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+//===- DeadStoreElimination.cpp - Fast Dead Store Elimination -------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a trivial dead store elimination that only considers
+// basic-block local redundant stores.
+//
+// FIXME: This should eventually be extended to be a post-dominator tree
+// traversal. Doing so would be pretty trivial.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "dse"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Constants.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/Pass.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/MemoryBuiltins.h"
+#include "llvm/Analysis/MemoryDependenceAnalysis.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Transforms/Utils/Local.h"
+using namespace llvm;
+
+STATISTIC(NumFastStores, "Number of stores deleted");
+STATISTIC(NumFastOther , "Number of other instrs removed");
+
+namespace {
+ struct DSE : public FunctionPass {
+ TargetData *TD;
+
+ static char ID; // Pass identification, replacement for typeid
+ DSE() : FunctionPass(&ID) {}
+
+ virtual bool runOnFunction(Function &F) {
+ bool Changed = false;
+
+ DominatorTree &DT = getAnalysis<DominatorTree>();
+
+ for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
+ // Only check non-dead blocks. Dead blocks may have strange pointer
+ // cycles that will confuse alias analysis.
+ if (DT.isReachableFromEntry(I))
+ Changed |= runOnBasicBlock(*I);
+ return Changed;
+ }
+
+ bool runOnBasicBlock(BasicBlock &BB);
+ bool handleFreeWithNonTrivialDependency(Instruction *F, MemDepResult Dep);
+ bool handleEndBlock(BasicBlock &BB);
+ bool RemoveUndeadPointers(Value *Ptr, uint64_t killPointerSize,
+ BasicBlock::iterator &BBI,
+ SmallPtrSet<Value*, 64> &deadPointers);
+ void DeleteDeadInstruction(Instruction *I,
+ SmallPtrSet<Value*, 64> *deadPointers = 0);
+
+
+ // getAnalysisUsage - We require post dominance frontiers (aka Control
+ // Dependence Graph)
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.setPreservesCFG();
+ AU.addRequired<DominatorTree>();
+ AU.addRequired<AliasAnalysis>();
+ AU.addRequired<MemoryDependenceAnalysis>();
+ AU.addPreserved<DominatorTree>();
+ AU.addPreserved<AliasAnalysis>();
+ AU.addPreserved<MemoryDependenceAnalysis>();
+ }
+
+ unsigned getPointerSize(Value *V) const;
+ };
+}
+
+char DSE::ID = 0;
+static RegisterPass<DSE> X("dse", "Dead Store Elimination");
+
+FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
+
+/// doesClobberMemory - Does this instruction clobber (write without reading)
+/// some memory?
+static bool doesClobberMemory(Instruction *I) {
+ if (isa<StoreInst>(I))
+ return true;
+ if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
+ switch (II->getIntrinsicID()) {
+ default:
+ return false;
+ case Intrinsic::memset:
+ case Intrinsic::memmove:
+ case Intrinsic::memcpy:
+ case Intrinsic::init_trampoline:
+ case Intrinsic::lifetime_end:
+ return true;
+ }
+ }
+ return false;
+}
+
+/// isElidable - If the value of this instruction and the memory it writes to is
+/// unused, may we delete this instrtction?
+static bool isElidable(Instruction *I) {
+ assert(doesClobberMemory(I));
+ if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
+ return II->getIntrinsicID() != Intrinsic::lifetime_end;
+ if (StoreInst *SI = dyn_cast<StoreInst>(I))
+ return !SI->isVolatile();
+ return true;
+}
+
+/// getPointerOperand - Return the pointer that is being clobbered.
+static Value *getPointerOperand(Instruction *I) {
+ assert(doesClobberMemory(I));
+ if (StoreInst *SI = dyn_cast<StoreInst>(I))
+ return SI->getPointerOperand();
+ if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
+ return MI->getOperand(1);
+
+ switch (cast<IntrinsicInst>(I)->getIntrinsicID()) {
+ default: assert(false && "Unexpected intrinsic!");
+ case Intrinsic::init_trampoline:
+ return I->getOperand(1);
+ case Intrinsic::lifetime_end:
+ return I->getOperand(2);
+ }
+}
+
+/// getStoreSize - Return the length in bytes of the write by the clobbering
+/// instruction. If variable or unknown, returns -1.
+static unsigned getStoreSize(Instruction *I, const TargetData *TD) {
+ assert(doesClobberMemory(I));
+ if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
+ if (!TD) return -1u;
+ return TD->getTypeStoreSize(SI->getOperand(0)->getType());
+ }
+
+ Value *Len;
+ if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I)) {
+ Len = MI->getLength();
+ } else {
+ switch (cast<IntrinsicInst>(I)->getIntrinsicID()) {
+ default: assert(false && "Unexpected intrinsic!");
+ case Intrinsic::init_trampoline:
+ return -1u;
+ case Intrinsic::lifetime_end:
+ Len = I->getOperand(1);
+ break;
+ }
+ }
+ if (ConstantInt *LenCI = dyn_cast<ConstantInt>(Len))
+ if (!LenCI->isAllOnesValue())
+ return LenCI->getZExtValue();
+ return -1u;
+}
+
+/// isStoreAtLeastAsWideAs - Return true if the size of the store in I1 is
+/// greater than or equal to the store in I2. This returns false if we don't
+/// know.
+///
+static bool isStoreAtLeastAsWideAs(Instruction *I1, Instruction *I2,
+ const TargetData *TD) {
+ const Type *I1Ty = getPointerOperand(I1)->getType();
+ const Type *I2Ty = getPointerOperand(I2)->getType();
+
+ // Exactly the same type, must have exactly the same size.
+ if (I1Ty == I2Ty) return true;
+
+ int I1Size = getStoreSize(I1, TD);
+ int I2Size = getStoreSize(I2, TD);
+
+ return I1Size != -1 && I2Size != -1 && I1Size >= I2Size;
+}
+
+bool DSE::runOnBasicBlock(BasicBlock &BB) {
+ MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
+ TD = getAnalysisIfAvailable<TargetData>();
+
+ bool MadeChange = false;
+
+ // Do a top-down walk on the BB.
+ for (BasicBlock::iterator BBI = BB.begin(), BBE = BB.end(); BBI != BBE; ) {
+ Instruction *Inst = BBI++;
+
+ // If we find a store or a free, get its memory dependence.
+ if (!doesClobberMemory(Inst) && !isFreeCall(Inst))
+ continue;
+
+ MemDepResult InstDep = MD.getDependency(Inst);
+
+ // Ignore non-local stores.
+ // FIXME: cross-block DSE would be fun. :)
+ if (InstDep.isNonLocal()) continue;
+
+ // Handle frees whose dependencies are non-trivial.
+ if (isFreeCall(Inst)) {
+ MadeChange |= handleFreeWithNonTrivialDependency(Inst, InstDep);
+ continue;
+ }
+
+ // If not a definite must-alias dependency, ignore it.
+ if (!InstDep.isDef())
+ continue;
+
+ // If this is a store-store dependence, then the previous store is dead so
+ // long as this store is at least as big as it.
+ if (doesClobberMemory(InstDep.getInst())) {
+ Instruction *DepStore = InstDep.getInst();
+ if (isStoreAtLeastAsWideAs(Inst, DepStore, TD) &&
+ isElidable(DepStore)) {
+ // Delete the store and now-dead instructions that feed it.
+ DeleteDeadInstruction(DepStore);
+ NumFastStores++;
+ MadeChange = true;
+
+ // DeleteDeadInstruction can delete the current instruction in loop
+ // cases, reset BBI.
+ BBI = Inst;
+ if (BBI != BB.begin())
+ --BBI;
+ continue;
+ }
+ }
+
+ if (!isElidable(Inst))
+ continue;
+
+ // If we're storing the same value back to a pointer that we just
+ // loaded from, then the store can be removed.
+ if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
+ if (LoadInst *DepLoad = dyn_cast<LoadInst>(InstDep.getInst())) {
+ if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
+ SI->getOperand(0) == DepLoad) {
+ // DeleteDeadInstruction can delete the current instruction. Save BBI
+ // in case we need it.
+ WeakVH NextInst(BBI);
+
+ DeleteDeadInstruction(SI);
+
+ if (NextInst == 0) // Next instruction deleted.
+ BBI = BB.begin();
+ else if (BBI != BB.begin()) // Revisit this instruction if possible.
+ --BBI;
+ NumFastStores++;
+ MadeChange = true;
+ continue;
+ }
+ }
+ }
+
+ // If this is a lifetime end marker, we can throw away the store.
+ if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(InstDep.getInst())) {
+ if (II->getIntrinsicID() == Intrinsic::lifetime_end) {
+ // Delete the store and now-dead instructions that feed it.
+ // DeleteDeadInstruction can delete the current instruction. Save BBI
+ // in case we need it.
+ WeakVH NextInst(BBI);
+
+ DeleteDeadInstruction(Inst);
+
+ if (NextInst == 0) // Next instruction deleted.
+ BBI = BB.begin();
+ else if (BBI != BB.begin()) // Revisit this instruction if possible.
+ --BBI;
+ NumFastStores++;
+ MadeChange = true;
+ continue;
+ }
+ }
+ }
+
+ // If this block ends in a return, unwind, or unreachable, all allocas are
+ // dead at its end, which means stores to them are also dead.
+ if (BB.getTerminator()->getNumSuccessors() == 0)
+ MadeChange |= handleEndBlock(BB);
+
+ return MadeChange;
+}
+
+/// handleFreeWithNonTrivialDependency - Handle frees of entire structures whose
+/// dependency is a store to a field of that structure.
+bool DSE::handleFreeWithNonTrivialDependency(Instruction *F, MemDepResult Dep) {
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+ Instruction *Dependency = Dep.getInst();
+ if (!Dependency || !doesClobberMemory(Dependency) || !isElidable(Dependency))
+ return false;
+
+ Value *DepPointer = getPointerOperand(Dependency)->getUnderlyingObject();
+
+ // Check for aliasing.
+ if (AA.alias(F->getOperand(1), 1, DepPointer, 1) !=
+ AliasAnalysis::MustAlias)
+ return false;
+
+ // DCE instructions only used to calculate that store
+ DeleteDeadInstruction(Dependency);
+ NumFastStores++;
+ return true;
+}
+
+/// handleEndBlock - Remove dead stores to stack-allocated locations in the
+/// function end block. Ex:
+/// %A = alloca i32
+/// ...
+/// store i32 1, i32* %A
+/// ret void
+bool DSE::handleEndBlock(BasicBlock &BB) {
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+ bool MadeChange = false;
+
+ // Pointers alloca'd in this function are dead in the end block
+ SmallPtrSet<Value*, 64> deadPointers;
+
+ // Find all of the alloca'd pointers in the entry block.
+ BasicBlock *Entry = BB.getParent()->begin();
+ for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I)
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
+ deadPointers.insert(AI);
+
+ // Treat byval arguments the same, stores to them are dead at the end of the
+ // function.
+ for (Function::arg_iterator AI = BB.getParent()->arg_begin(),
+ AE = BB.getParent()->arg_end(); AI != AE; ++AI)
+ if (AI->hasByValAttr())
+ deadPointers.insert(AI);
+
+ // Scan the basic block backwards
+ for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
+ --BBI;
+
+ // If we find a store whose pointer is dead.
+ if (doesClobberMemory(BBI)) {
+ if (isElidable(BBI)) {
+ // See through pointer-to-pointer bitcasts
+ Value *pointerOperand = getPointerOperand(BBI)->getUnderlyingObject();
+
+ // Alloca'd pointers or byval arguments (which are functionally like
+ // alloca's) are valid candidates for removal.
+ if (deadPointers.count(pointerOperand)) {
+ // DCE instructions only used to calculate that store.
+ Instruction *Dead = BBI;
+ BBI++;
+ DeleteDeadInstruction(Dead, &deadPointers);
+ NumFastStores++;
+ MadeChange = true;
+ continue;
+ }
+ }
+
+ // Because a memcpy or memmove is also a load, we can't skip it if we
+ // didn't remove it.
+ if (!isa<MemTransferInst>(BBI))
+ continue;
+ }
+
+ Value *killPointer = 0;
+ uint64_t killPointerSize = ~0UL;
+
+ // If we encounter a use of the pointer, it is no longer considered dead
+ if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
+ // However, if this load is unused and not volatile, we can go ahead and
+ // remove it, and not have to worry about it making our pointer undead!
+ if (L->use_empty() && !L->isVolatile()) {
+ BBI++;
+ DeleteDeadInstruction(L, &deadPointers);
+ NumFastOther++;
+ MadeChange = true;
+ continue;
+ }
+
+ killPointer = L->getPointerOperand();
+ } else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
+ killPointer = V->getOperand(0);
+ } else if (isa<MemTransferInst>(BBI) &&
+ isa<ConstantInt>(cast<MemTransferInst>(BBI)->getLength())) {
+ killPointer = cast<MemTransferInst>(BBI)->getSource();
+ killPointerSize = cast<ConstantInt>(
+ cast<MemTransferInst>(BBI)->getLength())->getZExtValue();
+ } else if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
+ deadPointers.erase(A);
+
+ // Dead alloca's can be DCE'd when we reach them
+ if (A->use_empty()) {
+ BBI++;
+ DeleteDeadInstruction(A, &deadPointers);
+ NumFastOther++;
+ MadeChange = true;
+ }
+
+ continue;
+ } else if (CallSite::get(BBI).getInstruction() != 0) {
+ // If this call does not access memory, it can't
+ // be undeadifying any of our pointers.
+ CallSite CS = CallSite::get(BBI);
+ if (AA.doesNotAccessMemory(CS))
+ continue;
+
+ unsigned modRef = 0;
+ unsigned other = 0;
+
+ // Remove any pointers made undead by the call from the dead set
+ std::vector<Value*> dead;
+ for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
+ E = deadPointers.end(); I != E; ++I) {
+ // HACK: if we detect that our AA is imprecise, it's not
+ // worth it to scan the rest of the deadPointers set. Just
+ // assume that the AA will return ModRef for everything, and
+ // go ahead and bail.
+ if (modRef >= 16 && other == 0) {
+ deadPointers.clear();
+ return MadeChange;
+ }
+
+ // See if the call site touches it
+ AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I,
+ getPointerSize(*I));
+
+ if (A == AliasAnalysis::ModRef)
+ modRef++;
+ else
+ other++;
+
+ if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
+ dead.push_back(*I);
+ }
+
+ for (std::vector<Value*>::iterator I = dead.begin(), E = dead.end();
+ I != E; ++I)
+ deadPointers.erase(*I);
+
+ continue;
+ } else if (isInstructionTriviallyDead(BBI)) {
+ // For any non-memory-affecting non-terminators, DCE them as we reach them
+ Instruction *Inst = BBI;
+ BBI++;
+ DeleteDeadInstruction(Inst, &deadPointers);
+ NumFastOther++;
+ MadeChange = true;
+ continue;
+ }
+
+ if (!killPointer)
+ continue;
+
+ killPointer = killPointer->getUnderlyingObject();
+
+ // Deal with undead pointers
+ MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
+ deadPointers);
+ }
+
+ return MadeChange;
+}
+
+/// RemoveUndeadPointers - check for uses of a pointer that make it
+/// undead when scanning for dead stores to alloca's.
+bool DSE::RemoveUndeadPointers(Value *killPointer, uint64_t killPointerSize,
+ BasicBlock::iterator &BBI,
+ SmallPtrSet<Value*, 64> &deadPointers) {
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+ // If the kill pointer can be easily reduced to an alloca,
+ // don't bother doing extraneous AA queries.
+ if (deadPointers.count(killPointer)) {
+ deadPointers.erase(killPointer);
+ return false;
+ }
+
+ // A global can't be in the dead pointer set.
+ if (isa<GlobalValue>(killPointer))
+ return false;
+
+ bool MadeChange = false;
+
+ SmallVector<Value*, 16> undead;
+
+ for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
+ E = deadPointers.end(); I != E; ++I) {
+ // See if this pointer could alias it
+ AliasAnalysis::AliasResult A = AA.alias(*I, getPointerSize(*I),
+ killPointer, killPointerSize);
+
+ // If it must-alias and a store, we can delete it
+ if (isa<StoreInst>(BBI) && A == AliasAnalysis::MustAlias) {
+ StoreInst *S = cast<StoreInst>(BBI);
+
+ // Remove it!
+ ++BBI;
+ DeleteDeadInstruction(S, &deadPointers);
+ NumFastStores++;
+ MadeChange = true;
+
+ continue;
+
+ // Otherwise, it is undead
+ } else if (A != AliasAnalysis::NoAlias)
+ undead.push_back(*I);
+ }
+
+ for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
+ I != E; ++I)
+ deadPointers.erase(*I);
+
+ return MadeChange;
+}
+
+/// DeleteDeadInstruction - Delete this instruction. Before we do, go through
+/// and zero out all the operands of this instruction. If any of them become
+/// dead, delete them and the computation tree that feeds them.
+///
+/// If ValueSet is non-null, remove any deleted instructions from it as well.
+///
+void DSE::DeleteDeadInstruction(Instruction *I,
+ SmallPtrSet<Value*, 64> *ValueSet) {
+ SmallVector<Instruction*, 32> NowDeadInsts;
+
+ NowDeadInsts.push_back(I);
+ --NumFastOther;
+
+ // Before we touch this instruction, remove it from memdep!
+ MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
+ do {
+ Instruction *DeadInst = NowDeadInsts.pop_back_val();
+
+ ++NumFastOther;
+
+ // This instruction is dead, zap it, in stages. Start by removing it from
+ // MemDep, which needs to know the operands and needs it to be in the
+ // function.
+ MDA.removeInstruction(DeadInst);
+
+ for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
+ Value *Op = DeadInst->getOperand(op);
+ DeadInst->setOperand(op, 0);
+
+ // If this operand just became dead, add it to the NowDeadInsts list.
+ if (!Op->use_empty()) continue;
+
+ if (Instruction *OpI = dyn_cast<Instruction>(Op))
+ if (isInstructionTriviallyDead(OpI))
+ NowDeadInsts.push_back(OpI);
+ }
+
+ DeadInst->eraseFromParent();
+
+ if (ValueSet) ValueSet->erase(DeadInst);
+ } while (!NowDeadInsts.empty());
+}
+
+unsigned DSE::getPointerSize(Value *V) const {
+ if (TD) {
+ if (AllocaInst *A = dyn_cast<AllocaInst>(V)) {
+ // Get size information for the alloca
+ if (ConstantInt *C = dyn_cast<ConstantInt>(A->getArraySize()))
+ return C->getZExtValue() * TD->getTypeAllocSize(A->getAllocatedType());
+ } else {
+ assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
+ const PointerType *PT = cast<PointerType>(V->getType());
+ return TD->getTypeAllocSize(PT->getElementType());
+ }
+ }
+ return ~0U;
+}
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