.
458 if (CallInst *F = isFreeCall(Inst, AA->getTargetLibraryInfo())) {
459 MadeChange |= HandleFree(F);
463 // If we find something that writes memory, get its memory dependence.
464 if (!hasMemoryWrite(Inst))
467 MemDepResult InstDep = MD->getDependency(Inst);
469 // Ignore any store where we can't find a local dependence.
470 // FIXME: cross-block DSE would be fun. :)
471 if (!InstDep.isDef() && !InstDep.isClobber())
474 // If we're storing the same value back to a pointer that we just
475 // loaded from, then the store can be removed.
476 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
477 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
478 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
479 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
480 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
481 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
483 // DeleteDeadInstruction can delete the current instruction. Save BBI
484 // in case we need it.
485 WeakVH NextInst(BBI);
487 DeleteDeadInstruction(SI, *MD, AA->getTargetLibraryInfo());
489 if (NextInst == 0) // Next instruction deleted.
491 else if (BBI != BB.begin()) // Revisit this instruction if possible.
500 // Figure out what location is being stored to.
501 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
503 // If we didn't get a useful location, fail.
507 while (InstDep.isDef() || InstDep.isClobber()) {
508 // Get the memory clobbered by the instruction we depend on. MemDep will
509 // skip any instructions that 'Loc' clearly doesn't interact with. If we
510 // end up depending on a may- or must-aliased load, then we can't optimize
511 // away the store and we bail out. However, if we depend on on something
512 // that overwrites the memory location we *can* potentially optimize it.
514 // Find out what memory location the dependent instruction stores.
515 Instruction *DepWrite = InstDep.getInst();
516 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
517 // If we didn't get a useful location, or if it isn't a size, bail out.
521 // If we find a write that is a) removable (i.e., non-volatile), b) is
522 // completely obliterated by the store to 'Loc', and c) which we know that
523 // 'Inst' doesn't load from, then we can remove it.
524 if (isRemovable(DepWrite) &&
525 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
526 int64_t InstWriteOffset, DepWriteOffset;
527 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
528 DepWriteOffset, InstWriteOffset);
529 if (OR == OverwriteComplete) {
530 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
531 << *DepWrite << "\n KILLER: " << *Inst << '\n');
533 // Delete the store and now-dead instructions that feed it.
534 DeleteDeadInstruction(DepWrite, *MD, AA->getTargetLibraryInfo());
538 // DeleteDeadInstruction can delete the current instruction in loop
541 if (BBI != BB.begin())
544 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
545 // TODO: base this on the target vector size so that if the earlier
546 // store was too small to get vector writes anyway then its likely
547 // a good idea to shorten it
548 // Power of 2 vector writes are probably always a bad idea to optimize
549 // as any store/memset/memcpy is likely using vector instructions so
550 // shortening it to not vector size is likely to be slower
551 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
552 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
553 if (llvm::isPowerOf2_64(InstWriteOffset) ||
554 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
556 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
557 << *DepWrite << "\n KILLER (offset "
558 << InstWriteOffset << ", "
559 << DepLoc.Size << ")"
562 Value* DepWriteLength = DepIntrinsic->getLength();
563 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
566 DepIntrinsic->setLength(TrimmedLength);
572 // If this is a may-aliased store that is clobbering the store value, we
573 // can keep searching past it for another must-aliased pointer that stores
574 // to the same location. For example, in:
578 // we can remove the first store to P even though we don't know if P and Q
580 if (DepWrite == &BB.front()) break;
582 // Can't look past this instruction if it might read 'Loc'.
583 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
586 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
590 // If this block ends in a return, unwind, or unreachable, all allocas are
591 // dead at its end, which means stores to them are also dead.
592 if (BB.getTerminator()->getNumSuccessors() == 0)
593 MadeChange |= handleEndBlock(BB);
598 /// Find all blocks that will unconditionally lead to the block BB and append
600 static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
601 BasicBlock *BB, DominatorTree *DT) {
602 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
603 BasicBlock *Pred = *I;
604 if (Pred == BB) continue;
605 TerminatorInst *PredTI = Pred->getTerminator();
606 if (PredTI->getNumSuccessors() != 1)
609 if (DT->isReachableFromEntry(Pred))
610 Blocks.push_back(Pred);
614 /// HandleFree - Handle frees of entire structures whose dependency is a store
615 /// to a field of that structure.
616 bool DSE::HandleFree(CallInst *F) {
617 bool MadeChange = false;
619 AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
620 SmallVector<BasicBlock *, 16> Blocks;
621 Blocks.push_back(F->getParent());
623 while (!Blocks.empty()) {
624 BasicBlock *BB = Blocks.pop_back_val();
625 Instruction *InstPt = BB->getTerminator();
626 if (BB == F->getParent()) InstPt = F;
628 MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
629 while (Dep.isDef() || Dep.isClobber()) {
630 Instruction *Dependency = Dep.getInst();
631 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
635 GetUnderlyingObject(getStoredPointerOperand(Dependency));
637 // Check for aliasing.
638 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
641 Instruction *Next = llvm::next(BasicBlock::iterator(Dependency));
643 // DCE instructions only used to calculate that store
644 DeleteDeadInstruction(Dependency, *MD, AA->getTargetLibraryInfo());
648 // Inst's old Dependency is now deleted. Compute the next dependency,
649 // which may also be dead, as in
651 // s[1] = 0; // This has just been deleted.
653 Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
656 if (Dep.isNonLocal())
657 FindUnconditionalPreds(Blocks, BB, DT);
663 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
664 /// function end block. Ex:
667 /// store i32 1, i32* %A
669 bool DSE::handleEndBlock(BasicBlock &BB) {
670 bool MadeChange = false;
672 // Keep track of all of the stack objects that are dead at the end of the
674 SmallSetVector<Value*, 16> DeadStackObjects;
676 // Find all of the alloca'd pointers in the entry block.
677 BasicBlock *Entry = BB.getParent()->begin();
678 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
679 if (isa<AllocaInst>(I))
680 DeadStackObjects.insert(I);
682 // Okay, so these are dead heap objects, but if the pointer never escapes
683 // then it's leaked by this function anyways.
684 else if (isAllocLikeFn(I, AA->getTargetLibraryInfo()) &&
685 !PointerMayBeCaptured(I, true, true))
686 DeadStackObjects.insert(I);
689 // Treat byval arguments the same, stores to them are dead at the end of the
691 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
692 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
693 if (AI->hasByValAttr())
694 DeadStackObjects.insert(AI);
696 // Scan the basic block backwards
697 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
700 // If we find a store, check to see if it points into a dead stack value.
701 if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
702 // See through pointer-to-pointer bitcasts
703 SmallVector<Value *, 4> Pointers;
704 GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers);
706 // Stores to stack values are valid candidates for removal.
708 for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
709 E = Pointers.end(); I != E; ++I)
710 if (!DeadStackObjects.count(*I)) {
716 Instruction *Dead = BBI++;
718 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
719 << *Dead << "\n Objects: ";
720 for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
721 E = Pointers.end(); I != E; ++I) {
723 if (llvm::next(I) != E)
728 // DCE instructions only used to calculate that store.
729 DeleteDeadInstruction(Dead, *MD, AA->getTargetLibraryInfo(),
737 // Remove any dead non-memory-mutating instructions.
738 if (isInstructionTriviallyDead(BBI, AA->getTargetLibraryInfo())) {
739 Instruction *Inst = BBI++;
740 DeleteDeadInstruction(Inst, *MD, AA->getTargetLibraryInfo(),
747 if (isa<AllocaInst>(BBI)) {
748 // Remove allocas from the list of dead stack objects; there can't be
749 // any references before the definition.
750 DeadStackObjects.remove(BBI);
754 if (CallSite CS = cast<Value>(BBI)) {
755 // Remove allocation function calls from the list of dead stack objects;
756 // there can't be any references before the definition.
757 if (isAllocLikeFn(BBI, AA->getTargetLibraryInfo()))
758 DeadStackObjects.remove(BBI);
760 // If this call does not access memory, it can't be loading any of our
762 if (AA->doesNotAccessMemory(CS))
765 // If the call might load from any of our allocas, then any store above
767 SmallVector<Value*, 8> LiveAllocas;
768 for (SmallSetVector<Value*, 16>::iterator I = DeadStackObjects.begin(),
769 E = DeadStackObjects.end(); I != E; ++I) {
770 // See if the call site touches it.
771 AliasAnalysis::ModRefResult A =
772 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
774 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
775 LiveAllocas.push_back(*I);
778 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
779 E = LiveAllocas.end(); I != E; ++I)
780 DeadStackObjects.remove(*I);
782 // If all of the allocas were clobbered by the call then we're not going
783 // to find anything else to process.
784 if (DeadStackObjects.empty())
790 AliasAnalysis::Location LoadedLoc;
792 // If we encounter a use of the pointer, it is no longer considered dead
793 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
794 if (!L->isUnordered()) // Be conservative with atomic/volatile load
796 LoadedLoc = AA->getLocation(L);
797 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
798 LoadedLoc = AA->getLocation(V);
799 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
800 LoadedLoc = AA->getLocationForSource(MTI);
801 } else if (!BBI->mayReadFromMemory()) {
802 // Instruction doesn't read memory. Note that stores that weren't removed
803 // above will hit this case.
806 // Unknown inst; assume it clobbers everything.
810 // Remove any allocas from the DeadPointer set that are loaded, as this
811 // makes any stores above the access live.
812 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
814 // If all of the allocas were clobbered by the access then we're not going
815 // to find anything else to process.
816 if (DeadStackObjects.empty())
823 /// RemoveAccessedObjects - Check to see if the specified location may alias any
824 /// of the stack objects in the DeadStackObjects set. If so, they become live
825 /// because the location is being loaded.
826 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
827 SmallSetVector<Value*, 16> &DeadStackObjects) {
828 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
830 // A constant can't be in the dead pointer set.
831 if (isa<Constant>(UnderlyingPointer))
834 // If the kill pointer can be easily reduced to an alloca, don't bother doing
835 // extraneous AA queries.
836 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
837 DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
841 SmallVector<Value*, 16> NowLive;
842 for (SmallSetVector<Value*, 16>::iterator I = DeadStackObjects.begin(),
843 E = DeadStackObjects.end(); I != E; ++I) {
844 // See if the loaded location could alias the stack location.
845 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
846 if (!AA->isNoAlias(StackLoc, LoadedLoc))
847 NowLive.push_back(*I);
850 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
852 DeadStackObjects.remove(*I);