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 if (!S->isVolatile() && S->getParent() == L->getParent() &&
152 S->getPointerOperand() == L->getPointerOperand()) {
153 MemDepResult dep = MD.getDependency(S);
154 if (dep.isDef() && dep.getInst() == L) {
155 DeleteDeadInstruction(S);
156 if (BBI != BB.begin())
161 // Update our most-recent-store map.
165 // Update our most-recent-store map.
169 // Update our most-recent-store map.
175 // If this block ends in a return, unwind, or unreachable, all allocas are
176 // dead at its end, which means stores to them are also dead.
177 if (BB.getTerminator()->getNumSuccessors() == 0)
178 MadeChange |= handleEndBlock(BB);
183 /// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
184 /// dependency is a store to a field of that structure.
185 bool DSE::handleFreeWithNonTrivialDependency(FreeInst* F, MemDepResult dep) {
186 TargetData &TD = getAnalysis<TargetData>();
187 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
189 StoreInst* dependency = dyn_cast_or_null<StoreInst>(dep.getInst());
192 else if (dependency->isVolatile())
195 Value* depPointer = dependency->getPointerOperand();
196 const Type* depType = dependency->getOperand(0)->getType();
197 unsigned depPointerSize = TD.getTypeStoreSize(depType);
199 // Check for aliasing
200 AliasAnalysis::AliasResult A = AA.alias(F->getPointerOperand(), ~0U,
201 depPointer, depPointerSize);
203 if (A != AliasAnalysis::MustAlias)
206 // DCE instructions only used to calculate that store
207 DeleteDeadInstruction(dependency);
212 /// handleEndBlock - Remove dead stores to stack-allocated locations in the
213 /// function end block. Ex:
216 /// store i32 1, i32* %A
218 bool DSE::handleEndBlock(BasicBlock &BB) {
219 TargetData &TD = getAnalysis<TargetData>();
220 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
222 bool MadeChange = false;
224 // Pointers alloca'd in this function are dead in the end block
225 SmallPtrSet<Value*, 64> deadPointers;
227 // Find all of the alloca'd pointers in the entry block.
228 BasicBlock *Entry = BB.getParent()->begin();
229 for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
230 if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
231 deadPointers.insert(AI);
233 // Treat byval arguments the same, stores to them are dead at the end of the
235 for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
236 AE = BB.getParent()->arg_end(); AI != AE; ++AI)
237 if (AI->hasByValAttr())
238 deadPointers.insert(AI);
240 // Scan the basic block backwards
241 for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
244 // If we find a store whose pointer is dead.
245 if (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
246 if (!S->isVolatile()) {
247 // See through pointer-to-pointer bitcasts
248 Value* pointerOperand = S->getPointerOperand()->getUnderlyingObject();
250 // Alloca'd pointers or byval arguments (which are functionally like
251 // alloca's) are valid candidates for removal.
252 if (deadPointers.count(pointerOperand)) {
253 // DCE instructions only used to calculate that store.
255 DeleteDeadInstruction(S, &deadPointers);
264 // We can also remove memcpy's to local variables at the end of a function.
265 if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
266 Value *dest = M->getDest()->getUnderlyingObject();
268 if (deadPointers.count(dest)) {
270 DeleteDeadInstruction(M, &deadPointers);
276 // Because a memcpy is also a load, we can't skip it if we didn't remove
280 Value* killPointer = 0;
281 uint64_t killPointerSize = ~0UL;
283 // If we encounter a use of the pointer, it is no longer considered dead
284 if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
285 // However, if this load is unused and not volatile, we can go ahead and
286 // remove it, and not have to worry about it making our pointer undead!
287 if (L->use_empty() && !L->isVolatile()) {
289 DeleteDeadInstruction(L, &deadPointers);
295 killPointer = L->getPointerOperand();
296 } else if (VAArgInst* V = dyn_cast<VAArgInst>(BBI)) {
297 killPointer = V->getOperand(0);
298 } else if (isa<MemCpyInst>(BBI) &&
299 isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
300 killPointer = cast<MemCpyInst>(BBI)->getSource();
301 killPointerSize = cast<ConstantInt>(
302 cast<MemCpyInst>(BBI)->getLength())->getZExtValue();
303 } else if (AllocaInst* A = dyn_cast<AllocaInst>(BBI)) {
304 deadPointers.erase(A);
306 // Dead alloca's can be DCE'd when we reach them
307 if (A->use_empty()) {
309 DeleteDeadInstruction(A, &deadPointers);
315 } else if (CallSite::get(BBI).getInstruction() != 0) {
316 // If this call does not access memory, it can't
317 // be undeadifying any of our pointers.
318 CallSite CS = CallSite::get(BBI);
319 if (AA.doesNotAccessMemory(CS))
325 // Remove any pointers made undead by the call from the dead set
326 std::vector<Value*> dead;
327 for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
328 E = deadPointers.end(); I != E; ++I) {
329 // HACK: if we detect that our AA is imprecise, it's not
330 // worth it to scan the rest of the deadPointers set. Just
331 // assume that the AA will return ModRef for everything, and
332 // go ahead and bail.
333 if (modRef >= 16 && other == 0) {
334 deadPointers.clear();
338 // Get size information for the alloca
339 unsigned pointerSize = ~0U;
340 if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
341 if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
342 pointerSize = C->getZExtValue() *
343 TD.getABITypeSize(A->getAllocatedType());
345 const PointerType* PT = cast<PointerType>(
346 cast<Argument>(*I)->getType());
347 pointerSize = TD.getABITypeSize(PT->getElementType());
350 // See if the call site touches it
351 AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
353 if (A == AliasAnalysis::ModRef)
358 if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
362 for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
364 deadPointers.erase(*I);
367 } else if (isInstructionTriviallyDead(BBI)) {
368 // For any non-memory-affecting non-terminators, DCE them as we reach them
369 Instruction *Inst = BBI;
371 DeleteDeadInstruction(Inst, &deadPointers);
380 killPointer = killPointer->getUnderlyingObject();
382 // Deal with undead pointers
383 MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
390 /// RemoveUndeadPointers - check for uses of a pointer that make it
391 /// undead when scanning for dead stores to alloca's.
392 bool DSE::RemoveUndeadPointers(Value* killPointer, uint64_t killPointerSize,
393 BasicBlock::iterator &BBI,
394 SmallPtrSet<Value*, 64>& deadPointers) {
395 TargetData &TD = getAnalysis<TargetData>();
396 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
398 // If the kill pointer can be easily reduced to an alloca,
399 // don't bother doing extraneous AA queries.
400 if (deadPointers.count(killPointer)) {
401 deadPointers.erase(killPointer);
405 // A global can't be in the dead pointer set.
406 if (isa<GlobalValue>(killPointer))
409 bool MadeChange = false;
411 SmallVector<Value*, 16> undead;
413 for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
414 E = deadPointers.end(); I != E; ++I) {
415 // Get size information for the alloca.
416 unsigned pointerSize = ~0U;
417 if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
418 if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
419 pointerSize = C->getZExtValue() *
420 TD.getABITypeSize(A->getAllocatedType());
422 const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
423 pointerSize = TD.getABITypeSize(PT->getElementType());
426 // See if this pointer could alias it
427 AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
428 killPointer, killPointerSize);
430 // If it must-alias and a store, we can delete it
431 if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
432 StoreInst* S = cast<StoreInst>(BBI);
436 DeleteDeadInstruction(S, &deadPointers);
442 // Otherwise, it is undead
443 } else if (A != AliasAnalysis::NoAlias)
444 undead.push_back(*I);
447 for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
449 deadPointers.erase(*I);
454 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
455 /// and zero out all the operands of this instruction. If any of them become
456 /// dead, delete them and the computation tree that feeds them.
458 /// If ValueSet is non-null, remove any deleted instructions from it as well.
460 void DSE::DeleteDeadInstruction(Instruction *I,
461 SmallPtrSet<Value*, 64> *ValueSet) {
462 SmallVector<Instruction*, 32> NowDeadInsts;
464 NowDeadInsts.push_back(I);
467 // Before we touch this instruction, remove it from memdep!
468 MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
469 while (!NowDeadInsts.empty()) {
470 Instruction *DeadInst = NowDeadInsts.back();
471 NowDeadInsts.pop_back();
475 // This instruction is dead, zap it, in stages. Start by removing it from
476 // MemDep, which needs to know the operands and needs it to be in the
478 MDA.removeInstruction(DeadInst);
480 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
481 Value *Op = DeadInst->getOperand(op);
482 DeadInst->setOperand(op, 0);
484 // If this operand just became dead, add it to the NowDeadInsts list.
485 if (!Op->use_empty()) continue;
487 if (Instruction *OpI = dyn_cast<Instruction>(Op))
488 if (isInstructionTriviallyDead(OpI))
489 NowDeadInsts.push_back(OpI);
492 DeadInst->eraseFromParent();
494 if (ValueSet) ValueSet->erase(DeadInst);