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/ADT/SmallPtrSet.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/AliasAnalysis.h"
29 #include "llvm/Analysis/Dominators.h"
30 #include "llvm/Analysis/MemoryBuiltins.h"
31 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
32 #include "llvm/Analysis/ValueTracking.h"
33 #include "llvm/Target/TargetData.h"
34 #include "llvm/Transforms/Utils/Local.h"
37 STATISTIC(NumFastStores, "Number of stores deleted");
38 STATISTIC(NumFastOther , "Number of other instrs removed");
41 struct DSE : public FunctionPass {
43 MemoryDependenceAnalysis *MD;
45 static char ID; // Pass identification, replacement for typeid
46 DSE() : FunctionPass(ID), AA(0), MD(0) {
47 initializeDSEPass(*PassRegistry::getPassRegistry());
50 virtual bool runOnFunction(Function &F) {
51 AA = &getAnalysis<AliasAnalysis>();
52 MD = &getAnalysis<MemoryDependenceAnalysis>();
53 DominatorTree &DT = getAnalysis<DominatorTree>();
56 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
57 // Only check non-dead blocks. Dead blocks may have strange pointer
58 // cycles that will confuse alias analysis.
59 if (DT.isReachableFromEntry(I))
60 Changed |= runOnBasicBlock(*I);
66 bool runOnBasicBlock(BasicBlock &BB);
67 bool HandleFree(CallInst *F);
68 bool handleEndBlock(BasicBlock &BB);
69 void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
70 SmallPtrSet<Value*, 16> &DeadStackObjects);
73 // getAnalysisUsage - We require post dominance frontiers (aka Control
75 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
77 AU.addRequired<DominatorTree>();
78 AU.addRequired<AliasAnalysis>();
79 AU.addRequired<MemoryDependenceAnalysis>();
80 AU.addPreserved<AliasAnalysis>();
81 AU.addPreserved<DominatorTree>();
82 AU.addPreserved<MemoryDependenceAnalysis>();
88 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
89 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
90 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
91 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
92 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
94 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
96 //===----------------------------------------------------------------------===//
98 //===----------------------------------------------------------------------===//
100 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
101 /// and zero out all the operands of this instruction. If any of them become
102 /// dead, delete them and the computation tree that feeds them.
104 /// If ValueSet is non-null, remove any deleted instructions from it as well.
106 static void DeleteDeadInstruction(Instruction *I,
107 MemoryDependenceAnalysis &MD,
108 SmallPtrSet<Value*, 16> *ValueSet = 0) {
109 SmallVector<Instruction*, 32> NowDeadInsts;
111 NowDeadInsts.push_back(I);
114 // Before we touch this instruction, remove it from memdep!
116 Instruction *DeadInst = NowDeadInsts.pop_back_val();
119 // This instruction is dead, zap it, in stages. Start by removing it from
120 // MemDep, which needs to know the operands and needs it to be in the
122 MD.removeInstruction(DeadInst);
124 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
125 Value *Op = DeadInst->getOperand(op);
126 DeadInst->setOperand(op, 0);
128 // If this operand just became dead, add it to the NowDeadInsts list.
129 if (!Op->use_empty()) continue;
131 if (Instruction *OpI = dyn_cast<Instruction>(Op))
132 if (isInstructionTriviallyDead(OpI))
133 NowDeadInsts.push_back(OpI);
136 DeadInst->eraseFromParent();
138 if (ValueSet) ValueSet->erase(DeadInst);
139 } while (!NowDeadInsts.empty());
143 /// hasMemoryWrite - Does this instruction write some memory? This only returns
144 /// true for things that we can analyze with other helpers below.
145 static bool hasMemoryWrite(Instruction *I) {
146 if (isa<StoreInst>(I))
148 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
149 switch (II->getIntrinsicID()) {
152 case Intrinsic::memset:
153 case Intrinsic::memmove:
154 case Intrinsic::memcpy:
155 case Intrinsic::init_trampoline:
156 case Intrinsic::lifetime_end:
163 /// getLocForWrite - Return a Location stored to by the specified instruction.
164 static AliasAnalysis::Location
165 getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
166 if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
167 return AA.getLocation(SI);
169 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
170 // memcpy/memmove/memset.
171 AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
172 // If we don't have target data around, an unknown size in Location means
173 // that we should use the size of the pointee type. This isn't valid for
174 // memset/memcpy, which writes more than an i8.
175 if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0)
176 return AliasAnalysis::Location();
180 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
181 if (II == 0) return AliasAnalysis::Location();
183 switch (II->getIntrinsicID()) {
184 default: return AliasAnalysis::Location(); // Unhandled intrinsic.
185 case Intrinsic::init_trampoline:
186 // If we don't have target data around, an unknown size in Location means
187 // that we should use the size of the pointee type. This isn't valid for
188 // init.trampoline, which writes more than an i8.
189 if (AA.getTargetData() == 0) return AliasAnalysis::Location();
191 // FIXME: We don't know the size of the trampoline, so we can't really
193 return AliasAnalysis::Location(II->getArgOperand(0));
194 case Intrinsic::lifetime_end: {
195 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
196 return AliasAnalysis::Location(II->getArgOperand(1), Len);
201 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
202 /// instruction if any.
203 static AliasAnalysis::Location
204 getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
205 assert(hasMemoryWrite(Inst) && "Unknown instruction case");
207 // The only instructions that both read and write are the mem transfer
208 // instructions (memcpy/memmove).
209 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
210 return AA.getLocationForSource(MTI);
211 return AliasAnalysis::Location();
215 /// isRemovable - If the value of this instruction and the memory it writes to
216 /// is unused, may we delete this instruction?
217 static bool isRemovable(Instruction *I) {
218 // Don't remove volatile stores.
219 if (StoreInst *SI = dyn_cast<StoreInst>(I))
220 return !SI->isVolatile();
222 IntrinsicInst *II = cast<IntrinsicInst>(I);
223 switch (II->getIntrinsicID()) {
224 default: assert(0 && "doesn't pass 'hasMemoryWrite' predicate");
225 case Intrinsic::lifetime_end:
226 // Never remove dead lifetime_end's, e.g. because it is followed by a
229 case Intrinsic::init_trampoline:
230 // Always safe to remove init_trampoline.
233 case Intrinsic::memset:
234 case Intrinsic::memmove:
235 case Intrinsic::memcpy:
236 // Don't remove volatile memory intrinsics.
237 return !cast<MemIntrinsic>(II)->isVolatile();
241 /// getStoredPointerOperand - Return the pointer that is being written to.
242 static Value *getStoredPointerOperand(Instruction *I) {
243 if (StoreInst *SI = dyn_cast<StoreInst>(I))
244 return SI->getPointerOperand();
245 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
246 return MI->getDest();
248 IntrinsicInst *II = cast<IntrinsicInst>(I);
249 switch (II->getIntrinsicID()) {
250 default: assert(false && "Unexpected intrinsic!");
251 case Intrinsic::init_trampoline:
252 return II->getArgOperand(0);
256 static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
257 const TargetData *TD = AA.getTargetData();
259 return AliasAnalysis::UnknownSize;
261 if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
262 // Get size information for the alloca
263 if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
264 return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
265 return AliasAnalysis::UnknownSize;
268 assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
269 const PointerType *PT = cast<PointerType>(V->getType());
270 return TD->getTypeAllocSize(PT->getElementType());
273 /// isObjectPointerWithTrustworthySize - Return true if the specified Value* is
274 /// pointing to an object with a pointer size we can trust.
275 static bool isObjectPointerWithTrustworthySize(const Value *V) {
276 if (const AllocaInst *AI = dyn_cast<AllocaInst>(V))
277 return !AI->isArrayAllocation();
278 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
279 return !GV->isWeakForLinker();
280 if (const Argument *A = dyn_cast<Argument>(V))
281 return A->hasByValAttr();
285 /// isCompleteOverwrite - Return true if a store to the 'Later' location
286 /// completely overwrites a store to the 'Earlier' location.
287 static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
288 const AliasAnalysis::Location &Earlier,
290 const Value *P1 = Earlier.Ptr->stripPointerCasts();
291 const Value *P2 = Later.Ptr->stripPointerCasts();
293 // If the start pointers are the same, we just have to compare sizes to see if
294 // the later store was larger than the earlier store.
296 // If we don't know the sizes of either access, then we can't do a
298 if (Later.Size == AliasAnalysis::UnknownSize ||
299 Earlier.Size == AliasAnalysis::UnknownSize) {
300 // If we have no TargetData information around, then the size of the store
301 // is inferrable from the pointee type. If they are the same type, then
302 // we know that the store is safe.
303 if (AA.getTargetData() == 0)
304 return Later.Ptr->getType() == Earlier.Ptr->getType();
308 // Make sure that the Later size is >= the Earlier size.
309 if (Later.Size < Earlier.Size)
314 // Otherwise, we have to have size information, and the later store has to be
315 // larger than the earlier one.
316 if (Later.Size == AliasAnalysis::UnknownSize ||
317 Earlier.Size == AliasAnalysis::UnknownSize ||
318 Later.Size <= Earlier.Size || AA.getTargetData() == 0)
321 // Check to see if the later store is to the entire object (either a global,
322 // an alloca, or a byval argument). If so, then it clearly overwrites any
323 // other store to the same object.
324 const TargetData &TD = *AA.getTargetData();
326 const Value *UO1 = P1->getUnderlyingObject(), *UO2 = P2->getUnderlyingObject();
328 // If we can't resolve the same pointers to the same object, then we can't
329 // analyze them at all.
333 // If the "Later" store is to a recognizable object, get its size.
334 if (isObjectPointerWithTrustworthySize(UO2)) {
335 uint64_t ObjectSize =
336 TD.getTypeAllocSize(cast<PointerType>(UO2->getType())->getElementType());
337 if (ObjectSize == Later.Size)
341 // Okay, we have stores to two completely different pointers. Try to
342 // decompose the pointer into a "base + constant_offset" form. If the base
343 // pointers are equal, then we can reason about the two stores.
344 int64_t Off1 = 0, Off2 = 0;
345 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, Off1, TD);
346 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, Off2, TD);
348 // If the base pointers still differ, we have two completely different stores.
352 // Otherwise, we might have a situation like:
353 // store i16 -> P + 1 Byte
355 // In this case, we see if the later store completely overlaps all bytes
356 // stored by the previous store.
357 if (Off1 < Off2 || // Earlier starts before Later.
358 Off1+Earlier.Size > Off2+Later.Size) // Earlier goes beyond Later.
360 // Otherwise, we have complete overlap.
364 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
365 /// memory region into an identical pointer) then it doesn't actually make its
366 /// input dead in the traditional sense. Consider this case:
371 /// In this case, the second store to A does not make the first store to A dead.
372 /// The usual situation isn't an explicit A<-A store like this (which can be
373 /// trivially removed) but a case where two pointers may alias.
375 /// This function detects when it is unsafe to remove a dependent instruction
376 /// because the DSE inducing instruction may be a self-read.
377 static bool isPossibleSelfRead(Instruction *Inst,
378 const AliasAnalysis::Location &InstStoreLoc,
379 Instruction *DepWrite, AliasAnalysis &AA) {
380 // Self reads can only happen for instructions that read memory. Get the
382 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
383 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
385 // If the read and written loc obviously don't alias, it isn't a read.
386 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
388 // Okay, 'Inst' may copy over itself. However, we can still remove a the
389 // DepWrite instruction if we can prove that it reads from the same location
390 // as Inst. This handles useful cases like:
393 // Here we don't know if A/B may alias, but we do know that B/B are must
394 // aliases, so removing the first memcpy is safe (assuming it writes <= #
395 // bytes as the second one.
396 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
398 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
401 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
402 // then it can't be considered dead.
407 //===----------------------------------------------------------------------===//
409 //===----------------------------------------------------------------------===//
411 bool DSE::runOnBasicBlock(BasicBlock &BB) {
412 bool MadeChange = false;
414 // Do a top-down walk on the BB.
415 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
416 Instruction *Inst = BBI++;
418 // Handle 'free' calls specially.
419 if (CallInst *F = isFreeCall(Inst)) {
420 MadeChange |= HandleFree(F);
424 // If we find something that writes memory, get its memory dependence.
425 if (!hasMemoryWrite(Inst))
428 MemDepResult InstDep = MD->getDependency(Inst);
430 // Ignore non-local store liveness.
431 // FIXME: cross-block DSE would be fun. :)
432 if (InstDep.isNonLocal() ||
433 // Ignore self dependence, which happens in the entry block of the
435 InstDep.getInst() == Inst)
438 // If we're storing the same value back to a pointer that we just
439 // loaded from, then the store can be removed.
440 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
441 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
442 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
443 SI->getOperand(0) == DepLoad && !SI->isVolatile()) {
444 // DeleteDeadInstruction can delete the current instruction. Save BBI
445 // in case we need it.
446 WeakVH NextInst(BBI);
448 DeleteDeadInstruction(SI, *MD);
450 if (NextInst == 0) // Next instruction deleted.
452 else if (BBI != BB.begin()) // Revisit this instruction if possible.
461 // Figure out what location is being stored to.
462 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
464 // If we didn't get a useful location, fail.
468 while (!InstDep.isNonLocal()) {
469 // Get the memory clobbered by the instruction we depend on. MemDep will
470 // skip any instructions that 'Loc' clearly doesn't interact with. If we
471 // end up depending on a may- or must-aliased load, then we can't optimize
472 // away the store and we bail out. However, if we depend on on something
473 // that overwrites the memory location we *can* potentially optimize it.
475 // Find out what memory location the dependant instruction stores.
476 Instruction *DepWrite = InstDep.getInst();
477 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
478 // If we didn't get a useful location, or if it isn't a size, bail out.
482 // If we find a write that is a) removable (i.e., non-volatile), b) is
483 // completely obliterated by the store to 'Loc', and c) which we know that
484 // 'Inst' doesn't load from, then we can remove it.
485 if (isRemovable(DepWrite) && isCompleteOverwrite(Loc, DepLoc, *AA) &&
486 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
487 // Delete the store and now-dead instructions that feed it.
488 DeleteDeadInstruction(DepWrite, *MD);
492 // DeleteDeadInstruction can delete the current instruction in loop
495 if (BBI != BB.begin())
500 // If this is a may-aliased store that is clobbering the store value, we
501 // can keep searching past it for another must-aliased pointer that stores
502 // to the same location. For example, in:
506 // we can remove the first store to P even though we don't know if P and Q
508 if (DepWrite == &BB.front()) break;
510 // Can't look past this instruction if it might read 'Loc'.
511 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
514 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
518 // If this block ends in a return, unwind, or unreachable, all allocas are
519 // dead at its end, which means stores to them are also dead.
520 if (BB.getTerminator()->getNumSuccessors() == 0)
521 MadeChange |= handleEndBlock(BB);
526 /// HandleFree - Handle frees of entire structures whose dependency is a store
527 /// to a field of that structure.
528 bool DSE::HandleFree(CallInst *F) {
529 MemDepResult Dep = MD->getDependency(F);
531 if (Dep.isNonLocal()) return false;
533 Instruction *Dependency = Dep.getInst();
534 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
538 getStoredPointerOperand(Dependency)->getUnderlyingObject();
540 // Check for aliasing.
541 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
544 // DCE instructions only used to calculate that store
545 DeleteDeadInstruction(Dependency, *MD);
548 // Inst's old Dependency is now deleted. Compute the next dependency,
549 // which may also be dead, as in
551 // s[1] = 0; // This has just been deleted.
553 Dep = MD->getDependency(F);
554 } while (!Dep.isNonLocal());
559 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
560 /// function end block. Ex:
563 /// store i32 1, i32* %A
565 bool DSE::handleEndBlock(BasicBlock &BB) {
566 bool MadeChange = false;
568 // Keep track of all of the stack objects that are dead at the end of the
570 SmallPtrSet<Value*, 16> DeadStackObjects;
572 // Find all of the alloca'd pointers in the entry block.
573 BasicBlock *Entry = BB.getParent()->begin();
574 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
575 if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
576 DeadStackObjects.insert(AI);
578 // Treat byval arguments the same, stores to them are dead at the end of the
580 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
581 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
582 if (AI->hasByValAttr())
583 DeadStackObjects.insert(AI);
585 // Scan the basic block backwards
586 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
589 // If we find a store, check to see if it points into a dead stack value.
590 if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
591 // See through pointer-to-pointer bitcasts
592 Value *Pointer = getStoredPointerOperand(BBI)->getUnderlyingObject();
594 // Stores to stack values are valid candidates for removal.
595 if (DeadStackObjects.count(Pointer)) {
596 // DCE instructions only used to calculate that store.
597 Instruction *Dead = BBI++;
598 DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
605 // Remove any dead non-memory-mutating instructions.
606 if (isInstructionTriviallyDead(BBI)) {
607 Instruction *Inst = BBI++;
608 DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
614 if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
615 DeadStackObjects.erase(A);
619 if (CallSite CS = cast<Value>(BBI)) {
620 // If this call does not access memory, it can't be loading any of our
622 if (AA->doesNotAccessMemory(CS))
625 unsigned NumModRef = 0, NumOther = 0;
627 // If the call might load from any of our allocas, then any store above
629 SmallVector<Value*, 8> LiveAllocas;
630 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
631 E = DeadStackObjects.end(); I != E; ++I) {
632 // If we detect that our AA is imprecise, it's not worth it to scan the
633 // rest of the DeadPointers set. Just assume that the AA will return
634 // ModRef for everything, and go ahead and bail out.
635 if (NumModRef >= 16 && NumOther == 0)
638 // See if the call site touches it.
639 AliasAnalysis::ModRefResult A =
640 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
642 if (A == AliasAnalysis::ModRef)
647 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
648 LiveAllocas.push_back(*I);
651 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
652 E = LiveAllocas.end(); I != E; ++I)
653 DeadStackObjects.erase(*I);
655 // If all of the allocas were clobbered by the call then we're not going
656 // to find anything else to process.
657 if (DeadStackObjects.empty())
663 AliasAnalysis::Location LoadedLoc;
665 // If we encounter a use of the pointer, it is no longer considered dead
666 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
667 LoadedLoc = AA->getLocation(L);
668 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
669 LoadedLoc = AA->getLocation(V);
670 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
671 LoadedLoc = AA->getLocationForSource(MTI);
673 // Not a loading instruction.
677 // Remove any allocas from the DeadPointer set that are loaded, as this
678 // makes any stores above the access live.
679 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
681 // If all of the allocas were clobbered by the access then we're not going
682 // to find anything else to process.
683 if (DeadStackObjects.empty())
690 /// RemoveAccessedObjects - Check to see if the specified location may alias any
691 /// of the stack objects in the DeadStackObjects set. If so, they become live
692 /// because the location is being loaded.
693 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
694 SmallPtrSet<Value*, 16> &DeadStackObjects) {
695 const Value *UnderlyingPointer = LoadedLoc.Ptr->getUnderlyingObject();
697 // A constant can't be in the dead pointer set.
698 if (isa<Constant>(UnderlyingPointer))
701 // If the kill pointer can be easily reduced to an alloca, don't bother doing
702 // extraneous AA queries.
703 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
704 DeadStackObjects.erase(const_cast<Value*>(UnderlyingPointer));
708 SmallVector<Value*, 16> NowLive;
709 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
710 E = DeadStackObjects.end(); I != E; ++I) {
711 // See if the loaded location could alias the stack location.
712 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
713 if (!AA->isNoAlias(StackLoc, LoadedLoc))
714 NowLive.push_back(*I);
717 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
719 DeadStackObjects.erase(*I);