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();
243 /// isShortenable - Returns true if this instruction can be safely shortened in
245 static bool isShortenable(Instruction *I) {
246 // Don't shorten stores for now
247 if (isa<StoreInst>(I))
250 IntrinsicInst *II = cast<IntrinsicInst>(I);
251 switch (II->getIntrinsicID()) {
252 default: return false;
253 case Intrinsic::memset:
254 case Intrinsic::memcpy:
255 // Do shorten memory intrinsics.
260 /// getStoredPointerOperand - Return the pointer that is being written to.
261 static Value *getStoredPointerOperand(Instruction *I) {
262 if (StoreInst *SI = dyn_cast<StoreInst>(I))
263 return SI->getPointerOperand();
264 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
265 return MI->getDest();
267 IntrinsicInst *II = cast<IntrinsicInst>(I);
268 switch (II->getIntrinsicID()) {
269 default: assert(false && "Unexpected intrinsic!");
270 case Intrinsic::init_trampoline:
271 return II->getArgOperand(0);
275 static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
276 const TargetData *TD = AA.getTargetData();
278 if (CallInst *CI = dyn_cast<CallInst>(V)) {
279 assert(isMalloc(CI) && "Expected Malloc call!");
280 if (ConstantInt *C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
281 return C->getZExtValue();
282 return AliasAnalysis::UnknownSize;
286 return AliasAnalysis::UnknownSize;
288 if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
289 // Get size information for the alloca
290 if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
291 return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
292 return AliasAnalysis::UnknownSize;
295 assert(isa<Argument>(V) && "Expected AllocaInst, malloc call or Argument!");
296 PointerType *PT = cast<PointerType>(V->getType());
297 return TD->getTypeAllocSize(PT->getElementType());
300 /// isObjectPointerWithTrustworthySize - Return true if the specified Value* is
301 /// pointing to an object with a pointer size we can trust.
302 static bool isObjectPointerWithTrustworthySize(const Value *V) {
303 if (const AllocaInst *AI = dyn_cast<AllocaInst>(V))
304 return !AI->isArrayAllocation();
305 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
306 return !GV->mayBeOverridden();
307 if (const Argument *A = dyn_cast<Argument>(V))
308 return A->hasByValAttr();
323 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
324 /// completely overwrites a store to the 'Earlier' location.
325 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
326 /// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
327 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
328 const AliasAnalysis::Location &Earlier,
332 const Value *P1 = Earlier.Ptr->stripPointerCasts();
333 const Value *P2 = Later.Ptr->stripPointerCasts();
335 // If the start pointers are the same, we just have to compare sizes to see if
336 // the later store was larger than the earlier store.
338 // If we don't know the sizes of either access, then we can't do a
340 if (Later.Size == AliasAnalysis::UnknownSize ||
341 Earlier.Size == AliasAnalysis::UnknownSize) {
342 // If we have no TargetData information around, then the size of the store
343 // is inferrable from the pointee type. If they are the same type, then
344 // we know that the store is safe.
345 if (AA.getTargetData() == 0 &&
346 Later.Ptr->getType() == Earlier.Ptr->getType())
347 return OverwriteComplete;
349 return OverwriteUnknown;
352 // Make sure that the Later size is >= the Earlier size.
353 if (Later.Size >= Earlier.Size)
354 return OverwriteComplete;
357 // Otherwise, we have to have size information, and the later store has to be
358 // larger than the earlier one.
359 if (Later.Size == AliasAnalysis::UnknownSize ||
360 Earlier.Size == AliasAnalysis::UnknownSize ||
361 AA.getTargetData() == 0)
362 return OverwriteUnknown;
364 // Check to see if the later store is to the entire object (either a global,
365 // an alloca, or a byval argument). If so, then it clearly overwrites any
366 // other store to the same object.
367 const TargetData &TD = *AA.getTargetData();
369 const Value *UO1 = GetUnderlyingObject(P1, &TD),
370 *UO2 = GetUnderlyingObject(P2, &TD);
372 // If we can't resolve the same pointers to the same object, then we can't
373 // analyze them at all.
375 return OverwriteUnknown;
377 // If the "Later" store is to a recognizable object, get its size.
378 if (isObjectPointerWithTrustworthySize(UO2)) {
379 uint64_t ObjectSize =
380 TD.getTypeAllocSize(cast<PointerType>(UO2->getType())->getElementType());
381 if (ObjectSize == Later.Size)
382 return OverwriteComplete;
385 // Okay, we have stores to two completely different pointers. Try to
386 // decompose the pointer into a "base + constant_offset" form. If the base
387 // pointers are equal, then we can reason about the two stores.
390 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
391 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
393 // If the base pointers still differ, we have two completely different stores.
395 return OverwriteUnknown;
397 // The later store completely overlaps the earlier store if:
399 // 1. Both start at the same offset and the later one's size is greater than
400 // or equal to the earlier one's, or
405 // 2. The earlier store has an offset greater than the later offset, but which
406 // still lies completely within the later store.
409 // |----- later ------|
411 // We have to be careful here as *Off is signed while *.Size is unsigned.
412 if (EarlierOff >= LaterOff &&
413 Later.Size > Earlier.Size &&
414 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
415 return OverwriteComplete;
417 // The other interesting case is if the later store overwrites the end of
423 // In this case we may want to trim the size of earlier to avoid generating
424 // writes to addresses which will definitely be overwritten later
425 if (LaterOff > EarlierOff &&
426 LaterOff + Later.Size >= EarlierOff + Earlier.Size)
429 // Otherwise, they don't completely overlap.
430 return OverwriteUnknown;
433 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
434 /// memory region into an identical pointer) then it doesn't actually make its
435 /// input dead in the traditional sense. Consider this case:
440 /// In this case, the second store to A does not make the first store to A dead.
441 /// The usual situation isn't an explicit A<-A store like this (which can be
442 /// trivially removed) but a case where two pointers may alias.
444 /// This function detects when it is unsafe to remove a dependent instruction
445 /// because the DSE inducing instruction may be a self-read.
446 static bool isPossibleSelfRead(Instruction *Inst,
447 const AliasAnalysis::Location &InstStoreLoc,
448 Instruction *DepWrite, AliasAnalysis &AA) {
449 // Self reads can only happen for instructions that read memory. Get the
451 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
452 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
454 // If the read and written loc obviously don't alias, it isn't a read.
455 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
457 // Okay, 'Inst' may copy over itself. However, we can still remove a the
458 // DepWrite instruction if we can prove that it reads from the same location
459 // as Inst. This handles useful cases like:
462 // Here we don't know if A/B may alias, but we do know that B/B are must
463 // aliases, so removing the first memcpy is safe (assuming it writes <= #
464 // bytes as the second one.
465 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
467 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
470 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
471 // then it can't be considered dead.
476 //===----------------------------------------------------------------------===//
478 //===----------------------------------------------------------------------===//
480 bool DSE::runOnBasicBlock(BasicBlock &BB) {
481 bool MadeChange = false;
483 // Do a top-down walk on the BB.
484 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
485 Instruction *Inst = BBI++;
487 // Handle 'free' calls specially.
488 if (CallInst *F = isFreeCall(Inst)) {
489 MadeChange |= HandleFree(F);
493 // If we find something that writes memory, get its memory dependence.
494 if (!hasMemoryWrite(Inst))
497 MemDepResult InstDep = MD->getDependency(Inst);
499 // Ignore any store where we can't find a local dependence.
500 // FIXME: cross-block DSE would be fun. :)
501 if (!InstDep.isDef() && !InstDep.isClobber())
504 // If we're storing the same value back to a pointer that we just
505 // loaded from, then the store can be removed.
506 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
507 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
508 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
509 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
510 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
511 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
513 // DeleteDeadInstruction can delete the current instruction. Save BBI
514 // in case we need it.
515 WeakVH NextInst(BBI);
517 DeleteDeadInstruction(SI, *MD);
519 if (NextInst == 0) // Next instruction deleted.
521 else if (BBI != BB.begin()) // Revisit this instruction if possible.
530 // Figure out what location is being stored to.
531 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
533 // If we didn't get a useful location, fail.
537 while (InstDep.isDef() || InstDep.isClobber()) {
538 // Get the memory clobbered by the instruction we depend on. MemDep will
539 // skip any instructions that 'Loc' clearly doesn't interact with. If we
540 // end up depending on a may- or must-aliased load, then we can't optimize
541 // away the store and we bail out. However, if we depend on on something
542 // that overwrites the memory location we *can* potentially optimize it.
544 // Find out what memory location the dependent instruction stores.
545 Instruction *DepWrite = InstDep.getInst();
546 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
547 // If we didn't get a useful location, or if it isn't a size, bail out.
551 // If we find a write that is a) removable (i.e., non-volatile), b) is
552 // completely obliterated by the store to 'Loc', and c) which we know that
553 // 'Inst' doesn't load from, then we can remove it.
554 if (isRemovable(DepWrite) &&
555 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
556 int64_t InstWriteOffset, DepWriteOffset;
557 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
558 DepWriteOffset, InstWriteOffset);
559 if (OR == OverwriteComplete) {
560 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
561 << *DepWrite << "\n KILLER: " << *Inst << '\n');
563 // Delete the store and now-dead instructions that feed it.
564 DeleteDeadInstruction(DepWrite, *MD);
568 // DeleteDeadInstruction can delete the current instruction in loop
571 if (BBI != BB.begin())
574 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
575 // TODO: base this on the target vector size so that if the earlier
576 // store was too small to get vector writes anyway then its likely
577 // a good idea to shorten it
578 // Power of 2 vector writes are probably always a bad idea to optimize
579 // as any store/memset/memcpy is likely using vector instructions so
580 // shortening it to not vector size is likely to be slower
581 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
582 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
583 if (llvm::isPowerOf2_64(InstWriteOffset) ||
584 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
586 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
587 << *DepWrite << "\n KILLER (offset "
588 << InstWriteOffset << ", "
589 << DepLoc.Size << ")"
592 Value* DepWriteLength = DepIntrinsic->getLength();
593 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
596 DepIntrinsic->setLength(TrimmedLength);
602 // If this is a may-aliased store that is clobbering the store value, we
603 // can keep searching past it for another must-aliased pointer that stores
604 // to the same location. For example, in:
608 // we can remove the first store to P even though we don't know if P and Q
610 if (DepWrite == &BB.front()) break;
612 // Can't look past this instruction if it might read 'Loc'.
613 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
616 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
620 // If this block ends in a return, unwind, or unreachable, all allocas are
621 // dead at its end, which means stores to them are also dead.
622 if (BB.getTerminator()->getNumSuccessors() == 0)
623 MadeChange |= handleEndBlock(BB);
628 /// HandleFree - Handle frees of entire structures whose dependency is a store
629 /// to a field of that structure.
630 bool DSE::HandleFree(CallInst *F) {
631 bool MadeChange = false;
633 MemDepResult Dep = MD->getDependency(F);
635 while (Dep.isDef() || Dep.isClobber()) {
636 Instruction *Dependency = Dep.getInst();
637 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
641 GetUnderlyingObject(getStoredPointerOperand(Dependency));
643 // Check for aliasing.
644 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
647 // DCE instructions only used to calculate that store
648 DeleteDeadInstruction(Dependency, *MD);
652 // Inst's old Dependency is now deleted. Compute the next dependency,
653 // which may also be dead, as in
655 // s[1] = 0; // This has just been deleted.
657 Dep = MD->getDependency(F);
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 SmallPtrSet<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 (AllocaInst *AI = dyn_cast<AllocaInst>(I))
680 DeadStackObjects.insert(AI);
682 // Okay, so these are dead heap objects, but if the pointer never escapes
683 // then it's leaked by this function anyways.
684 if (CallInst *CI = extractMallocCall(I))
685 if (!PointerMayBeCaptured(CI, true, true))
686 DeadStackObjects.insert(CI);
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 Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));
705 // Stores to stack values are valid candidates for removal.
706 if (DeadStackObjects.count(Pointer)) {
707 Instruction *Dead = BBI++;
709 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
710 << *Dead << "\n Object: " << *Pointer << '\n');
712 // DCE instructions only used to calculate that store.
713 DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
720 // Remove any dead non-memory-mutating instructions.
721 if (isInstructionTriviallyDead(BBI)) {
722 Instruction *Inst = BBI++;
723 DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
729 if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
730 DeadStackObjects.erase(A);
734 if (CallInst *CI = extractMallocCall(BBI)) {
735 DeadStackObjects.erase(CI);
739 if (CallSite CS = cast<Value>(BBI)) {
740 // If this call does not access memory, it can't be loading any of our
742 if (AA->doesNotAccessMemory(CS))
745 // If the call might load from any of our allocas, then any store above
747 SmallVector<Value*, 8> LiveAllocas;
748 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
749 E = DeadStackObjects.end(); I != E; ++I) {
750 // See if the call site touches it.
751 AliasAnalysis::ModRefResult A =
752 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
754 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
755 LiveAllocas.push_back(*I);
758 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
759 E = LiveAllocas.end(); I != E; ++I)
760 DeadStackObjects.erase(*I);
762 // If all of the allocas were clobbered by the call then we're not going
763 // to find anything else to process.
764 if (DeadStackObjects.empty())
770 AliasAnalysis::Location LoadedLoc;
772 // If we encounter a use of the pointer, it is no longer considered dead
773 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
774 if (!L->isUnordered()) // Be conservative with atomic/volatile load
776 LoadedLoc = AA->getLocation(L);
777 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
778 LoadedLoc = AA->getLocation(V);
779 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
780 LoadedLoc = AA->getLocationForSource(MTI);
781 } else if (!BBI->mayReadFromMemory()) {
782 // Instruction doesn't read memory. Note that stores that weren't removed
783 // above will hit this case.
786 // Unknown inst; assume it clobbers everything.
790 // Remove any allocas from the DeadPointer set that are loaded, as this
791 // makes any stores above the access live.
792 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
794 // If all of the allocas were clobbered by the access then we're not going
795 // to find anything else to process.
796 if (DeadStackObjects.empty())
803 /// RemoveAccessedObjects - Check to see if the specified location may alias any
804 /// of the stack objects in the DeadStackObjects set. If so, they become live
805 /// because the location is being loaded.
806 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
807 SmallPtrSet<Value*, 16> &DeadStackObjects) {
808 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
810 // A constant can't be in the dead pointer set.
811 if (isa<Constant>(UnderlyingPointer))
814 // If the kill pointer can be easily reduced to an alloca, don't bother doing
815 // extraneous AA queries.
816 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
817 DeadStackObjects.erase(const_cast<Value*>(UnderlyingPointer));
821 SmallVector<Value*, 16> NowLive;
822 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
823 E = DeadStackObjects.end(); I != E; ++I) {
824 // See if the loaded location could alias the stack location.
825 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
826 if (!AA->isNoAlias(StackLoc, LoadedLoc))
827 NowLive.push_back(*I);
830 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
832 DeadStackObjects.erase(*I);