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"
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 SmallPtrSet<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 SmallPtrSet<Value*, 16> *ValueSet = 0) {
110 SmallVector<Instruction*, 32> NowDeadInsts;
112 NowDeadInsts.push_back(I);
115 // Before we touch this instruction, remove it from memdep!
117 Instruction *DeadInst = NowDeadInsts.pop_back_val();
120 // This instruction is dead, zap it, in stages. Start by removing it from
121 // MemDep, which needs to know the operands and needs it to be in the
123 MD.removeInstruction(DeadInst);
125 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
126 Value *Op = DeadInst->getOperand(op);
127 DeadInst->setOperand(op, 0);
129 // If this operand just became dead, add it to the NowDeadInsts list.
130 if (!Op->use_empty()) continue;
132 if (Instruction *OpI = dyn_cast<Instruction>(Op))
133 if (isInstructionTriviallyDead(OpI))
134 NowDeadInsts.push_back(OpI);
137 DeadInst->eraseFromParent();
139 if (ValueSet) ValueSet->erase(DeadInst);
140 } while (!NowDeadInsts.empty());
144 /// hasMemoryWrite - Does this instruction write some memory? This only returns
145 /// true for things that we can analyze with other helpers below.
146 static bool hasMemoryWrite(Instruction *I) {
147 if (isa<StoreInst>(I))
149 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
150 switch (II->getIntrinsicID()) {
153 case Intrinsic::memset:
154 case Intrinsic::memmove:
155 case Intrinsic::memcpy:
156 case Intrinsic::init_trampoline:
157 case Intrinsic::lifetime_end:
164 /// getLocForWrite - Return a Location stored to by the specified instruction.
165 /// If isRemovable returns true, this function and getLocForRead completely
166 /// describe the memory operations for this instruction.
167 static AliasAnalysis::Location
168 getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
169 if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
170 return AA.getLocation(SI);
172 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
173 // memcpy/memmove/memset.
174 AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
175 // If we don't have target data around, an unknown size in Location means
176 // that we should use the size of the pointee type. This isn't valid for
177 // memset/memcpy, which writes more than an i8.
178 if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0)
179 return AliasAnalysis::Location();
183 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
184 if (II == 0) return AliasAnalysis::Location();
186 switch (II->getIntrinsicID()) {
187 default: return AliasAnalysis::Location(); // Unhandled intrinsic.
188 case Intrinsic::init_trampoline:
189 // If we don't have target data around, an unknown size in Location means
190 // that we should use the size of the pointee type. This isn't valid for
191 // init.trampoline, which writes more than an i8.
192 if (AA.getTargetData() == 0) return AliasAnalysis::Location();
194 // FIXME: We don't know the size of the trampoline, so we can't really
196 return AliasAnalysis::Location(II->getArgOperand(0));
197 case Intrinsic::lifetime_end: {
198 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
199 return AliasAnalysis::Location(II->getArgOperand(1), Len);
204 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
205 /// instruction if any.
206 static AliasAnalysis::Location
207 getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
208 assert(hasMemoryWrite(Inst) && "Unknown instruction case");
210 // The only instructions that both read and write are the mem transfer
211 // instructions (memcpy/memmove).
212 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
213 return AA.getLocationForSource(MTI);
214 return AliasAnalysis::Location();
218 /// isRemovable - If the value of this instruction and the memory it writes to
219 /// is unused, may we delete this instruction?
220 static bool isRemovable(Instruction *I) {
221 // Don't remove volatile/atomic stores.
222 if (StoreInst *SI = dyn_cast<StoreInst>(I))
223 return SI->isUnordered();
225 IntrinsicInst *II = cast<IntrinsicInst>(I);
226 switch (II->getIntrinsicID()) {
227 default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
228 case Intrinsic::lifetime_end:
229 // Never remove dead lifetime_end's, e.g. because it is followed by a
232 case Intrinsic::init_trampoline:
233 // Always safe to remove init_trampoline.
236 case Intrinsic::memset:
237 case Intrinsic::memmove:
238 case Intrinsic::memcpy:
239 // Don't remove volatile memory intrinsics.
240 return !cast<MemIntrinsic>(II)->isVolatile();
245 /// isShortenable - Returns true if this instruction can be safely shortened in
247 static bool isShortenable(Instruction *I) {
248 // Don't shorten stores for now
249 if (isa<StoreInst>(I))
252 IntrinsicInst *II = cast<IntrinsicInst>(I);
253 switch (II->getIntrinsicID()) {
254 default: return false;
255 case Intrinsic::memset:
256 case Intrinsic::memcpy:
257 // Do shorten memory intrinsics.
263 /// isMemset - Returns true if this instruction is an intrinsic memset
264 static bool isMemset(Instruction *I) {
265 IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
266 return II && II->getIntrinsicID() == Intrinsic::memset;
269 /// getStoredPointerOperand - Return the pointer that is being written to.
270 static Value *getStoredPointerOperand(Instruction *I) {
271 if (StoreInst *SI = dyn_cast<StoreInst>(I))
272 return SI->getPointerOperand();
273 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
274 return MI->getDest();
276 IntrinsicInst *II = cast<IntrinsicInst>(I);
277 switch (II->getIntrinsicID()) {
278 default: llvm_unreachable("Unexpected intrinsic!");
279 case Intrinsic::init_trampoline:
280 return II->getArgOperand(0);
284 static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
285 const TargetData *TD = AA.getTargetData();
287 if (const CallInst *CI = extractMallocCall(V)) {
288 if (const ConstantInt *C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
289 return C->getZExtValue();
293 return AliasAnalysis::UnknownSize;
295 if (const AllocaInst *A = dyn_cast<AllocaInst>(V)) {
296 // Get size information for the alloca
297 if (const ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
298 return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
301 if (const Argument *A = dyn_cast<Argument>(V)) {
302 if (A->hasByValAttr())
303 if (PointerType *PT = dyn_cast<PointerType>(A->getType()))
304 return TD->getTypeAllocSize(PT->getElementType());
307 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
308 if (!GV->mayBeOverridden())
309 return TD->getTypeAllocSize(GV->getType()->getElementType());
312 return AliasAnalysis::UnknownSize;
325 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
326 /// completely overwrites a store to the 'Earlier' location.
327 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
328 /// overwritten by 'Later', 'OverWriteStart' if the start of 'Earlier'
329 /// is completely overwritten by 'Later' or 'OverwriteUnknown' if nothing
330 /// can be determined
331 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
332 const AliasAnalysis::Location &Earlier,
336 const Value *P1 = Earlier.Ptr->stripPointerCasts();
337 const Value *P2 = Later.Ptr->stripPointerCasts();
339 // If the start pointers are the same, we just have to compare sizes to see if
340 // the later store was larger than the earlier store.
342 // If we don't know the sizes of either access, then we can't do a
344 if (Later.Size == AliasAnalysis::UnknownSize ||
345 Earlier.Size == AliasAnalysis::UnknownSize) {
346 // If we have no TargetData information around, then the size of the store
347 // is inferrable from the pointee type. If they are the same type, then
348 // we know that the store is safe.
349 if (AA.getTargetData() == 0 &&
350 Later.Ptr->getType() == Earlier.Ptr->getType())
351 return OverwriteComplete;
353 return OverwriteUnknown;
356 // Make sure that the Later size is >= the Earlier size.
357 if (Later.Size >= Earlier.Size)
358 return OverwriteComplete;
361 // Otherwise, we have to have size information, and the later store has to be
362 // larger than the earlier one.
363 if (Later.Size == AliasAnalysis::UnknownSize ||
364 Earlier.Size == AliasAnalysis::UnknownSize ||
365 AA.getTargetData() == 0)
366 return OverwriteUnknown;
368 // Check to see if the later store is to the entire object (either a global,
369 // an alloca, or a byval argument). If so, then it clearly overwrites any
370 // other store to the same object.
371 const TargetData &TD = *AA.getTargetData();
373 const Value *UO1 = GetUnderlyingObject(P1, &TD),
374 *UO2 = GetUnderlyingObject(P2, &TD);
376 // If we can't resolve the same pointers to the same object, then we can't
377 // analyze them at all.
379 return OverwriteUnknown;
381 // If the "Later" store is to a recognizable object, get its size.
382 uint64_t ObjectSize = getPointerSize(UO2, AA);
383 if (ObjectSize != AliasAnalysis::UnknownSize)
384 if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
385 return OverwriteComplete;
387 // Okay, we have stores to two completely different pointers. Try to
388 // decompose the pointer into a "base + constant_offset" form. If the base
389 // pointers are equal, then we can reason about the two stores.
392 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
393 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
395 // If the base pointers still differ, we have two completely different stores.
397 return OverwriteUnknown;
399 // The later store completely overlaps the earlier store if:
401 // 1. Both start at the same offset and the later one's size is greater than
402 // or equal to the earlier one's, or
407 // 2. The earlier store has an offset greater than the later offset, but which
408 // still lies completely within the later store.
411 // |----- later ------|
413 // We have to be careful here as *Off is signed while *.Size is unsigned.
414 if (EarlierOff >= LaterOff &&
415 Later.Size > Earlier.Size &&
416 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
417 return OverwriteComplete;
419 // The other interesting case is if the later store overwrites the end of
425 // In this case we may want to trim the size of earlier to avoid generating
426 // writes to addresses which will definitely be overwritten later
427 if (LaterOff > EarlierOff &&
428 LaterOff < int64_t(EarlierOff + Earlier.Size) &&
429 int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
432 // The other interesting case is if the later store overwrites the end of
438 // In this case we may want to trim the size of earlier to avoid generating
439 // writes to addresses which will definitely be overwritten later
440 if (EarlierOff >= LaterOff &&
441 EarlierOff < int64_t(LaterOff + Later.Size) &&
442 int64_t(EarlierOff + Earlier.Size) >= int64_t(LaterOff + Later.Size)) {
443 LaterOff = LaterOff + Later.Size;
444 return OverwriteStart;
447 // Otherwise, they don't completely overlap.
448 return OverwriteUnknown;
451 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
452 /// memory region into an identical pointer) then it doesn't actually make its
453 /// input dead in the traditional sense. Consider this case:
458 /// In this case, the second store to A does not make the first store to A dead.
459 /// The usual situation isn't an explicit A<-A store like this (which can be
460 /// trivially removed) but a case where two pointers may alias.
462 /// This function detects when it is unsafe to remove a dependent instruction
463 /// because the DSE inducing instruction may be a self-read.
464 static bool isPossibleSelfRead(Instruction *Inst,
465 const AliasAnalysis::Location &InstStoreLoc,
466 Instruction *DepWrite, AliasAnalysis &AA) {
467 // Self reads can only happen for instructions that read memory. Get the
469 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
470 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
472 // If the read and written loc obviously don't alias, it isn't a read.
473 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
475 // Okay, 'Inst' may copy over itself. However, we can still remove a the
476 // DepWrite instruction if we can prove that it reads from the same location
477 // as Inst. This handles useful cases like:
480 // Here we don't know if A/B may alias, but we do know that B/B are must
481 // aliases, so removing the first memcpy is safe (assuming it writes <= #
482 // bytes as the second one.
483 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
485 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
488 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
489 // then it can't be considered dead.
494 //===----------------------------------------------------------------------===//
496 //===----------------------------------------------------------------------===//
498 bool DSE::runOnBasicBlock(BasicBlock &BB) {
499 bool MadeChange = false;
501 // Do a top-down walk on the BB.
502 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
503 Instruction *Inst = BBI++;
505 // Handle 'free' calls specially.
506 if (CallInst *F = isFreeCall(Inst)) {
507 MadeChange |= HandleFree(F);
511 // If we find something that writes memory, get its memory dependence.
512 if (!hasMemoryWrite(Inst))
515 MemDepResult InstDep = MD->getDependency(Inst);
517 // Ignore any store where we can't find a local dependence.
518 // FIXME: cross-block DSE would be fun. :)
519 if (!InstDep.isDef() && !InstDep.isClobber())
522 // If we're storing the same value back to a pointer that we just
523 // loaded from, then the store can be removed.
524 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
525 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
526 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
527 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
528 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
529 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
531 // DeleteDeadInstruction can delete the current instruction. Save BBI
532 // in case we need it.
533 WeakVH NextInst(BBI);
535 DeleteDeadInstruction(SI, *MD);
537 if (NextInst == 0) // Next instruction deleted.
539 else if (BBI != BB.begin()) // Revisit this instruction if possible.
548 // Figure out what location is being stored to.
549 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
551 // If we didn't get a useful location, fail.
555 while (InstDep.isDef() || InstDep.isClobber()) {
556 // Get the memory clobbered by the instruction we depend on. MemDep will
557 // skip any instructions that 'Loc' clearly doesn't interact with. If we
558 // end up depending on a may- or must-aliased load, then we can't optimize
559 // away the store and we bail out. However, if we depend on on something
560 // that overwrites the memory location we *can* potentially optimize it.
562 // Find out what memory location the dependent instruction stores.
563 Instruction *DepWrite = InstDep.getInst();
564 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
565 // If we didn't get a useful location, or if it isn't a size, bail out.
569 // If we find a write that is a) removable (i.e., non-volatile), b) is
570 // completely obliterated by the store to 'Loc', and c) which we know that
571 // 'Inst' doesn't load from, then we can remove it.
572 if (isRemovable(DepWrite) &&
573 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
574 int64_t InstWriteOffset, DepWriteOffset;
575 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
576 DepWriteOffset, InstWriteOffset);
577 if (OR == OverwriteComplete) {
578 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
579 << *DepWrite << "\n KILLER: " << *Inst << '\n');
581 // Delete the store and now-dead instructions that feed it.
582 DeleteDeadInstruction(DepWrite, *MD);
586 // DeleteDeadInstruction can delete the current instruction in loop
589 if (BBI != BB.begin())
592 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
593 // TODO: base this on the target vector size so that if the earlier
594 // store was too small to get vector writes anyway then its likely
595 // a good idea to shorten it
596 // Power of 2 vector writes are probably always a bad idea to optimize
597 // as any store/memset/memcpy is likely using vector instructions so
598 // shortening it to not vector size is likely to be slower
599 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
600 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
601 if (llvm::isPowerOf2_64(InstWriteOffset) ||
602 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
604 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
605 << *DepWrite << "\n KILLER (offset "
606 << InstWriteOffset << ", "
607 << DepLoc.Size << ")"
610 Value* DepWriteLength = DepIntrinsic->getLength();
611 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
614 DepIntrinsic->setLength(TrimmedLength);
617 } else if (OR == OverwriteStart && isMemset(DepWrite)) {
618 // TODO: base this on the target vector size so that if the earlier
619 // store was too small to get vector writes anyway then its likely
620 // a good idea to shorten it
621 // Power of 2 vector writes are probably always a bad idea to optimize
622 // as any store/memset/memcpy is likely using vector instructions so
623 // shortening it to not vector size is likely to be slower
624 // TODO: shorten memcpy and memmove by offsetting the source address.
625 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
626 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
627 if (llvm::isPowerOf2_64(InstWriteOffset) ||
628 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
630 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW START: "
631 << *DepWrite << "\n KILLER (offset "
632 << InstWriteOffset << ", "
633 << DepWriteOffset << ", "
634 << DepLoc.Size << ")"
637 Value* DepWriteLength = DepIntrinsic->getLength();
638 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
642 DepIntrinsic->setLength(TrimmedLength);
643 const TargetData *TD = AA->getTargetData();
644 Type *IntPtrTy = TD->getIntPtrType(BB.getContext());
645 Value* Offset = ConstantInt::get(IntPtrTy,
646 InstWriteOffset - DepWriteOffset);
647 // Offset the start of the memset with a GEP. As the memset type is
648 // i8* a GEP will do this without needing to use ptrtoint, etc.
649 Value *Dest = GetElementPtrInst::Create(DepIntrinsic->getRawDest(),
653 DepIntrinsic->setDest(Dest);
659 // If this is a may-aliased store that is clobbering the store value, we
660 // can keep searching past it for another must-aliased pointer that stores
661 // to the same location. For example, in:
665 // we can remove the first store to P even though we don't know if P and Q
667 if (DepWrite == &BB.front()) break;
669 // Can't look past this instruction if it might read 'Loc'.
670 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
673 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
677 // If this block ends in a return, unwind, or unreachable, all allocas are
678 // dead at its end, which means stores to them are also dead.
679 if (BB.getTerminator()->getNumSuccessors() == 0)
680 MadeChange |= handleEndBlock(BB);
685 /// Find all blocks that will unconditionally lead to the block BB and append
687 static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
688 BasicBlock *BB, DominatorTree *DT) {
689 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
690 BasicBlock *Pred = *I;
691 if (Pred == BB) continue;
692 TerminatorInst *PredTI = Pred->getTerminator();
693 if (PredTI->getNumSuccessors() != 1)
696 if (DT->isReachableFromEntry(Pred))
697 Blocks.push_back(Pred);
701 /// HandleFree - Handle frees of entire structures whose dependency is a store
702 /// to a field of that structure.
703 bool DSE::HandleFree(CallInst *F) {
704 bool MadeChange = false;
706 AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
707 SmallVector<BasicBlock *, 16> Blocks;
708 Blocks.push_back(F->getParent());
710 while (!Blocks.empty()) {
711 BasicBlock *BB = Blocks.pop_back_val();
712 Instruction *InstPt = BB->getTerminator();
713 if (BB == F->getParent()) InstPt = F;
715 MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
716 while (Dep.isDef() || Dep.isClobber()) {
717 Instruction *Dependency = Dep.getInst();
718 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
722 GetUnderlyingObject(getStoredPointerOperand(Dependency));
724 // Check for aliasing.
725 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
728 Instruction *Next = llvm::next(BasicBlock::iterator(Dependency));
730 // DCE instructions only used to calculate that store
731 DeleteDeadInstruction(Dependency, *MD);
735 // Inst's old Dependency is now deleted. Compute the next dependency,
736 // which may also be dead, as in
738 // s[1] = 0; // This has just been deleted.
740 Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
743 if (Dep.isNonLocal())
744 FindUnconditionalPreds(Blocks, BB, DT);
750 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
751 /// function end block. Ex:
754 /// store i32 1, i32* %A
756 bool DSE::handleEndBlock(BasicBlock &BB) {
757 bool MadeChange = false;
759 // Keep track of all of the stack objects that are dead at the end of the
761 SmallPtrSet<Value*, 16> DeadStackObjects;
763 // Find all of the alloca'd pointers in the entry block.
764 BasicBlock *Entry = BB.getParent()->begin();
765 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
766 if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
767 DeadStackObjects.insert(AI);
769 // Okay, so these are dead heap objects, but if the pointer never escapes
770 // then it's leaked by this function anyways.
771 if (CallInst *CI = extractMallocCall(I))
772 if (!PointerMayBeCaptured(CI, true, true))
773 DeadStackObjects.insert(CI);
776 // Treat byval arguments the same, stores to them are dead at the end of the
778 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
779 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
780 if (AI->hasByValAttr())
781 DeadStackObjects.insert(AI);
783 // Scan the basic block backwards
784 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
787 // If we find a store, check to see if it points into a dead stack value.
788 if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
789 // See through pointer-to-pointer bitcasts
790 Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));
792 // Stores to stack values are valid candidates for removal.
793 if (DeadStackObjects.count(Pointer)) {
794 Instruction *Dead = BBI++;
796 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
797 << *Dead << "\n Object: " << *Pointer << '\n');
799 // DCE instructions only used to calculate that store.
800 DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
807 // Remove any dead non-memory-mutating instructions.
808 if (isInstructionTriviallyDead(BBI)) {
809 Instruction *Inst = BBI++;
810 DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
816 if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
817 DeadStackObjects.erase(A);
821 if (CallInst *CI = extractMallocCall(BBI)) {
822 DeadStackObjects.erase(CI);
826 if (CallSite CS = cast<Value>(BBI)) {
827 // If this call does not access memory, it can't be loading any of our
829 if (AA->doesNotAccessMemory(CS))
832 // If the call might load from any of our allocas, then any store above
834 SmallVector<Value*, 8> LiveAllocas;
835 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
836 E = DeadStackObjects.end(); I != E; ++I) {
837 // See if the call site touches it.
838 AliasAnalysis::ModRefResult A =
839 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
841 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
842 LiveAllocas.push_back(*I);
845 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
846 E = LiveAllocas.end(); I != E; ++I)
847 DeadStackObjects.erase(*I);
849 // If all of the allocas were clobbered by the call then we're not going
850 // to find anything else to process.
851 if (DeadStackObjects.empty())
857 AliasAnalysis::Location LoadedLoc;
859 // If we encounter a use of the pointer, it is no longer considered dead
860 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
861 if (!L->isUnordered()) // Be conservative with atomic/volatile load
863 LoadedLoc = AA->getLocation(L);
864 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
865 LoadedLoc = AA->getLocation(V);
866 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
867 LoadedLoc = AA->getLocationForSource(MTI);
868 } else if (!BBI->mayReadFromMemory()) {
869 // Instruction doesn't read memory. Note that stores that weren't removed
870 // above will hit this case.
873 // Unknown inst; assume it clobbers everything.
877 // Remove any allocas from the DeadPointer set that are loaded, as this
878 // makes any stores above the access live.
879 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
881 // If all of the allocas were clobbered by the access then we're not going
882 // to find anything else to process.
883 if (DeadStackObjects.empty())
890 /// RemoveAccessedObjects - Check to see if the specified location may alias any
891 /// of the stack objects in the DeadStackObjects set. If so, they become live
892 /// because the location is being loaded.
893 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
894 SmallPtrSet<Value*, 16> &DeadStackObjects) {
895 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
897 // A constant can't be in the dead pointer set.
898 if (isa<Constant>(UnderlyingPointer))
901 // If the kill pointer can be easily reduced to an alloca, don't bother doing
902 // extraneous AA queries.
903 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
904 DeadStackObjects.erase(const_cast<Value*>(UnderlyingPointer));
908 SmallVector<Value*, 16> NowLive;
909 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
910 E = DeadStackObjects.end(); I != E; ++I) {
911 // See if the loaded location could alias the stack location.
912 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
913 if (!AA->isNoAlias(StackLoc, LoadedLoc))
914 NowLive.push_back(*I);
917 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
919 DeadStackObjects.erase(*I);