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/Instructions.h"
23 #include "llvm/IntrinsicInst.h"
24 #include "llvm/Pass.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/Analysis/Dominators.h"
29 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
30 #include "llvm/Target/TargetData.h"
31 #include "llvm/Transforms/Utils/Local.h"
32 #include "llvm/Support/Compiler.h"
35 STATISTIC(NumFastStores, "Number of stores deleted");
36 STATISTIC(NumFastOther , "Number of other instrs removed");
39 struct VISIBILITY_HIDDEN DSE : public FunctionPass {
40 static char ID; // Pass identification, replacement for typeid
41 DSE() : FunctionPass(&ID) {}
43 virtual bool runOnFunction(Function &F) {
45 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
46 Changed |= runOnBasicBlock(*I);
50 bool runOnBasicBlock(BasicBlock &BB);
51 bool handleFreeWithNonTrivialDependency(FreeInst *F, MemDepResult Dep);
52 bool handleEndBlock(BasicBlock &BB);
53 bool RemoveUndeadPointers(Value* pointer, uint64_t killPointerSize,
54 BasicBlock::iterator& BBI,
55 SmallPtrSet<Value*, 64>& deadPointers);
56 void DeleteDeadInstruction(Instruction *I,
57 SmallPtrSet<Value*, 64> *deadPointers = 0);
60 // getAnalysisUsage - We require post dominance frontiers (aka Control
62 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
64 AU.addRequired<DominatorTree>();
65 AU.addRequired<TargetData>();
66 AU.addRequired<AliasAnalysis>();
67 AU.addRequired<MemoryDependenceAnalysis>();
68 AU.addPreserved<DominatorTree>();
69 AU.addPreserved<AliasAnalysis>();
70 AU.addPreserved<MemoryDependenceAnalysis>();
76 static RegisterPass<DSE> X("dse", "Dead Store Elimination");
78 FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
80 bool DSE::runOnBasicBlock(BasicBlock &BB) {
81 MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
82 TargetData &TD = getAnalysis<TargetData>();
84 // Record the last-seen store to this pointer
85 DenseMap<Value*, StoreInst*> lastStore;
87 bool MadeChange = false;
89 // Do a top-down walk on the BB
90 for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
91 Instruction *Inst = BBI++;
93 // If we find a store or a free...
94 if (!isa<StoreInst>(Inst) && !isa<FreeInst>(Inst))
98 if (StoreInst* S = dyn_cast<StoreInst>(Inst)) {
101 pointer = S->getPointerOperand();
103 pointer = cast<FreeInst>(Inst)->getPointerOperand();
106 pointer = pointer->stripPointerCasts();
107 StoreInst *&last = lastStore[pointer];
109 // ... to a pointer that has been stored to before...
111 MemDepResult dep = MD.getDependency(Inst);
112 bool deletedStore = false;
114 // ... and no other memory dependencies are between them....
115 while (StoreInst *DepStore = dyn_cast_or_null<StoreInst>(dep.getInst())) {
116 if (DepStore != last ||
117 TD.getTypeStoreSize(last->getOperand(0)->getType()) >
118 TD.getTypeStoreSize(Inst->getOperand(0)->getType())) {
119 dep = MD.getDependencyFrom(Inst, DepStore, DepStore->getParent());
123 // Delete the store and now-dead instructions that feed it.
124 DeleteDeadInstruction(last);
131 // If we deleted a store, reinvestigate this instruction.
133 if (BBI != BB.begin())
139 // Handle frees whose dependencies are non-trivial.
140 if (FreeInst* F = dyn_cast<FreeInst>(Inst)) {
141 MadeChange |= handleFreeWithNonTrivialDependency(F, MD.getDependency(F));
143 // No known stores after the free.
146 StoreInst* S = cast<StoreInst>(Inst);
148 // If we're storing the same value back to a pointer that we just
149 // loaded from, then the store can be removed;
150 if (LoadInst* L = dyn_cast<LoadInst>(S->getOperand(0))) {
151 // FIXME: Don't do dep query if Parents don't match and other stuff!
152 MemDepResult dep = MD.getDependency(S);
153 DominatorTree& DT = getAnalysis<DominatorTree>();
155 if (!S->isVolatile() && S->getParent() == L->getParent() &&
156 S->getPointerOperand() == L->getPointerOperand() &&
157 (!dep.isNormal() || DT.dominates(dep.getInst(), L))) {
159 DeleteDeadInstruction(S);
160 if (BBI != BB.begin())
165 // Update our most-recent-store map.
168 // Update our most-recent-store map.
173 // If this block ends in a return, unwind, or unreachable, all allocas are
174 // dead at its end, which means stores to them are also dead.
175 if (BB.getTerminator()->getNumSuccessors() == 0)
176 MadeChange |= handleEndBlock(BB);
181 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
182 /// dependency is a store to a field of that structure.
183 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, MemDepResult dep) {
184 TargetData &TD = getAnalysis<TargetData>();
185 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
187 StoreInst* dependency = dyn_cast_or_null<StoreInst>(dep.getInst());
190 else if (dependency->isVolatile())
193 Value* depPointer = dependency->getPointerOperand();
194 const Type* depType = dependency->getOperand(0)->getType();
195 unsigned depPointerSize = TD.getTypeStoreSize(depType);
197 // Check for aliasing
198 AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0U,
199 depPointer, depPointerSize);
201 if (A != AliasAnalysis::MustAlias)
204 // DCE instructions only used to calculate that store
205 DeleteDeadInstruction(dependency);
210 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
211 /// function end block. Ex:
214 /// store i32 1, i32* %A
216 bool DSE::handleEndBlock(BasicBlock &BB) {
217 TargetData &TD = getAnalysis<TargetData>();
218 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
220 bool MadeChange = false;
222 // Pointers alloca'd in this function are dead in the end block
223 SmallPtrSet<Value*, 64> deadPointers;
225 // Find all of the alloca'd pointers in the entry block.
226 BasicBlock *Entry = BB.getParent()->begin();
227 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
228 if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
229 deadPointers.insert(AI);
231 // Treat byval arguments the same, stores to them are dead at the end of the
233 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
234 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
235 if (AI->hasByValAttr())
236 deadPointers.insert(AI);
238 // Scan the basic block backwards
239 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
242 // If we find a store whose pointer is dead.
243 if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
244 if (!S->isVolatile()) {
245 // See through pointer-to-pointer bitcasts
246 Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();
248 // Alloca'd pointers or byval arguments (which are functionally like
249 // alloca's) are valid candidates for removal.
250 if (deadPointers.count(pointerOperand)) {
251 // DCE instructions only used to calculate that store.
253 DeleteDeadInstruction(S, &deadPointers);
262 // We can also remove memcpy's to local variables at the end of a function.
263 if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
264 Value *dest = M->getDest()->getUnderlyingObject();
266 if (deadPointers.count(dest)) {
268 DeleteDeadInstruction(M, &deadPointers);
274 // Because a memcpy is also a load, we can't skip it if we didn't remove
278 Value* killPointer = 0;
279 uint64_t killPointerSize = ~0UL;
281 // If we encounter a use of the pointer, it is no longer considered dead
282 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
283 // However, if this load is unused and not volatile, we can go ahead and
284 // remove it, and not have to worry about it making our pointer undead!
285 if (L->use_empty() && !L->isVolatile()) {
287 DeleteDeadInstruction(L, &deadPointers);
293 killPointer = L->getPointerOperand();
294 } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
295 killPointer = V->getOperand(0);
296 } else if (isa<MemCpyInst>(BBI) &&
297 isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
298 killPointer = cast<MemCpyInst>(BBI)->getSource();
299 killPointerSize = cast<ConstantInt>(
300 cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
301 } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
302 deadPointers.erase(A);
304 // Dead alloca's can be DCE'd when we reach them
305 if (A->use_empty()) {
307 DeleteDeadInstruction(A, &deadPointers);
313 } else if (CallSite::get(BBI).getInstruction() != 0) {
314 // If this call does not access memory, it can't
315 // be undeadifying any of our pointers.
316 CallSite CS = CallSite::get(BBI);
317 if (AA.doesNotAccessMemory(CS))
323 // Remove any pointers made undead by the call from the dead set
324 std::vector<Value*> dead;
325 for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
326 E = deadPointers.end(); I != E; ++I) {
327 // HACK: if we detect that our AA is imprecise, it's not
328 // worth it to scan the rest of the deadPointers set. Just
329 // assume that the AA will return ModRef for everything, and
330 // go ahead and bail.
331 if (modRef >= 16 && other == 0) {
332 deadPointers.clear();
336 // Get size information for the alloca
337 unsigned pointerSize = ~0U;
338 if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
339 if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
340 pointerSize = C->getZExtValue() *
341 TD.getABITypeSize(A->getAllocatedType());
343 const PointerType* PT = cast<PointerType>(
344 cast<Argument>(*I)->getType());
345 pointerSize = TD.getABITypeSize(PT->getElementType());
348 // See if the call site touches it
349 AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
351 if (A == AliasAnalysis::ModRef)
356 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
360 for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
362 deadPointers.erase(*I);
365 } else if (isInstructionTriviallyDead(BBI)) {
366 // For any non-memory-affecting non-terminators, DCE them as we reach them
367 Instruction *Inst = BBI;
369 DeleteDeadInstruction(Inst, &deadPointers);
378 killPointer = killPointer->getUnderlyingObject();
380 // Deal with undead pointers
381 MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
388 /// RemoveUndeadPointers - check for uses of a pointer that make it
389 /// undead when scanning for dead stores to alloca's.
390 bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
391 BasicBlock::iterator &BBI,
392 SmallPtrSet<Value*, 64>& deadPointers) {
393 TargetData &TD = getAnalysis<TargetData>();
394 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
396 // If the kill pointer can be easily reduced to an alloca,
397 // don't bother doing extraneous AA queries.
398 if (deadPointers.count(killPointer)) {
399 deadPointers.erase(killPointer);
403 // A global can't be in the dead pointer set.
404 if (isa<GlobalValue>(killPointer))
407 bool MadeChange = false;
409 SmallVector<Value*, 16> undead;
411 for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
412 E = deadPointers.end(); I != E; ++I) {
413 // Get size information for the alloca.
414 unsigned pointerSize = ~0U;
415 if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
416 if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
417 pointerSize = C->getZExtValue() *
418 TD.getABITypeSize(A->getAllocatedType());
420 const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
421 pointerSize = TD.getABITypeSize(PT->getElementType());
424 // See if this pointer could alias it
425 AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
426 killPointer, killPointerSize);
428 // If it must-alias and a store, we can delete it
429 if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
430 StoreInst* S = cast<StoreInst>(BBI);
434 DeleteDeadInstruction(S, &deadPointers);
440 // Otherwise, it is undead
441 } else if (A != AliasAnalysis::NoAlias)
442 undead.push_back(*I);
445 for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
447 deadPointers.erase(*I);
452 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
453 /// and zero out all the operands of this instruction. If any of them become
454 /// dead, delete them and the computation tree that feeds them.
456 /// If ValueSet is non-null, remove any deleted instructions from it as well.
458 void DSE::DeleteDeadInstruction(Instruction *I,
459 SmallPtrSet<Value*, 64> *ValueSet) {
460 SmallVector<Instruction*, 32> NowDeadInsts;
462 NowDeadInsts.push_back(I);
465 // Before we touch this instruction, remove it from memdep!
466 MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
467 while (!NowDeadInsts.empty()) {
468 Instruction *DeadInst = NowDeadInsts.back();
469 NowDeadInsts.pop_back();
473 // This instruction is dead, zap it, in stages. Start by removing it from
474 // MemDep, which needs to know the operands and needs it to be in the
476 MDA.removeInstruction(DeadInst);
478 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
479 Value *Op = DeadInst->getOperand(op);
480 DeadInst->setOperand(op, 0);
482 // If this operand just became dead, add it to the NowDeadInsts list.
483 if (!Op->use_empty()) continue;
485 if (Instruction *OpI = dyn_cast<Instruction>(Op))
486 if (isInstructionTriviallyDead(OpI))
487 NowDeadInsts.push_back(OpI);
490 DeadInst->eraseFromParent();
492 if (ValueSet) ValueSet->erase(DeadInst);