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/ADT/STLExtras.h"
21 #include "llvm/ADT/SetVector.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/Analysis/AliasAnalysis.h"
24 #include "llvm/Analysis/CaptureTracking.h"
25 #include "llvm/Analysis/Dominators.h"
26 #include "llvm/Analysis/MemoryBuiltins.h"
27 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
28 #include "llvm/Analysis/ValueTracking.h"
29 #include "llvm/Constants.h"
30 #include "llvm/DataLayout.h"
31 #include "llvm/Function.h"
32 #include "llvm/GlobalVariable.h"
33 #include "llvm/Instructions.h"
34 #include "llvm/IntrinsicInst.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Target/TargetLibraryInfo.h"
38 #include "llvm/Transforms/Utils/Local.h"
41 STATISTIC(NumFastStores, "Number of stores deleted");
42 STATISTIC(NumFastOther , "Number of other instrs removed");
45 struct DSE : public FunctionPass {
47 MemoryDependenceAnalysis *MD;
49 const TargetLibraryInfo *TLI;
51 static char ID; // Pass identification, replacement for typeid
52 DSE() : FunctionPass(ID), AA(0), MD(0), DT(0) {
53 initializeDSEPass(*PassRegistry::getPassRegistry());
56 virtual bool runOnFunction(Function &F) {
57 AA = &getAnalysis<AliasAnalysis>();
58 MD = &getAnalysis<MemoryDependenceAnalysis>();
59 DT = &getAnalysis<DominatorTree>();
60 TLI = AA->getTargetLibraryInfo();
63 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
64 // Only check non-dead blocks. Dead blocks may have strange pointer
65 // cycles that will confuse alias analysis.
66 if (DT->isReachableFromEntry(I))
67 Changed |= runOnBasicBlock(*I);
69 AA = 0; MD = 0; DT = 0;
73 bool runOnBasicBlock(BasicBlock &BB);
74 bool HandleFree(CallInst *F);
75 bool handleEndBlock(BasicBlock &BB);
76 void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
77 SmallSetVector<Value*, 16> &DeadStackObjects);
79 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
81 AU.addRequired<DominatorTree>();
82 AU.addRequired<AliasAnalysis>();
83 AU.addRequired<MemoryDependenceAnalysis>();
84 AU.addPreserved<AliasAnalysis>();
85 AU.addPreserved<DominatorTree>();
86 AU.addPreserved<MemoryDependenceAnalysis>();
92 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
93 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
94 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
95 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
96 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
98 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
100 //===----------------------------------------------------------------------===//
102 //===----------------------------------------------------------------------===//
104 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
105 /// and zero out all the operands of this instruction. If any of them become
106 /// dead, delete them and the computation tree that feeds them.
108 /// If ValueSet is non-null, remove any deleted instructions from it as well.
110 static void DeleteDeadInstruction(Instruction *I,
111 MemoryDependenceAnalysis &MD,
112 const TargetLibraryInfo *TLI,
113 SmallSetVector<Value*, 16> *ValueSet = 0) {
114 SmallVector<Instruction*, 32> NowDeadInsts;
116 NowDeadInsts.push_back(I);
119 // Before we touch this instruction, remove it from memdep!
121 Instruction *DeadInst = NowDeadInsts.pop_back_val();
124 // This instruction is dead, zap it, in stages. Start by removing it from
125 // MemDep, which needs to know the operands and needs it to be in the
127 MD.removeInstruction(DeadInst);
129 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
130 Value *Op = DeadInst->getOperand(op);
131 DeadInst->setOperand(op, 0);
133 // If this operand just became dead, add it to the NowDeadInsts list.
134 if (!Op->use_empty()) continue;
136 if (Instruction *OpI = dyn_cast<Instruction>(Op))
137 if (isInstructionTriviallyDead(OpI, TLI))
138 NowDeadInsts.push_back(OpI);
141 DeadInst->eraseFromParent();
143 if (ValueSet) ValueSet->remove(DeadInst);
144 } while (!NowDeadInsts.empty());
148 /// hasMemoryWrite - Does this instruction write some memory? This only returns
149 /// true for things that we can analyze with other helpers below.
150 static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo *TLI) {
151 if (isa<StoreInst>(I))
153 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
154 switch (II->getIntrinsicID()) {
157 case Intrinsic::memset:
158 case Intrinsic::memmove:
159 case Intrinsic::memcpy:
160 case Intrinsic::init_trampoline:
161 case Intrinsic::lifetime_end:
165 if (CallSite CS = I) {
166 if (Function *F = CS.getCalledFunction()) {
167 if (TLI && TLI->has(LibFunc::strcpy) &&
168 F->getName() == TLI->getName(LibFunc::strcpy)) {
171 if (TLI && TLI->has(LibFunc::strncpy) &&
172 F->getName() == TLI->getName(LibFunc::strncpy)) {
175 if (TLI && TLI->has(LibFunc::strcat) &&
176 F->getName() == TLI->getName(LibFunc::strcat)) {
179 if (TLI && TLI->has(LibFunc::strncat) &&
180 F->getName() == TLI->getName(LibFunc::strncat)) {
188 /// getLocForWrite - Return a Location stored to by the specified instruction.
189 /// If isRemovable returns true, this function and getLocForRead completely
190 /// describe the memory operations for this instruction.
191 static AliasAnalysis::Location
192 getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
193 if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
194 return AA.getLocation(SI);
196 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
197 // memcpy/memmove/memset.
198 AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
199 // If we don't have target data around, an unknown size in Location means
200 // that we should use the size of the pointee type. This isn't valid for
201 // memset/memcpy, which writes more than an i8.
202 if (Loc.Size == AliasAnalysis::UnknownSize && AA.getDataLayout() == 0)
203 return AliasAnalysis::Location();
207 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
208 if (II == 0) return AliasAnalysis::Location();
210 switch (II->getIntrinsicID()) {
211 default: return AliasAnalysis::Location(); // Unhandled intrinsic.
212 case Intrinsic::init_trampoline:
213 // If we don't have target data around, an unknown size in Location means
214 // that we should use the size of the pointee type. This isn't valid for
215 // init.trampoline, which writes more than an i8.
216 if (AA.getDataLayout() == 0) return AliasAnalysis::Location();
218 // FIXME: We don't know the size of the trampoline, so we can't really
220 return AliasAnalysis::Location(II->getArgOperand(0));
221 case Intrinsic::lifetime_end: {
222 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
223 return AliasAnalysis::Location(II->getArgOperand(1), Len);
228 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
229 /// instruction if any.
230 static AliasAnalysis::Location
231 getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
232 assert(hasMemoryWrite(Inst, AA.getTargetLibraryInfo()) &&
233 "Unknown instruction case");
235 // The only instructions that both read and write are the mem transfer
236 // instructions (memcpy/memmove).
237 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
238 return AA.getLocationForSource(MTI);
239 return AliasAnalysis::Location();
243 /// isRemovable - If the value of this instruction and the memory it writes to
244 /// is unused, may we delete this instruction?
245 static bool isRemovable(Instruction *I) {
246 // Don't remove volatile/atomic stores.
247 if (StoreInst *SI = dyn_cast<StoreInst>(I))
248 return SI->isUnordered();
250 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
251 switch (II->getIntrinsicID()) {
252 default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
253 case Intrinsic::lifetime_end:
254 // Never remove dead lifetime_end's, e.g. because it is followed by a
257 case Intrinsic::init_trampoline:
258 // Always safe to remove init_trampoline.
261 case Intrinsic::memset:
262 case Intrinsic::memmove:
263 case Intrinsic::memcpy:
264 // Don't remove volatile memory intrinsics.
265 return !cast<MemIntrinsic>(II)->isVolatile();
270 return CS.getInstruction()->use_empty();
276 /// isShortenable - Returns true if this instruction can be safely shortened in
278 static bool isShortenable(Instruction *I) {
279 // Don't shorten stores for now
280 if (isa<StoreInst>(I))
283 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
284 switch (II->getIntrinsicID()) {
285 default: return false;
286 case Intrinsic::memset:
287 case Intrinsic::memcpy:
288 // Do shorten memory intrinsics.
293 // Don't shorten libcalls calls for now.
298 /// getStoredPointerOperand - Return the pointer that is being written to.
299 static Value *getStoredPointerOperand(Instruction *I) {
300 if (StoreInst *SI = dyn_cast<StoreInst>(I))
301 return SI->getPointerOperand();
302 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
303 return MI->getDest();
305 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
306 switch (II->getIntrinsicID()) {
307 default: llvm_unreachable("Unexpected intrinsic!");
308 case Intrinsic::init_trampoline:
309 return II->getArgOperand(0);
314 // All the supported functions so far happen to have dest as their first
316 return CS.getArgument(0);
319 static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
321 if (getObjectSize(V, Size, AA.getDataLayout(), AA.getTargetLibraryInfo()))
323 return AliasAnalysis::UnknownSize;
335 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
336 /// completely overwrites a store to the 'Earlier' location.
337 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
338 /// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
339 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
340 const AliasAnalysis::Location &Earlier,
344 const Value *P1 = Earlier.Ptr->stripPointerCasts();
345 const Value *P2 = Later.Ptr->stripPointerCasts();
347 // If the start pointers are the same, we just have to compare sizes to see if
348 // the later store was larger than the earlier store.
350 // If we don't know the sizes of either access, then we can't do a
352 if (Later.Size == AliasAnalysis::UnknownSize ||
353 Earlier.Size == AliasAnalysis::UnknownSize) {
354 // If we have no DataLayout information around, then the size of the store
355 // is inferrable from the pointee type. If they are the same type, then
356 // we know that the store is safe.
357 if (AA.getDataLayout() == 0 &&
358 Later.Ptr->getType() == Earlier.Ptr->getType())
359 return OverwriteComplete;
361 return OverwriteUnknown;
364 // Make sure that the Later size is >= the Earlier size.
365 if (Later.Size >= Earlier.Size)
366 return OverwriteComplete;
369 // Otherwise, we have to have size information, and the later store has to be
370 // larger than the earlier one.
371 if (Later.Size == AliasAnalysis::UnknownSize ||
372 Earlier.Size == AliasAnalysis::UnknownSize ||
373 AA.getDataLayout() == 0)
374 return OverwriteUnknown;
376 // Check to see if the later store is to the entire object (either a global,
377 // an alloca, or a byval argument). If so, then it clearly overwrites any
378 // other store to the same object.
379 const DataLayout &TD = *AA.getDataLayout();
381 const Value *UO1 = GetUnderlyingObject(P1, &TD),
382 *UO2 = GetUnderlyingObject(P2, &TD);
384 // If we can't resolve the same pointers to the same object, then we can't
385 // analyze them at all.
387 return OverwriteUnknown;
389 // If the "Later" store is to a recognizable object, get its size.
390 uint64_t ObjectSize = getPointerSize(UO2, AA);
391 if (ObjectSize != AliasAnalysis::UnknownSize)
392 if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
393 return OverwriteComplete;
395 // Okay, we have stores to two completely different pointers. Try to
396 // decompose the pointer into a "base + constant_offset" form. If the base
397 // pointers are equal, then we can reason about the two stores.
400 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
401 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
403 // If the base pointers still differ, we have two completely different stores.
405 return OverwriteUnknown;
407 // The later store completely overlaps the earlier store if:
409 // 1. Both start at the same offset and the later one's size is greater than
410 // or equal to the earlier one's, or
415 // 2. The earlier store has an offset greater than the later offset, but which
416 // still lies completely within the later store.
419 // |----- later ------|
421 // We have to be careful here as *Off is signed while *.Size is unsigned.
422 if (EarlierOff >= LaterOff &&
423 Later.Size >= Earlier.Size &&
424 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
425 return OverwriteComplete;
427 // The other interesting case is if the later store overwrites the end of
433 // In this case we may want to trim the size of earlier to avoid generating
434 // writes to addresses which will definitely be overwritten later
435 if (LaterOff > EarlierOff &&
436 LaterOff < int64_t(EarlierOff + Earlier.Size) &&
437 int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
440 // Otherwise, they don't completely overlap.
441 return OverwriteUnknown;
444 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
445 /// memory region into an identical pointer) then it doesn't actually make its
446 /// input dead in the traditional sense. Consider this case:
451 /// In this case, the second store to A does not make the first store to A dead.
452 /// The usual situation isn't an explicit A<-A store like this (which can be
453 /// trivially removed) but a case where two pointers may alias.
455 /// This function detects when it is unsafe to remove a dependent instruction
456 /// because the DSE inducing instruction may be a self-read.
457 static bool isPossibleSelfRead(Instruction *Inst,
458 const AliasAnalysis::Location &InstStoreLoc,
459 Instruction *DepWrite, AliasAnalysis &AA) {
460 // Self reads can only happen for instructions that read memory. Get the
462 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
463 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
465 // If the read and written loc obviously don't alias, it isn't a read.
466 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
468 // Okay, 'Inst' may copy over itself. However, we can still remove a the
469 // DepWrite instruction if we can prove that it reads from the same location
470 // as Inst. This handles useful cases like:
473 // Here we don't know if A/B may alias, but we do know that B/B are must
474 // aliases, so removing the first memcpy is safe (assuming it writes <= #
475 // bytes as the second one.
476 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
478 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
481 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
482 // then it can't be considered dead.
487 //===----------------------------------------------------------------------===//
489 //===----------------------------------------------------------------------===//
491 bool DSE::runOnBasicBlock(BasicBlock &BB) {
492 bool MadeChange = false;
494 // Do a top-down walk on the BB.
495 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
496 Instruction *Inst = BBI++;
498 // Handle 'free' calls specially.
499 if (CallInst *F = isFreeCall(Inst, TLI)) {
500 MadeChange |= HandleFree(F);
504 // If we find something that writes memory, get its memory dependence.
505 if (!hasMemoryWrite(Inst, TLI))
508 MemDepResult InstDep = MD->getDependency(Inst);
510 // Ignore any store where we can't find a local dependence.
511 // FIXME: cross-block DSE would be fun. :)
512 if (!InstDep.isDef() && !InstDep.isClobber())
515 // If we're storing the same value back to a pointer that we just
516 // loaded from, then the store can be removed.
517 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
518 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
519 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
520 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
521 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
522 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
524 // DeleteDeadInstruction can delete the current instruction. Save BBI
525 // in case we need it.
526 WeakVH NextInst(BBI);
528 DeleteDeadInstruction(SI, *MD, TLI);
530 if (NextInst == 0) // Next instruction deleted.
532 else if (BBI != BB.begin()) // Revisit this instruction if possible.
541 // Figure out what location is being stored to.
542 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
544 // If we didn't get a useful location, fail.
548 while (InstDep.isDef() || InstDep.isClobber()) {
549 // Get the memory clobbered by the instruction we depend on. MemDep will
550 // skip any instructions that 'Loc' clearly doesn't interact with. If we
551 // end up depending on a may- or must-aliased load, then we can't optimize
552 // away the store and we bail out. However, if we depend on on something
553 // that overwrites the memory location we *can* potentially optimize it.
555 // Find out what memory location the dependent instruction stores.
556 Instruction *DepWrite = InstDep.getInst();
557 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
558 // If we didn't get a useful location, or if it isn't a size, bail out.
562 // If we find a write that is a) removable (i.e., non-volatile), b) is
563 // completely obliterated by the store to 'Loc', and c) which we know that
564 // 'Inst' doesn't load from, then we can remove it.
565 if (isRemovable(DepWrite) &&
566 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
567 int64_t InstWriteOffset, DepWriteOffset;
568 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
569 DepWriteOffset, InstWriteOffset);
570 if (OR == OverwriteComplete) {
571 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
572 << *DepWrite << "\n KILLER: " << *Inst << '\n');
574 // Delete the store and now-dead instructions that feed it.
575 DeleteDeadInstruction(DepWrite, *MD, TLI);
579 // DeleteDeadInstruction can delete the current instruction in loop
582 if (BBI != BB.begin())
585 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
586 // TODO: base this on the target vector size so that if the earlier
587 // store was too small to get vector writes anyway then its likely
588 // a good idea to shorten it
589 // Power of 2 vector writes are probably always a bad idea to optimize
590 // as any store/memset/memcpy is likely using vector instructions so
591 // shortening it to not vector size is likely to be slower
592 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
593 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
594 if (llvm::isPowerOf2_64(InstWriteOffset) ||
595 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
597 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
598 << *DepWrite << "\n KILLER (offset "
599 << InstWriteOffset << ", "
600 << DepLoc.Size << ")"
603 Value* DepWriteLength = DepIntrinsic->getLength();
604 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
607 DepIntrinsic->setLength(TrimmedLength);
613 // If this is a may-aliased store that is clobbering the store value, we
614 // can keep searching past it for another must-aliased pointer that stores
615 // to the same location. For example, in:
619 // we can remove the first store to P even though we don't know if P and Q
621 if (DepWrite == &BB.front()) break;
623 // Can't look past this instruction if it might read 'Loc'.
624 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
627 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
631 // If this block ends in a return, unwind, or unreachable, all allocas are
632 // dead at its end, which means stores to them are also dead.
633 if (BB.getTerminator()->getNumSuccessors() == 0)
634 MadeChange |= handleEndBlock(BB);
639 /// Find all blocks that will unconditionally lead to the block BB and append
641 static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
642 BasicBlock *BB, DominatorTree *DT) {
643 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
644 BasicBlock *Pred = *I;
645 if (Pred == BB) continue;
646 TerminatorInst *PredTI = Pred->getTerminator();
647 if (PredTI->getNumSuccessors() != 1)
650 if (DT->isReachableFromEntry(Pred))
651 Blocks.push_back(Pred);
655 /// HandleFree - Handle frees of entire structures whose dependency is a store
656 /// to a field of that structure.
657 bool DSE::HandleFree(CallInst *F) {
658 bool MadeChange = false;
660 AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
661 SmallVector<BasicBlock *, 16> Blocks;
662 Blocks.push_back(F->getParent());
664 while (!Blocks.empty()) {
665 BasicBlock *BB = Blocks.pop_back_val();
666 Instruction *InstPt = BB->getTerminator();
667 if (BB == F->getParent()) InstPt = F;
669 MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
670 while (Dep.isDef() || Dep.isClobber()) {
671 Instruction *Dependency = Dep.getInst();
672 if (!hasMemoryWrite(Dependency, TLI) || !isRemovable(Dependency))
676 GetUnderlyingObject(getStoredPointerOperand(Dependency));
678 // Check for aliasing.
679 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
682 Instruction *Next = llvm::next(BasicBlock::iterator(Dependency));
684 // DCE instructions only used to calculate that store
685 DeleteDeadInstruction(Dependency, *MD, TLI);
689 // Inst's old Dependency is now deleted. Compute the next dependency,
690 // which may also be dead, as in
692 // s[1] = 0; // This has just been deleted.
694 Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
697 if (Dep.isNonLocal())
698 FindUnconditionalPreds(Blocks, BB, DT);
706 typedef Value *argument_type;
710 bool operator()(Value *I) {
711 // See if the call site touches the value.
712 AliasAnalysis::ModRefResult A =
713 AA->getModRefInfo(CS, I, getPointerSize(I, *AA));
715 return A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref;
720 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
721 /// function end block. Ex:
724 /// store i32 1, i32* %A
726 bool DSE::handleEndBlock(BasicBlock &BB) {
727 bool MadeChange = false;
729 // Keep track of all of the stack objects that are dead at the end of the
731 SmallSetVector<Value*, 16> DeadStackObjects;
733 // Find all of the alloca'd pointers in the entry block.
734 BasicBlock *Entry = BB.getParent()->begin();
735 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
736 if (isa<AllocaInst>(I))
737 DeadStackObjects.insert(I);
739 // Okay, so these are dead heap objects, but if the pointer never escapes
740 // then it's leaked by this function anyways.
741 else if (isAllocLikeFn(I, TLI) && !PointerMayBeCaptured(I, true, true))
742 DeadStackObjects.insert(I);
745 // Treat byval arguments the same, stores to them are dead at the end of the
747 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
748 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
749 if (AI->hasByValAttr())
750 DeadStackObjects.insert(AI);
752 // Scan the basic block backwards
753 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
756 // If we find a store, check to see if it points into a dead stack value.
757 if (hasMemoryWrite(BBI, TLI) && isRemovable(BBI)) {
758 // See through pointer-to-pointer bitcasts
759 SmallVector<Value *, 4> Pointers;
760 GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers);
762 // Stores to stack values are valid candidates for removal.
764 for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
765 E = Pointers.end(); I != E; ++I)
766 if (!DeadStackObjects.count(*I)) {
772 Instruction *Dead = BBI++;
774 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
775 << *Dead << "\n Objects: ";
776 for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
777 E = Pointers.end(); I != E; ++I) {
779 if (llvm::next(I) != E)
784 // DCE instructions only used to calculate that store.
785 DeleteDeadInstruction(Dead, *MD, TLI, &DeadStackObjects);
792 // Remove any dead non-memory-mutating instructions.
793 if (isInstructionTriviallyDead(BBI, TLI)) {
794 Instruction *Inst = BBI++;
795 DeleteDeadInstruction(Inst, *MD, TLI, &DeadStackObjects);
801 if (isa<AllocaInst>(BBI)) {
802 // Remove allocas from the list of dead stack objects; there can't be
803 // any references before the definition.
804 DeadStackObjects.remove(BBI);
808 if (CallSite CS = cast<Value>(BBI)) {
809 // Remove allocation function calls from the list of dead stack objects;
810 // there can't be any references before the definition.
811 if (isAllocLikeFn(BBI, TLI))
812 DeadStackObjects.remove(BBI);
814 // If this call does not access memory, it can't be loading any of our
816 if (AA->doesNotAccessMemory(CS))
819 // If the call might load from any of our allocas, then any store above
821 CouldRef Pred = { CS, AA };
822 DeadStackObjects.remove_if(Pred);
824 // If all of the allocas were clobbered by the call then we're not going
825 // to find anything else to process.
826 if (DeadStackObjects.empty())
832 AliasAnalysis::Location LoadedLoc;
834 // If we encounter a use of the pointer, it is no longer considered dead
835 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
836 if (!L->isUnordered()) // Be conservative with atomic/volatile load
838 LoadedLoc = AA->getLocation(L);
839 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
840 LoadedLoc = AA->getLocation(V);
841 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
842 LoadedLoc = AA->getLocationForSource(MTI);
843 } else if (!BBI->mayReadFromMemory()) {
844 // Instruction doesn't read memory. Note that stores that weren't removed
845 // above will hit this case.
848 // Unknown inst; assume it clobbers everything.
852 // Remove any allocas from the DeadPointer set that are loaded, as this
853 // makes any stores above the access live.
854 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
856 // If all of the allocas were clobbered by the access then we're not going
857 // to find anything else to process.
858 if (DeadStackObjects.empty())
867 typedef Value *argument_type;
868 const AliasAnalysis::Location &LoadedLoc;
871 bool operator()(Value *I) {
872 // See if the loaded location could alias the stack location.
873 AliasAnalysis::Location StackLoc(I, getPointerSize(I, *AA));
874 return !AA->isNoAlias(StackLoc, LoadedLoc);
879 /// RemoveAccessedObjects - Check to see if the specified location may alias any
880 /// of the stack objects in the DeadStackObjects set. If so, they become live
881 /// because the location is being loaded.
882 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
883 SmallSetVector<Value*, 16> &DeadStackObjects) {
884 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
886 // A constant can't be in the dead pointer set.
887 if (isa<Constant>(UnderlyingPointer))
890 // If the kill pointer can be easily reduced to an alloca, don't bother doing
891 // extraneous AA queries.
892 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
893 DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
897 // Remove objects that could alias LoadedLoc.
898 CouldAlias Pred = { LoadedLoc, AA };
899 DeadStackObjects.remove_if(Pred);