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.
262 /// getStoredPointerOperand - Return the pointer that is being written to.
263 static Value *getStoredPointerOperand(Instruction *I) {
264 if (StoreInst *SI = dyn_cast<StoreInst>(I))
265 return SI->getPointerOperand();
266 if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
267 return MI->getDest();
269 IntrinsicInst *II = cast<IntrinsicInst>(I);
270 switch (II->getIntrinsicID()) {
271 default: llvm_unreachable("Unexpected intrinsic!");
272 case Intrinsic::init_trampoline:
273 return II->getArgOperand(0);
277 static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
278 const TargetData *TD = AA.getTargetData();
280 if (const CallInst *CI = extractMallocCall(V)) {
281 if (const ConstantInt *C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
282 return C->getZExtValue();
286 return AliasAnalysis::UnknownSize;
288 if (const AllocaInst *A = dyn_cast<AllocaInst>(V)) {
289 // Get size information for the alloca
290 if (const ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
291 return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
294 if (const Argument *A = dyn_cast<Argument>(V)) {
295 if (A->hasByValAttr())
296 if (PointerType *PT = dyn_cast<PointerType>(A->getType()))
297 return TD->getTypeAllocSize(PT->getElementType());
300 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
301 if (!GV->mayBeOverridden())
302 return TD->getTypeAllocSize(GV->getType()->getElementType());
305 return AliasAnalysis::UnknownSize;
317 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
318 /// completely overwrites a store to the 'Earlier' location.
319 /// 'OverwriteEnd' if the end of the 'Earlier' location is completely
320 /// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
321 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
322 const AliasAnalysis::Location &Earlier,
326 const Value *P1 = Earlier.Ptr->stripPointerCasts();
327 const Value *P2 = Later.Ptr->stripPointerCasts();
329 // If the start pointers are the same, we just have to compare sizes to see if
330 // the later store was larger than the earlier store.
332 // If we don't know the sizes of either access, then we can't do a
334 if (Later.Size == AliasAnalysis::UnknownSize ||
335 Earlier.Size == AliasAnalysis::UnknownSize) {
336 // If we have no TargetData information around, then the size of the store
337 // is inferrable from the pointee type. If they are the same type, then
338 // we know that the store is safe.
339 if (AA.getTargetData() == 0 &&
340 Later.Ptr->getType() == Earlier.Ptr->getType())
341 return OverwriteComplete;
343 return OverwriteUnknown;
346 // Make sure that the Later size is >= the Earlier size.
347 if (Later.Size >= Earlier.Size)
348 return OverwriteComplete;
351 // Otherwise, we have to have size information, and the later store has to be
352 // larger than the earlier one.
353 if (Later.Size == AliasAnalysis::UnknownSize ||
354 Earlier.Size == AliasAnalysis::UnknownSize ||
355 AA.getTargetData() == 0)
356 return OverwriteUnknown;
358 // Check to see if the later store is to the entire object (either a global,
359 // an alloca, or a byval argument). If so, then it clearly overwrites any
360 // other store to the same object.
361 const TargetData &TD = *AA.getTargetData();
363 const Value *UO1 = GetUnderlyingObject(P1, &TD),
364 *UO2 = GetUnderlyingObject(P2, &TD);
366 // If we can't resolve the same pointers to the same object, then we can't
367 // analyze them at all.
369 return OverwriteUnknown;
371 // If the "Later" store is to a recognizable object, get its size.
372 uint64_t ObjectSize = getPointerSize(UO2, AA);
373 if (ObjectSize != AliasAnalysis::UnknownSize)
374 if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
375 return OverwriteComplete;
377 // Okay, we have stores to two completely different pointers. Try to
378 // decompose the pointer into a "base + constant_offset" form. If the base
379 // pointers are equal, then we can reason about the two stores.
382 const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
383 const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
385 // If the base pointers still differ, we have two completely different stores.
387 return OverwriteUnknown;
389 // The later store completely overlaps the earlier store if:
391 // 1. Both start at the same offset and the later one's size is greater than
392 // or equal to the earlier one's, or
397 // 2. The earlier store has an offset greater than the later offset, but which
398 // still lies completely within the later store.
401 // |----- later ------|
403 // We have to be careful here as *Off is signed while *.Size is unsigned.
404 if (EarlierOff >= LaterOff &&
405 Later.Size > Earlier.Size &&
406 uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
407 return OverwriteComplete;
409 // The other interesting case is if the later store overwrites the end of
415 // In this case we may want to trim the size of earlier to avoid generating
416 // writes to addresses which will definitely be overwritten later
417 if (LaterOff > EarlierOff &&
418 LaterOff < int64_t(EarlierOff + Earlier.Size) &&
419 int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
422 // Otherwise, they don't completely overlap.
423 return OverwriteUnknown;
426 /// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
427 /// memory region into an identical pointer) then it doesn't actually make its
428 /// input dead in the traditional sense. Consider this case:
433 /// In this case, the second store to A does not make the first store to A dead.
434 /// The usual situation isn't an explicit A<-A store like this (which can be
435 /// trivially removed) but a case where two pointers may alias.
437 /// This function detects when it is unsafe to remove a dependent instruction
438 /// because the DSE inducing instruction may be a self-read.
439 static bool isPossibleSelfRead(Instruction *Inst,
440 const AliasAnalysis::Location &InstStoreLoc,
441 Instruction *DepWrite, AliasAnalysis &AA) {
442 // Self reads can only happen for instructions that read memory. Get the
444 AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
445 if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
447 // If the read and written loc obviously don't alias, it isn't a read.
448 if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
450 // Okay, 'Inst' may copy over itself. However, we can still remove a the
451 // DepWrite instruction if we can prove that it reads from the same location
452 // as Inst. This handles useful cases like:
455 // Here we don't know if A/B may alias, but we do know that B/B are must
456 // aliases, so removing the first memcpy is safe (assuming it writes <= #
457 // bytes as the second one.
458 AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
460 if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
463 // If DepWrite doesn't read memory or if we can't prove it is a must alias,
464 // then it can't be considered dead.
469 //===----------------------------------------------------------------------===//
471 //===----------------------------------------------------------------------===//
473 bool DSE::runOnBasicBlock(BasicBlock &BB) {
474 bool MadeChange = false;
476 // Do a top-down walk on the BB.
477 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
478 Instruction *Inst = BBI++;
480 // Handle 'free' calls specially.
481 if (CallInst *F = isFreeCall(Inst)) {
482 MadeChange |= HandleFree(F);
486 // If we find something that writes memory, get its memory dependence.
487 if (!hasMemoryWrite(Inst))
490 MemDepResult InstDep = MD->getDependency(Inst);
492 // Ignore any store where we can't find a local dependence.
493 // FIXME: cross-block DSE would be fun. :)
494 if (!InstDep.isDef() && !InstDep.isClobber())
497 // If we're storing the same value back to a pointer that we just
498 // loaded from, then the store can be removed.
499 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
500 if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
501 if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
502 SI->getOperand(0) == DepLoad && isRemovable(SI)) {
503 DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
504 << "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
506 // DeleteDeadInstruction can delete the current instruction. Save BBI
507 // in case we need it.
508 WeakVH NextInst(BBI);
510 DeleteDeadInstruction(SI, *MD);
512 if (NextInst == 0) // Next instruction deleted.
514 else if (BBI != BB.begin()) // Revisit this instruction if possible.
523 // Figure out what location is being stored to.
524 AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
526 // If we didn't get a useful location, fail.
530 while (InstDep.isDef() || InstDep.isClobber()) {
531 // Get the memory clobbered by the instruction we depend on. MemDep will
532 // skip any instructions that 'Loc' clearly doesn't interact with. If we
533 // end up depending on a may- or must-aliased load, then we can't optimize
534 // away the store and we bail out. However, if we depend on on something
535 // that overwrites the memory location we *can* potentially optimize it.
537 // Find out what memory location the dependent instruction stores.
538 Instruction *DepWrite = InstDep.getInst();
539 AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
540 // If we didn't get a useful location, or if it isn't a size, bail out.
544 // If we find a write that is a) removable (i.e., non-volatile), b) is
545 // completely obliterated by the store to 'Loc', and c) which we know that
546 // 'Inst' doesn't load from, then we can remove it.
547 if (isRemovable(DepWrite) &&
548 !isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
549 int64_t InstWriteOffset, DepWriteOffset;
550 OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
551 DepWriteOffset, InstWriteOffset);
552 if (OR == OverwriteComplete) {
553 DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
554 << *DepWrite << "\n KILLER: " << *Inst << '\n');
556 // Delete the store and now-dead instructions that feed it.
557 DeleteDeadInstruction(DepWrite, *MD);
561 // DeleteDeadInstruction can delete the current instruction in loop
564 if (BBI != BB.begin())
567 } else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
568 // TODO: base this on the target vector size so that if the earlier
569 // store was too small to get vector writes anyway then its likely
570 // a good idea to shorten it
571 // Power of 2 vector writes are probably always a bad idea to optimize
572 // as any store/memset/memcpy is likely using vector instructions so
573 // shortening it to not vector size is likely to be slower
574 MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
575 unsigned DepWriteAlign = DepIntrinsic->getAlignment();
576 if (llvm::isPowerOf2_64(InstWriteOffset) ||
577 ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
579 DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
580 << *DepWrite << "\n KILLER (offset "
581 << InstWriteOffset << ", "
582 << DepLoc.Size << ")"
585 Value* DepWriteLength = DepIntrinsic->getLength();
586 Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
589 DepIntrinsic->setLength(TrimmedLength);
595 // If this is a may-aliased store that is clobbering the store value, we
596 // can keep searching past it for another must-aliased pointer that stores
597 // to the same location. For example, in:
601 // we can remove the first store to P even though we don't know if P and Q
603 if (DepWrite == &BB.front()) break;
605 // Can't look past this instruction if it might read 'Loc'.
606 if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
609 InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
613 // If this block ends in a return, unwind, or unreachable, all allocas are
614 // dead at its end, which means stores to them are also dead.
615 if (BB.getTerminator()->getNumSuccessors() == 0)
616 MadeChange |= handleEndBlock(BB);
621 /// Find all blocks that will unconditionally lead to the block BB and append
623 static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
624 BasicBlock *BB, DominatorTree *DT) {
625 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
626 BasicBlock *Pred = *I;
627 if (Pred == BB) continue;
628 TerminatorInst *PredTI = Pred->getTerminator();
629 if (PredTI->getNumSuccessors() != 1)
632 if (DT->isReachableFromEntry(Pred))
633 Blocks.push_back(Pred);
637 /// HandleFree - Handle frees of entire structures whose dependency is a store
638 /// to a field of that structure.
639 bool DSE::HandleFree(CallInst *F) {
640 bool MadeChange = false;
642 AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
643 SmallVector<BasicBlock *, 16> Blocks;
644 Blocks.push_back(F->getParent());
646 while (!Blocks.empty()) {
647 BasicBlock *BB = Blocks.pop_back_val();
648 Instruction *InstPt = BB->getTerminator();
649 if (BB == F->getParent()) InstPt = F;
651 MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
652 while (Dep.isDef() || Dep.isClobber()) {
653 Instruction *Dependency = Dep.getInst();
654 if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
658 GetUnderlyingObject(getStoredPointerOperand(Dependency));
660 // Check for aliasing.
661 if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
664 Instruction *Next = llvm::next(BasicBlock::iterator(Dependency));
666 // DCE instructions only used to calculate that store
667 DeleteDeadInstruction(Dependency, *MD);
671 // Inst's old Dependency is now deleted. Compute the next dependency,
672 // which may also be dead, as in
674 // s[1] = 0; // This has just been deleted.
676 Dep = MD->getPointerDependencyFrom(Loc, false, Next, BB);
679 if (Dep.isNonLocal())
680 FindUnconditionalPreds(Blocks, BB, DT);
686 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
687 /// function end block. Ex:
690 /// store i32 1, i32* %A
692 bool DSE::handleEndBlock(BasicBlock &BB) {
693 bool MadeChange = false;
695 // Keep track of all of the stack objects that are dead at the end of the
697 SmallPtrSet<Value*, 16> DeadStackObjects;
699 // Find all of the alloca'd pointers in the entry block.
700 BasicBlock *Entry = BB.getParent()->begin();
701 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
702 if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
703 DeadStackObjects.insert(AI);
705 // Okay, so these are dead heap objects, but if the pointer never escapes
706 // then it's leaked by this function anyways.
707 if (CallInst *CI = extractMallocCall(I))
708 if (!PointerMayBeCaptured(CI, true, true))
709 DeadStackObjects.insert(CI);
712 // Treat byval arguments the same, stores to them are dead at the end of the
714 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
715 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
716 if (AI->hasByValAttr())
717 DeadStackObjects.insert(AI);
719 // Scan the basic block backwards
720 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
723 // If we find a store, check to see if it points into a dead stack value.
724 if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
725 // See through pointer-to-pointer bitcasts
726 Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));
728 // Stores to stack values are valid candidates for removal.
729 if (DeadStackObjects.count(Pointer)) {
730 Instruction *Dead = BBI++;
732 DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
733 << *Dead << "\n Object: " << *Pointer << '\n');
735 // DCE instructions only used to calculate that store.
736 DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
743 // Remove any dead non-memory-mutating instructions.
744 if (isInstructionTriviallyDead(BBI)) {
745 Instruction *Inst = BBI++;
746 DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
752 if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
753 DeadStackObjects.erase(A);
757 if (CallInst *CI = extractMallocCall(BBI)) {
758 DeadStackObjects.erase(CI);
762 if (CallSite CS = cast<Value>(BBI)) {
763 // If this call does not access memory, it can't be loading any of our
765 if (AA->doesNotAccessMemory(CS))
768 // If the call might load from any of our allocas, then any store above
770 SmallVector<Value*, 8> LiveAllocas;
771 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
772 E = DeadStackObjects.end(); I != E; ++I) {
773 // See if the call site touches it.
774 AliasAnalysis::ModRefResult A =
775 AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
777 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
778 LiveAllocas.push_back(*I);
781 for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
782 E = LiveAllocas.end(); I != E; ++I)
783 DeadStackObjects.erase(*I);
785 // If all of the allocas were clobbered by the call then we're not going
786 // to find anything else to process.
787 if (DeadStackObjects.empty())
793 AliasAnalysis::Location LoadedLoc;
795 // If we encounter a use of the pointer, it is no longer considered dead
796 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
797 if (!L->isUnordered()) // Be conservative with atomic/volatile load
799 LoadedLoc = AA->getLocation(L);
800 } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
801 LoadedLoc = AA->getLocation(V);
802 } else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
803 LoadedLoc = AA->getLocationForSource(MTI);
804 } else if (!BBI->mayReadFromMemory()) {
805 // Instruction doesn't read memory. Note that stores that weren't removed
806 // above will hit this case.
809 // Unknown inst; assume it clobbers everything.
813 // Remove any allocas from the DeadPointer set that are loaded, as this
814 // makes any stores above the access live.
815 RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
817 // If all of the allocas were clobbered by the access then we're not going
818 // to find anything else to process.
819 if (DeadStackObjects.empty())
826 /// RemoveAccessedObjects - Check to see if the specified location may alias any
827 /// of the stack objects in the DeadStackObjects set. If so, they become live
828 /// because the location is being loaded.
829 void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
830 SmallPtrSet<Value*, 16> &DeadStackObjects) {
831 const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
833 // A constant can't be in the dead pointer set.
834 if (isa<Constant>(UnderlyingPointer))
837 // If the kill pointer can be easily reduced to an alloca, don't bother doing
838 // extraneous AA queries.
839 if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
840 DeadStackObjects.erase(const_cast<Value*>(UnderlyingPointer));
844 SmallVector<Value*, 16> NowLive;
845 for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
846 E = DeadStackObjects.end(); I != E; ++I) {
847 // See if the loaded location could alias the stack location.
848 AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
849 if (!AA->isNoAlias(StackLoc, LoadedLoc))
850 NowLive.push_back(*I);
853 for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
855 DeadStackObjects.erase(*I);