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/SmallPtrSet.h"
36 #include "llvm/ADT/Statistic.h"
39 STATISTIC(NumFastStores, "Number of stores deleted");
40 STATISTIC(NumFastOther , "Number of other instrs removed");
43 struct DSE : public FunctionPass {
45 MemoryDependenceAnalysis *MD;
47 static char ID; // Pass identification, replacement for typeid
48 DSE() : FunctionPass(ID), AA(0), MD(0) {
49 initializeDSEPass(*PassRegistry::getPassRegistry());
52 virtual bool runOnFunction(Function &F) {
53 AA = &getAnalysis<AliasAnalysis>();
54 MD = &getAnalysis<MemoryDependenceAnalysis>();
55 DominatorTree &DT = getAnalysis<DominatorTree>();
58 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
59 // Only check non-dead blocks. Dead blocks may have strange pointer
60 // cycles that will confuse alias analysis.
61 if (DT.isReachableFromEntry(I))
62 Changed |= runOnBasicBlock(*I);
68 bool runOnBasicBlock(BasicBlock &BB);
69 bool HandleFree(CallInst *F);
70 bool handleEndBlock(BasicBlock &BB);
71 void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
72 SmallPtrSet<Value*, 16> &DeadStackObjects);
74 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
76 AU.addRequired<DominatorTree>();
77 AU.addRequired<AliasAnalysis>();
78 AU.addRequired<MemoryDependenceAnalysis>();
79 AU.addPreserved<AliasAnalysis>();
80 AU.addPreserved<DominatorTree>();
81 AU.addPreserved<MemoryDependenceAnalysis>();
87 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
88 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
89 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
90 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
91 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
93 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
95 //===----------------------------------------------------------------------===//
97 //===----------------------------------------------------------------------===//
99 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
100 /// and zero out all the operands of this instruction. If any of them become
101 /// dead, delete them and the computation tree that feeds them.
103 /// If ValueSet is non-null, remove any deleted instructions from it as well.
105 static void DeleteDeadInstruction(Instruction *I,
106 MemoryDependenceAnalysis &MD,
107 SmallPtrSet<Value*, 16> *ValueSet = 0) {
108 SmallVector<Instruction*, 32> NowDeadInsts;
110 NowDeadInsts.push_back(I);
113 // Before we touch this instruction, remove it from memdep!
115 Instruction *DeadInst = NowDeadInsts.pop_back_val();
118 // This instruction is dead, zap it, in stages. Start by removing it from
119 // MemDep, which needs to know the operands and needs it to be in the
121 MD.removeInstruction(DeadInst);
123 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
124 Value *Op = DeadInst->getOperand(op);
125 DeadInst->setOperand(op, 0);
127 // If this operand just became dead, add it to the NowDeadInsts list.
128 if (!Op->use_empty()) continue;
130 if (Instruction *OpI = dyn_cast<Instruction>(Op))
131 if (isInstructionTriviallyDead(OpI))
132 NowDeadInsts.push_back(OpI);
135 DeadInst->eraseFromParent();
137 if (ValueSet) ValueSet->erase(DeadInst);
138 } while (!NowDeadInsts.empty());
142 /// hasMemoryWrite - Does this instruction write some memory? This only returns
143 /// true for things that we can analyze with other helpers below.
144 static bool hasMemoryWrite(Instruction *I) {
145 if (isa<StoreInst>(I))
147 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
148 switch (II->getIntrinsicID()) {
151 case Intrinsic::memset:
152 case Intrinsic::memmove:
153 case Intrinsic::memcpy:
154 case Intrinsic::init_trampoline:
155 case Intrinsic::lifetime_end:
162 /// getLocForWrite - Return a Location stored to by the specified instruction.
163 /// If isRemovable returns true, this function and getLocForRead completely
164 /// describe the memory operations for this instruction.
165 static AliasAnalysis::Location
166 getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
167 if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
168 return AA.getLocation(SI);
170 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
171 // memcpy/memmove/memset.
172 AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
173 // If we don't have target data around, an unknown size in Location means
174 // that we should use the size of the pointee type. This isn't valid for
175 // memset/memcpy, which writes more than an i8.
176 if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0)
177 return AliasAnalysis::Location();
181 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
182 if (II == 0) return AliasAnalysis::Location();
184 switch (II->getIntrinsicID()) {
185 default: return AliasAnalysis::Location(); // Unhandled intrinsic.
186 case Intrinsic::init_trampoline:
187 // If we don't have target data around, an unknown size in Location means
188 // that we should use the size of the pointee type. This isn't valid for
189 // init.trampoline, which writes more than an i8.
190 if (AA.getTargetData() == 0) return AliasAnalysis::Location();
192 // FIXME: We don't know the size of the trampoline, so we can't really
194 return AliasAnalysis::Location(II->getArgOperand(0));
195 case Intrinsic::lifetime_end: {
196 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
197 return AliasAnalysis::Location(II->getArgOperand(1), Len);
202 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
203 /// instruction if any.
204 static AliasAnalysis::Location
205 getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
206 assert(hasMemoryWrite(Inst) && "Unknown instruction case");
208 // The only instructions that both read and write are the mem transfer
209 // instructions (memcpy/memmove).
210 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
211 return AA.getLocationForSource(MTI);
212 return AliasAnalysis::Location();
216 /// isRemovable - If the value of this instruction and the memory it writes to
217 /// is unused, may we delete this instruction?
218 static bool isRemovable(Instruction *I) {
219 // Don't remove volatile/atomic stores.
220 if (StoreInst *SI = dyn_cast<StoreInst>(I))
221 return SI->isUnordered();
223 IntrinsicInst *II = cast<IntrinsicInst>(I);
224 switch (II->getIntrinsicID()) {
225 default: assert(0 && "doesn't pass 'hasMemoryWrite' predicate");
226 case Intrinsic::lifetime_end:
227 // Never remove dead lifetime_end's, e.g. because it is followed by a
230 case Intrinsic::init_trampoline:
231 // Always safe to remove init_trampoline.
234 case Intrinsic::memset:
235 case Intrinsic::memmove:
236 case Intrinsic::memcpy:
237 // Don't remove volatile memory intrinsics.
238 return !cast<MemIntrinsic>(II)->isVolatile();
242 /// getStoredPointerOperand - Return the pointer that is being written to.
243 static Value *getStoredPointerOperand(Instruction *I) {
244 if (StoreInst *SI = dyn_cast<StoreInst>(I))
245 return SI->getPointerOperand();
246 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
247 return MI->getDest();
249 IntrinsicInst *II = cast<IntrinsicInst>(I);
250 switch (II->getIntrinsicID()) {
251 default: assert(false && "Unexpected intrinsic!");
252 case Intrinsic::init_trampoline:
253 return II->getArgOperand(0);
257 static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
258 const TargetData *TD = AA.getTargetData();
260 if (CallInst *CI = dyn_cast<CallInst>(V)) {
261 assert(isMalloc(CI) && "Expected Malloc call!");
262 if (ConstantInt *C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
263 return C->getZExtValue();
264 return AliasAnalysis::UnknownSize;
268 return AliasAnalysis::UnknownSize;
270 if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
271 // Get size information for the alloca
272 if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
273 return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
274 return AliasAnalysis::UnknownSize;
277 assert(isa<Argument>(V) && "Expected AllocaInst, malloc call or Argument!");
278 PointerType *PT = cast<PointerType>(V->getType());
279 return TD->getTypeAllocSize(PT->getElementType());
282 /// isObjectPointerWithTrustworthySize - Return true if the specified Value* is
283 /// pointing to an object with a pointer size we can trust.
284 static bool isObjectPointerWithTrustworthySize(const Value *V) {
285 if (const AllocaInst *AI = dyn_cast<AllocaInst>(V))
286 return !AI->isArrayAllocation();
287 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
288 return !GV->mayBeOverridden();
289 if (const Argument *A = dyn_cast<Argument>(V))
290 return A->hasByValAttr();
296 /// isCompleteOverwrite - Return true if a store to the 'Later' location
297 /// completely overwrites a store to the 'Earlier' location.
298 static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
299 const AliasAnalysis::Location &Earlier,
301 const Value *P1 = Earlier.Ptr->stripPointerCasts();
302 const Value *P2 = Later.Ptr->stripPointerCasts();
304 // If the start pointers are the same, we just have to compare sizes to see if
305 // the later store was larger than the earlier store.
307 // If we don't know the sizes of either access, then we can't do a
309 if (Later.Size == AliasAnalysis::UnknownSize ||
310 Earlier.Size == AliasAnalysis::UnknownSize) {
311 // If we have no TargetData information around, then the size of the store
312 // is inferrable from the pointee type. If they are the same type, then
313 // we know that the store is safe.
314 if (AA.getTargetData() == 0)
315 return Later.Ptr->getType() == Earlier.Ptr->getType();
319 // Make sure that the Later size is >= the Earlier size.
320 if (Later.Size < Earlier.Size)
325 // Otherwise, we have to have size information, and the later store has to be
326 // larger than the earlier one.
327 if (Later.Size == AliasAnalysis::UnknownSize ||
328 Earlier.Size == AliasAnalysis::UnknownSize ||
329 Later.Size <= Earlier.Size || AA.getTargetData() == 0)
332 // Check to see if the later store is to the entire object (either a global,
333 // an alloca, or a byval argument). If so, then it clearly overwrites any
334 // other store to the same object.
335 const TargetData &TD = *AA.getTargetData();
337 const Value *UO1 = GetUnderlyingObject(P1, &TD),
338 *UO2 = GetUnderlyingObject(P2, &TD);
340 // If we can't resolve the same pointers to the same object, then we can't
341 // analyze them at all.
345 // If the "Later" store is to a recognizable object, get its size.
346 if (isObjectPointerWithTrustworthySize(UO2)) {
347 uint64_t ObjectSize =
348 TD.getTypeAllocSize(cast<PointerType>(UO2->getType())->getElementType());
349 if (ObjectSize == Later.Size)
353 // Okay, we have stores to two completely different pointers. Try to
354 // decompose the pointer into a "base + constant_offset" form. If the base
355 // pointers are equal, then we can reason about the two stores.
356 int64_t EarlierOff = 0, LaterOff = 0;
357 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
358 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
360 // If the base pointers still differ, we have two completely different stores.
364 // The later store completely overlaps the earlier store if:
366 // 1. Both start at the same offset and the later one's size is greater than
367 // or equal to the earlier one's, or
372 // 2. The earlier store has an offset greater than the later offset, but which
373 // still lies completely within the later store.
376 // |----- later ------|
378 // We have to be careful here as *Off is signed while *.Size is unsigned.
379 if (EarlierOff >= LaterOff &&
380 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
383 // Otherwise, they don't completely overlap.
387 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
388 /// memory region into an identical pointer) then it doesn't actually make its
389 /// input dead in the traditional sense. Consider this case:
394 /// In this case, the second store to A does not make the first store to A dead.
395 /// The usual situation isn't an explicit A<-A store like this (which can be
396 /// trivially removed) but a case where two pointers may alias.
398 /// This function detects when it is unsafe to remove a dependent instruction
399 /// because the DSE inducing instruction may be a self-read.
400 static bool isPossibleSelfRead(Instruction *Inst,
401 const AliasAnalysis::Location &InstStoreLoc,
402 Instruction *DepWrite, AliasAnalysis &AA) {
403 // Self reads can only happen for instructions that read memory. Get the
405 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
406 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
408 // If the read and written loc obviously don't alias, it isn't a read.
409 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
411 // Okay, 'Inst' may copy over itself. However, we can still remove a the
412 // DepWrite instruction if we can prove that it reads from the same location
413 // as Inst. This handles useful cases like:
416 // Here we don't know if A/B may alias, but we do know that B/B are must
417 // aliases, so removing the first memcpy is safe (assuming it writes <= #
418 // bytes as the second one.
419 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
421 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
424 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
425 // then it can't be considered dead.
430 //===----------------------------------------------------------------------===//
432 //===----------------------------------------------------------------------===//
434 bool DSE::runOnBasicBlock(BasicBlock &BB) {
435 bool MadeChange = false;
437 // Do a top-down walk on the BB.
438 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
439 Instruction *Inst = BBI++;
441 // Handle 'free' calls specially.
442 if (CallInst *F = isFreeCall(Inst)) {
443 MadeChange |= HandleFree(F);
447 // If we find something that writes memory, get its memory dependence.
448 if (!hasMemoryWrite(Inst))
451 MemDepResult InstDep = MD->getDependency(Inst);
453 // Ignore any store where we can't find a local dependence.
454 // FIXME: cross-block DSE would be fun. :)
455 if (!InstDep.isDef() && !InstDep.isClobber())
458 // If we're storing the same value back to a pointer that we just
459 // loaded from, then the store can be removed.
460 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
461 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
462 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
463 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
464 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
465 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
467 // DeleteDeadInstruction can delete the current instruction. Save BBI
468 // in case we need it.
469 WeakVH NextInst(BBI);
471 DeleteDeadInstruction(SI, *MD);
473 if (NextInst == 0) // Next instruction deleted.
475 else if (BBI != BB.begin()) // Revisit this instruction if possible.
484 // Figure out what location is being stored to.
485 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
487 // If we didn't get a useful location, fail.
491 while (InstDep.isDef() || InstDep.isClobber()) {
492 // Get the memory clobbered by the instruction we depend on. MemDep will
493 // skip any instructions that 'Loc' clearly doesn't interact with. If we
494 // end up depending on a may- or must-aliased load, then we can't optimize
495 // away the store and we bail out. However, if we depend on on something
496 // that overwrites the memory location we *can* potentially optimize it.
498 // Find out what memory location the dependent instruction stores.
499 Instruction *DepWrite = InstDep.getInst();
500 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
501 // If we didn't get a useful location, or if it isn't a size, bail out.
505 // If we find a write that is a) removable (i.e., non-volatile), b) is
506 // completely obliterated by the store to 'Loc', and c) which we know that
507 // 'Inst' doesn't load from, then we can remove it.
508 if (isRemovable(DepWrite) && isCompleteOverwrite(Loc, DepLoc, *AA) &&
509 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
510 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
511 << *DepWrite << "\n KILLER: " << *Inst << '\n');
513 // Delete the store and now-dead instructions that feed it.
514 DeleteDeadInstruction(DepWrite, *MD);
518 // DeleteDeadInstruction can delete the current instruction in loop
521 if (BBI != BB.begin())
526 // If this is a may-aliased store that is clobbering the store value, we
527 // can keep searching past it for another must-aliased pointer that stores
528 // to the same location. For example, in:
532 // we can remove the first store to P even though we don't know if P and Q
534 if (DepWrite == &BB.front()) break;
536 // Can't look past this instruction if it might read 'Loc'.
537 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
540 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
544 // If this block ends in a return, unwind, or unreachable, all allocas are
545 // dead at its end, which means stores to them are also dead.
546 if (BB.getTerminator()->getNumSuccessors() == 0)
547 MadeChange |= handleEndBlock(BB);
552 /// HandleFree - Handle frees of entire structures whose dependency is a store
553 /// to a field of that structure.
554 bool DSE::HandleFree(CallInst *F) {
555 bool MadeChange = false;
557 MemDepResult Dep = MD->getDependency(F);
559 while (Dep.isDef() || Dep.isClobber()) {
560 Instruction *Dependency = Dep.getInst();
561 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
565 GetUnderlyingObject(getStoredPointerOperand(Dependency));
567 // Check for aliasing.
568 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
571 // DCE instructions only used to calculate that store
572 DeleteDeadInstruction(Dependency, *MD);
576 // Inst's old Dependency is now deleted. Compute the next dependency,
577 // which may also be dead, as in
579 // s[1] = 0; // This has just been deleted.
581 Dep = MD->getDependency(F);
587 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
588 /// function end block. Ex:
591 /// store i32 1, i32* %A
593 bool DSE::handleEndBlock(BasicBlock &BB) {
594 bool MadeChange = false;
596 // Keep track of all of the stack objects that are dead at the end of the
598 SmallPtrSet<Value*, 16> DeadStackObjects;
600 // Find all of the alloca'd pointers in the entry block.
601 BasicBlock *Entry = BB.getParent()->begin();
602 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
603 if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
604 DeadStackObjects.insert(AI);
606 // Okay, so these are dead heap objects, but if the pointer never escapes
607 // then it's leaked by this function anyways.
608 if (CallInst *CI = extractMallocCall(I))
609 if (!PointerMayBeCaptured(CI, true, true))
610 DeadStackObjects.insert(CI);
613 // Treat byval arguments the same, stores to them are dead at the end of the
615 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
616 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
617 if (AI->hasByValAttr())
618 DeadStackObjects.insert(AI);
620 // Scan the basic block backwards
621 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
624 // If we find a store, check to see if it points into a dead stack value.
625 if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
626 // See through pointer-to-pointer bitcasts
627 Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));
629 // Stores to stack values are valid candidates for removal.
630 if (DeadStackObjects.count(Pointer)) {
631 Instruction *Dead = BBI++;
633 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
634 << *Dead << "\n Object: " << *Pointer << '\n');
636 // DCE instructions only used to calculate that store.
637 DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
644 // Remove any dead non-memory-mutating instructions.
645 if (isInstructionTriviallyDead(BBI)) {
646 Instruction *Inst = BBI++;
647 DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
653 if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
654 DeadStackObjects.erase(A);
658 if (CallInst *CI = extractMallocCall(BBI)) {
659 DeadStackObjects.erase(CI);
663 if (CallSite CS = cast<Value>(BBI)) {
664 // If this call does not access memory, it can't be loading any of our
666 if (AA->doesNotAccessMemory(CS))
669 // If the call might load from any of our allocas, then any store above
671 SmallVector<Value*, 8> LiveAllocas;
672 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
673 E = DeadStackObjects.end(); I != E; ++I) {
674 // See if the call site touches it.
675 AliasAnalysis::ModRefResult A =
676 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
678 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
679 LiveAllocas.push_back(*I);
682 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
683 E = LiveAllocas.end(); I != E; ++I)
684 DeadStackObjects.erase(*I);
686 // If all of the allocas were clobbered by the call then we're not going
687 // to find anything else to process.
688 if (DeadStackObjects.empty())
694 AliasAnalysis::Location LoadedLoc;
696 // If we encounter a use of the pointer, it is no longer considered dead
697 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
698 if (!L->isUnordered()) // Be conservative with atomic/volatile load
700 LoadedLoc = AA->getLocation(L);
701 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
702 LoadedLoc = AA->getLocation(V);
703 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
704 LoadedLoc = AA->getLocationForSource(MTI);
705 } else if (!BBI->mayReadFromMemory()) {
706 // Instruction doesn't read memory. Note that stores that weren't removed
707 // above will hit this case.
710 // Unknown inst; assume it clobbers everything.
714 // Remove any allocas from the DeadPointer set that are loaded, as this
715 // makes any stores above the access live.
716 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
718 // If all of the allocas were clobbered by the access then we're not going
719 // to find anything else to process.
720 if (DeadStackObjects.empty())
727 /// RemoveAccessedObjects - Check to see if the specified location may alias any
728 /// of the stack objects in the DeadStackObjects set. If so, they become live
729 /// because the location is being loaded.
730 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
731 SmallPtrSet<Value*, 16> &DeadStackObjects) {
732 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
734 // A constant can't be in the dead pointer set.
735 if (isa<Constant>(UnderlyingPointer))
738 // If the kill pointer can be easily reduced to an alloca, don't bother doing
739 // extraneous AA queries.
740 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
741 DeadStackObjects.erase(const_cast<Value*>(UnderlyingPointer));
745 SmallVector<Value*, 16> NowLive;
746 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
747 E = DeadStackObjects.end(); I != E; ++I) {
748 // See if the loaded location could alias the stack location.
749 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
750 if (!AA->isNoAlias(StackLoc, LoadedLoc))
751 NowLive.push_back(*I);
754 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
756 DeadStackObjects.erase(*I);