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/MemoryBuiltins.h"
26 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
27 #include "llvm/Analysis/ValueTracking.h"
28 #include "llvm/IR/Constants.h"
29 #include "llvm/IR/DataLayout.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/Function.h"
32 #include "llvm/IR/GlobalVariable.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/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 bool runOnFunction(Function &F) override {
57 if (skipOptnoneFunction(F))
60 AA = &getAnalysis<AliasAnalysis>();
61 MD = &getAnalysis<MemoryDependenceAnalysis>();
62 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
63 TLI = AA->getTargetLibraryInfo();
66 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
67 // Only check non-dead blocks. Dead blocks may have strange pointer
68 // cycles that will confuse alias analysis.
69 if (DT->isReachableFromEntry(I))
70 Changed |= runOnBasicBlock(*I);
72 AA = 0; MD = 0; DT = 0;
76 bool runOnBasicBlock(BasicBlock &BB);
77 bool HandleFree(CallInst *F);
78 bool handleEndBlock(BasicBlock &BB);
79 void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
80 SmallSetVector<Value*, 16> &DeadStackObjects);
82 void getAnalysisUsage(AnalysisUsage &AU) const override {
84 AU.addRequired<DominatorTreeWrapperPass>();
85 AU.addRequired<AliasAnalysis>();
86 AU.addRequired<MemoryDependenceAnalysis>();
87 AU.addPreserved<AliasAnalysis>();
88 AU.addPreserved<DominatorTreeWrapperPass>();
89 AU.addPreserved<MemoryDependenceAnalysis>();
95 INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
96 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
97 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
98 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
99 INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
101 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
103 //===----------------------------------------------------------------------===//
105 //===----------------------------------------------------------------------===//
107 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
108 /// and zero out all the operands of this instruction. If any of them become
109 /// dead, delete them and the computation tree that feeds them.
111 /// If ValueSet is non-null, remove any deleted instructions from it as well.
113 static void DeleteDeadInstruction(Instruction *I,
114 MemoryDependenceAnalysis &MD,
115 const TargetLibraryInfo *TLI,
116 SmallSetVector<Value*, 16> *ValueSet = 0) {
117 SmallVector<Instruction*, 32> NowDeadInsts;
119 NowDeadInsts.push_back(I);
122 // Before we touch this instruction, remove it from memdep!
124 Instruction *DeadInst = NowDeadInsts.pop_back_val();
127 // This instruction is dead, zap it, in stages. Start by removing it from
128 // MemDep, which needs to know the operands and needs it to be in the
130 MD.removeInstruction(DeadInst);
132 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
133 Value *Op = DeadInst->getOperand(op);
134 DeadInst->setOperand(op, 0);
136 // If this operand just became dead, add it to the NowDeadInsts list.
137 if (!Op->use_empty()) continue;
139 if (Instruction *OpI = dyn_cast<Instruction>(Op))
140 if (isInstructionTriviallyDead(OpI, TLI))
141 NowDeadInsts.push_back(OpI);
144 DeadInst->eraseFromParent();
146 if (ValueSet) ValueSet->remove(DeadInst);
147 } while (!NowDeadInsts.empty());
151 /// hasMemoryWrite - Does this instruction write some memory? This only returns
152 /// true for things that we can analyze with other helpers below.
153 static bool hasMemoryWrite(Instruction *I, const TargetLibraryInfo *TLI) {
154 if (isa<StoreInst>(I))
156 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
157 switch (II->getIntrinsicID()) {
160 case Intrinsic::memset:
161 case Intrinsic::memmove:
162 case Intrinsic::memcpy:
163 case Intrinsic::init_trampoline:
164 case Intrinsic::lifetime_end:
168 if (CallSite CS = I) {
169 if (Function *F = CS.getCalledFunction()) {
170 if (TLI && TLI->has(LibFunc::strcpy) &&
171 F->getName() == TLI->getName(LibFunc::strcpy)) {
174 if (TLI && TLI->has(LibFunc::strncpy) &&
175 F->getName() == TLI->getName(LibFunc::strncpy)) {
178 if (TLI && TLI->has(LibFunc::strcat) &&
179 F->getName() == TLI->getName(LibFunc::strcat)) {
182 if (TLI && TLI->has(LibFunc::strncat) &&
183 F->getName() == TLI->getName(LibFunc::strncat)) {
191 /// getLocForWrite - Return a Location stored to by the specified instruction.
192 /// If isRemovable returns true, this function and getLocForRead completely
193 /// describe the memory operations for this instruction.
194 static AliasAnalysis::Location
195 getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
196 const DataLayout *DL = AA.getDataLayout();
197 if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
198 return AA.getLocation(SI);
200 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
201 // memcpy/memmove/memset.
202 AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
203 // If we don't have target data around, an unknown size in Location means
204 // that we should use the size of the pointee type. This isn't valid for
205 // memset/memcpy, which writes more than an i8.
206 if (Loc.Size == AliasAnalysis::UnknownSize && DL == 0)
207 return AliasAnalysis::Location();
211 IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
212 if (II == 0) return AliasAnalysis::Location();
214 switch (II->getIntrinsicID()) {
215 default: return AliasAnalysis::Location(); // Unhandled intrinsic.
216 case Intrinsic::init_trampoline:
217 // If we don't have target data around, an unknown size in Location means
218 // that we should use the size of the pointee type. This isn't valid for
219 // init.trampoline, which writes more than an i8.
220 if (DL == 0) return AliasAnalysis::Location();
222 // FIXME: We don't know the size of the trampoline, so we can't really
224 return AliasAnalysis::Location(II->getArgOperand(0));
225 case Intrinsic::lifetime_end: {
226 uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
227 return AliasAnalysis::Location(II->getArgOperand(1), Len);
232 /// getLocForRead - Return the location read by the specified "hasMemoryWrite"
233 /// instruction if any.
234 static AliasAnalysis::Location
235 getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
236 assert(hasMemoryWrite(Inst, AA.getTargetLibraryInfo()) &&
237 "Unknown instruction case");
239 // The only instructions that both read and write are the mem transfer
240 // instructions (memcpy/memmove).
241 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
242 return AA.getLocationForSource(MTI);
243 return AliasAnalysis::Location();
247 /// isRemovable - If the value of this instruction and the memory it writes to
248 /// is unused, may we delete this instruction?
249 static bool isRemovable(Instruction *I) {
250 // Don't remove volatile/atomic stores.
251 if (StoreInst *SI = dyn_cast<StoreInst>(I))
252 return SI->isUnordered();
254 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
255 switch (II->getIntrinsicID()) {
256 default: llvm_unreachable("doesn't pass 'hasMemoryWrite' predicate");
257 case Intrinsic::lifetime_end:
258 // Never remove dead lifetime_end's, e.g. because it is followed by a
261 case Intrinsic::init_trampoline:
262 // Always safe to remove init_trampoline.
265 case Intrinsic::memset:
266 case Intrinsic::memmove:
267 case Intrinsic::memcpy:
268 // Don't remove volatile memory intrinsics.
269 return !cast<MemIntrinsic>(II)->isVolatile();
274 return CS.getInstruction()->use_empty();
280 /// isShortenable - Returns true if this instruction can be safely shortened in
282 static bool isShortenable(Instruction *I) {
283 // Don't shorten stores for now
284 if (isa<StoreInst>(I))
287 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
288 switch (II->getIntrinsicID()) {
289 default: return false;
290 case Intrinsic::memset:
291 case Intrinsic::memcpy:
292 // Do shorten memory intrinsics.
297 // Don't shorten libcalls calls for now.
302 /// getStoredPointerOperand - Return the pointer that is being written to.
303 static Value *getStoredPointerOperand(Instruction *I) {
304 if (StoreInst *SI = dyn_cast<StoreInst>(I))
305 return SI->getPointerOperand();
306 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
307 return MI->getDest();
309 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
310 switch (II->getIntrinsicID()) {
311 default: llvm_unreachable("Unexpected intrinsic!");
312 case Intrinsic::init_trampoline:
313 return II->getArgOperand(0);
318 // All the supported functions so far happen to have dest as their first
320 return CS.getArgument(0);
323 static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
325 if (getObjectSize(V, Size, AA.getDataLayout(), AA.getTargetLibraryInfo()))
327 return AliasAnalysis::UnknownSize;
339 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
340 /// completely overwrites a store to the 'Earlier' location.
341 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
342 /// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
343 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
344 const AliasAnalysis::Location &Earlier,
348 const DataLayout *DL = AA.getDataLayout();
349 const Value *P1 = Earlier.Ptr->stripPointerCasts();
350 const Value *P2 = Later.Ptr->stripPointerCasts();
352 // If the start pointers are the same, we just have to compare sizes to see if
353 // the later store was larger than the earlier store.
355 // If we don't know the sizes of either access, then we can't do a
357 if (Later.Size == AliasAnalysis::UnknownSize ||
358 Earlier.Size == AliasAnalysis::UnknownSize) {
359 // If we have no DataLayout information around, then the size of the store
360 // is inferrable from the pointee type. If they are the same type, then
361 // we know that the store is safe.
362 if (DL == 0 && Later.Ptr->getType() == Earlier.Ptr->getType())
363 return OverwriteComplete;
365 return OverwriteUnknown;
368 // Make sure that the Later size is >= the Earlier size.
369 if (Later.Size >= Earlier.Size)
370 return OverwriteComplete;
373 // Otherwise, we have to have size information, and the later store has to be
374 // larger than the earlier one.
375 if (Later.Size == AliasAnalysis::UnknownSize ||
376 Earlier.Size == AliasAnalysis::UnknownSize || DL == 0)
377 return OverwriteUnknown;
379 // Check to see if the later store is to the entire object (either a global,
380 // an alloca, or a byval/inalloca argument). If so, then it clearly
381 // overwrites any other store to the same object.
382 const Value *UO1 = GetUnderlyingObject(P1, DL),
383 *UO2 = GetUnderlyingObject(P2, DL);
385 // If we can't resolve the same pointers to the same object, then we can't
386 // analyze them at all.
388 return OverwriteUnknown;
390 // If the "Later" store is to a recognizable object, get its size.
391 uint64_t ObjectSize = getPointerSize(UO2, AA);
392 if (ObjectSize != AliasAnalysis::UnknownSize)
393 if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
394 return OverwriteComplete;
396 // Okay, we have stores to two completely different pointers. Try to
397 // decompose the pointer into a "base + constant_offset" form. If the base
398 // pointers are equal, then we can reason about the two stores.
401 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
402 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);
404 // If the base pointers still differ, we have two completely different stores.
406 return OverwriteUnknown;
408 // The later store completely overlaps the earlier store if:
410 // 1. Both start at the same offset and the later one's size is greater than
411 // or equal to the earlier one's, or
416 // 2. The earlier store has an offset greater than the later offset, but which
417 // still lies completely within the later store.
420 // |----- later ------|
422 // We have to be careful here as *Off is signed while *.Size is unsigned.
423 if (EarlierOff >= LaterOff &&
424 Later.Size >= Earlier.Size &&
425 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
426 return OverwriteComplete;
428 // The other interesting case is if the later store overwrites the end of
434 // In this case we may want to trim the size of earlier to avoid generating
435 // writes to addresses which will definitely be overwritten later
436 if (LaterOff > EarlierOff &&
437 LaterOff < int64_t(EarlierOff + Earlier.Size) &&
438 int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
441 // Otherwise, they don't completely overlap.
442 return OverwriteUnknown;
445 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
446 /// memory region into an identical pointer) then it doesn't actually make its
447 /// input dead in the traditional sense. Consider this case:
452 /// In this case, the second store to A does not make the first store to A dead.
453 /// The usual situation isn't an explicit A<-A store like this (which can be
454 /// trivially removed) but a case where two pointers may alias.
456 /// This function detects when it is unsafe to remove a dependent instruction
457 /// because the DSE inducing instruction may be a self-read.
458 static bool isPossibleSelfRead(Instruction *Inst,
459 const AliasAnalysis::Location &InstStoreLoc,
460 Instruction *DepWrite, AliasAnalysis &AA) {
461 // Self reads can only happen for instructions that read memory. Get the
463 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
464 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
466 // If the read and written loc obviously don't alias, it isn't a read.
467 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
469 // Okay, 'Inst' may copy over itself. However, we can still remove a the
470 // DepWrite instruction if we can prove that it reads from the same location
471 // as Inst. This handles useful cases like:
474 // Here we don't know if A/B may alias, but we do know that B/B are must
475 // aliases, so removing the first memcpy is safe (assuming it writes <= #
476 // bytes as the second one.
477 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
479 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
482 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
483 // then it can't be considered dead.
488 //===----------------------------------------------------------------------===//
490 //===----------------------------------------------------------------------===//
492 bool DSE::runOnBasicBlock(BasicBlock &BB) {
493 bool MadeChange = false;
495 // Do a top-down walk on the BB.
496 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
497 Instruction *Inst = BBI++;
499 // Handle 'free' calls specially.
500 if (CallInst *F = isFreeCall(Inst, TLI)) {
501 MadeChange |= HandleFree(F);
505 // If we find something that writes memory, get its memory dependence.
506 if (!hasMemoryWrite(Inst, TLI))
509 MemDepResult InstDep = MD->getDependency(Inst);
511 // Ignore any store where we can't find a local dependence.
512 // FIXME: cross-block DSE would be fun. :)
513 if (!InstDep.isDef() && !InstDep.isClobber())
516 // If we're storing the same value back to a pointer that we just
517 // loaded from, then the store can be removed.
518 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
519 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
520 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
521 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
522 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
523 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
525 // DeleteDeadInstruction can delete the current instruction. Save BBI
526 // in case we need it.
527 WeakVH NextInst(BBI);
529 DeleteDeadInstruction(SI, *MD, TLI);
531 if (NextInst == 0) // Next instruction deleted.
533 else if (BBI != BB.begin()) // Revisit this instruction if possible.
542 // Figure out what location is being stored to.
543 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
545 // If we didn't get a useful location, fail.
549 while (InstDep.isDef() || InstDep.isClobber()) {
550 // Get the memory clobbered by the instruction we depend on. MemDep will
551 // skip any instructions that 'Loc' clearly doesn't interact with. If we
552 // end up depending on a may- or must-aliased load, then we can't optimize
553 // away the store and we bail out. However, if we depend on on something
554 // that overwrites the memory location we *can* potentially optimize it.
556 // Find out what memory location the dependent instruction stores.
557 Instruction *DepWrite = InstDep.getInst();
558 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
559 // If we didn't get a useful location, or if it isn't a size, bail out.
563 // If we find a write that is a) removable (i.e., non-volatile), b) is
564 // completely obliterated by the store to 'Loc', and c) which we know that
565 // 'Inst' doesn't load from, then we can remove it.
566 if (isRemovable(DepWrite) &&
567 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
568 int64_t InstWriteOffset, DepWriteOffset;
569 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
570 DepWriteOffset, InstWriteOffset);
571 if (OR == OverwriteComplete) {
572 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
573 << *DepWrite << "\n KILLER: " << *Inst << '\n');
575 // Delete the store and now-dead instructions that feed it.
576 DeleteDeadInstruction(DepWrite, *MD, TLI);
580 // DeleteDeadInstruction can delete the current instruction in loop
583 if (BBI != BB.begin())
586 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
587 // TODO: base this on the target vector size so that if the earlier
588 // store was too small to get vector writes anyway then its likely
589 // a good idea to shorten it
590 // Power of 2 vector writes are probably always a bad idea to optimize
591 // as any store/memset/memcpy is likely using vector instructions so
592 // shortening it to not vector size is likely to be slower
593 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
594 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
595 if (llvm::isPowerOf2_64(InstWriteOffset) ||
596 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
598 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
599 << *DepWrite << "\n KILLER (offset "
600 << InstWriteOffset << ", "
601 << DepLoc.Size << ")"
604 Value* DepWriteLength = DepIntrinsic->getLength();
605 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
608 DepIntrinsic->setLength(TrimmedLength);
614 // If this is a may-aliased store that is clobbering the store value, we
615 // can keep searching past it for another must-aliased pointer that stores
616 // to the same location. For example, in:
620 // we can remove the first store to P even though we don't know if P and Q
622 if (DepWrite == &BB.front()) break;
624 // Can't look past this instruction if it might read 'Loc'.
625 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
628 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
632 // If this block ends in a return, unwind, or unreachable, all allocas are
633 // dead at its end, which means stores to them are also dead.
634 if (BB.getTerminator()->getNumSuccessors() == 0)
635 MadeChange |= handleEndBlock(BB);
640 /// Find all blocks that will unconditionally lead to the block BB and append
642 static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
643 BasicBlock *BB, DominatorTree *DT) {
644 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
645 BasicBlock *Pred = *I;
646 if (Pred == BB) continue;
647 TerminatorInst *PredTI = Pred->getTerminator();
648 if (PredTI->getNumSuccessors() != 1)
651 if (DT->isReachableFromEntry(Pred))
652 Blocks.push_back(Pred);
656 /// HandleFree - Handle frees of entire structures whose dependency is a store
657 /// to a field of that structure.
658 bool DSE::HandleFree(CallInst *F) {
659 bool MadeChange = false;
661 AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
662 SmallVector<BasicBlock *, 16> Blocks;
663 Blocks.push_back(F->getParent());
665 while (!Blocks.empty()) {
666 BasicBlock *BB = Blocks.pop_back_val();
667 Instruction *InstPt = BB->getTerminator();
668 if (BB == F->getParent()) InstPt = F;
670 MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
671 while (Dep.isDef() || Dep.isClobber()) {
672 Instruction *Dependency = Dep.getInst();
673 if (!hasMemoryWrite(Dependency, TLI) || !isRemovable(Dependency))
677 GetUnderlyingObject(getStoredPointerOperand(Dependency));
679 // Check for aliasing.
680 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
683 Instruction *Next = std::next(BasicBlock::iterator(Dependency));
685 // DCE instructions only used to calculate that store
686 DeleteDeadInstruction(Dependency, *MD, TLI);
690 // Inst's old Dependency is now deleted. Compute the next dependency,
691 // which may also be dead, as in
693 // s[1] = 0; // This has just been deleted.
695 Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
698 if (Dep.isNonLocal())
699 FindUnconditionalPreds(Blocks, BB, DT);
705 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
706 /// function end block. Ex:
709 /// store i32 1, i32* %A
711 bool DSE::handleEndBlock(BasicBlock &BB) {
712 bool MadeChange = false;
714 // Keep track of all of the stack objects that are dead at the end of the
716 SmallSetVector<Value*, 16> DeadStackObjects;
718 // Find all of the alloca'd pointers in the entry block.
719 BasicBlock *Entry = BB.getParent()->begin();
720 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
721 if (isa<AllocaInst>(I))
722 DeadStackObjects.insert(I);
724 // Okay, so these are dead heap objects, but if the pointer never escapes
725 // then it's leaked by this function anyways.
726 else if (isAllocLikeFn(I, TLI) && !PointerMayBeCaptured(I, true, true))
727 DeadStackObjects.insert(I);
730 // Treat byval or inalloca arguments the same, stores to them are dead at the
731 // end of the function.
732 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
733 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
734 if (AI->hasByValOrInAllocaAttr())
735 DeadStackObjects.insert(AI);
737 // Scan the basic block backwards
738 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
741 // If we find a store, check to see if it points into a dead stack value.
742 if (hasMemoryWrite(BBI, TLI) && isRemovable(BBI)) {
743 // See through pointer-to-pointer bitcasts
744 SmallVector<Value *, 4> Pointers;
745 GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers);
747 // Stores to stack values are valid candidates for removal.
749 for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
750 E = Pointers.end(); I != E; ++I)
751 if (!DeadStackObjects.count(*I)) {
757 Instruction *Dead = BBI++;
759 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
760 << *Dead << "\n Objects: ";
761 for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
762 E = Pointers.end(); I != E; ++I) {
764 if (std::next(I) != E)
769 // DCE instructions only used to calculate that store.
770 DeleteDeadInstruction(Dead, *MD, TLI, &DeadStackObjects);
777 // Remove any dead non-memory-mutating instructions.
778 if (isInstructionTriviallyDead(BBI, TLI)) {
779 Instruction *Inst = BBI++;
780 DeleteDeadInstruction(Inst, *MD, TLI, &DeadStackObjects);
786 if (isa<AllocaInst>(BBI)) {
787 // Remove allocas from the list of dead stack objects; there can't be
788 // any references before the definition.
789 DeadStackObjects.remove(BBI);
793 if (CallSite CS = cast<Value>(BBI)) {
794 // Remove allocation function calls from the list of dead stack objects;
795 // there can't be any references before the definition.
796 if (isAllocLikeFn(BBI, TLI))
797 DeadStackObjects.remove(BBI);
799 // If this call does not access memory, it can't be loading any of our
801 if (AA->doesNotAccessMemory(CS))
804 // If the call might load from any of our allocas, then any store above
806 DeadStackObjects.remove_if([&](Value *I) {
807 // See if the call site touches the value.
808 AliasAnalysis::ModRefResult A =
809 AA->getModRefInfo(CS, I, getPointerSize(I, *AA));
811 return A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref;
814 // If all of the allocas were clobbered by the call then we're not going
815 // to find anything else to process.
816 if (DeadStackObjects.empty())
822 AliasAnalysis::Location LoadedLoc;
824 // If we encounter a use of the pointer, it is no longer considered dead
825 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
826 if (!L->isUnordered()) // Be conservative with atomic/volatile load
828 LoadedLoc = AA->getLocation(L);
829 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
830 LoadedLoc = AA->getLocation(V);
831 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
832 LoadedLoc = AA->getLocationForSource(MTI);
833 } else if (!BBI->mayReadFromMemory()) {
834 // Instruction doesn't read memory. Note that stores that weren't removed
835 // above will hit this case.
838 // Unknown inst; assume it clobbers everything.
842 // Remove any allocas from the DeadPointer set that are loaded, as this
843 // makes any stores above the access live.
844 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
846 // If all of the allocas were clobbered by the access then we're not going
847 // to find anything else to process.
848 if (DeadStackObjects.empty())
855 /// RemoveAccessedObjects - Check to see if the specified location may alias any
856 /// of the stack objects in the DeadStackObjects set. If so, they become live
857 /// because the location is being loaded.
858 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
859 SmallSetVector<Value*, 16> &DeadStackObjects) {
860 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
862 // A constant can't be in the dead pointer set.
863 if (isa<Constant>(UnderlyingPointer))
866 // If the kill pointer can be easily reduced to an alloca, don't bother doing
867 // extraneous AA queries.
868 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
869 DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
873 // Remove objects that could alias LoadedLoc.
874 DeadStackObjects.remove_if([&](Value *I) {
875 // See if the loaded location could alias the stack location.
876 AliasAnalysis::Location StackLoc(I, getPointerSize(I, *AA));
877 return !AA->isNoAlias(StackLoc, LoadedLoc);