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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/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");
namespace {
struct DSE : public FunctionPass {
AliasAnalysis *AA;
MemoryDependenceAnalysis *MD;
static char ID; // Pass identification, replacement for typeid
DSE() : FunctionPass(ID), AA(0), MD(0) {
virtual bool runOnFunction(Function &F) {
AA = &getAnalysis<AliasAnalysis>();
MD = &getAnalysis<MemoryDependenceAnalysis>();
DominatorTree &DT = getAnalysis<DominatorTree>();
bool Changed = false;
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);
AA = 0; MD = 0;
return Changed;
bool runOnBasicBlock(BasicBlock &BB);
bool HandleFree(CallInst *F);
bool handleEndBlock(BasicBlock &BB);
void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
SmallPtrSet<Value*, 16> &DeadStackObjects);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
// Helper functions
/// 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.
static void DeleteDeadInstruction(Instruction *I,
MemoryDependenceAnalysis &MD,
SmallPtrSet<Value*, 16> *ValueSet = 0) {
SmallVector<Instruction*, 32> NowDeadInsts;
// Before we touch this instruction, remove it from memdep!
do {
Instruction *DeadInst = NowDeadInsts.pop_back_val();
// 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.
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))
if (ValueSet) ValueSet->erase(DeadInst);
} while (!NowDeadInsts.empty());
/// hasMemoryWrite - Does this instruction write some memory? This only returns
/// true for things that we can analyze with other helpers below.
static bool hasMemoryWrite(Instruction *I) {
if (isa<StoreInst>(I))
return true;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
return false;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
case Intrinsic::init_trampoline:
case Intrinsic::lifetime_end:
return true;
return false;
/// getLocForWrite - Return a Location stored to by the specified instruction.
static AliasAnalysis::Location
getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
return AA.getLocation(SI);
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
// memcpy/memmove/memset.
AliasAnalysis::Location Loc = AA.getLocationForDest(MI);
// If we don't have target data around, an unknown size in Location means
// that we should use the size of the pointee type. This isn't valid for
// memset/memcpy, which writes more than an i8.
if (Loc.Size == AliasAnalysis::UnknownSize && AA.getTargetData() == 0)
return AliasAnalysis::Location();
return Loc;
IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
if (II == 0) return AliasAnalysis::Location();
switch (II->getIntrinsicID()) {
default: return AliasAnalysis::Location(); // Unhandled intrinsic.
case Intrinsic::init_trampoline:
// If we don't have target data around, an unknown size in Location means
// that we should use the size of the pointee type. This isn't valid for
// init.trampoline, which writes more than an i8.
if (AA.getTargetData() == 0) return AliasAnalysis::Location();
// FIXME: We don't know the size of the trampoline, so we can't really
// handle it here.
return AliasAnalysis::Location(II->getArgOperand(0));
case Intrinsic::lifetime_end: {
uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
return AliasAnalysis::Location(II->getArgOperand(1), Len);
/// getLocForRead - Return the location read by the specified "hasMemoryWrite"
/// instruction if any.
static AliasAnalysis::Location
getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
assert(hasMemoryWrite(Inst) && "Unknown instruction case");
// The only instructions that both read and write are the mem transfer
// instructions (memcpy/memmove).
if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(Inst))
return AA.getLocationForSource(MTI);
return AliasAnalysis::Location();
/// isRemovable - If the value of this instruction and the memory it writes to
/// is unused, may we delete this instruction?
static bool isRemovable(Instruction *I) {
// Don't remove volatile stores.
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return !SI->isVolatile();
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: assert(0 && "doesn't pass 'hasMemoryWrite' predicate");
case Intrinsic::lifetime_end:
// Never remove dead lifetime_end's, e.g. because it is followed by a
// free.
return false;
case Intrinsic::init_trampoline:
// Always safe to remove init_trampoline.
return true;
case Intrinsic::memset:
case Intrinsic::memmove:
case Intrinsic::memcpy:
// Don't remove volatile memory intrinsics.
return !cast<MemIntrinsic>(II)->isVolatile();
/// getStoredPointerOperand - Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->getPointerOperand();
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
return MI->getDest();
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: assert(false && "Unexpected intrinsic!");
case Intrinsic::init_trampoline:
return II->getArgOperand(0);
static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
const TargetData *TD = AA.getTargetData();
if (TD == 0)
return AliasAnalysis::UnknownSize;
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());
return AliasAnalysis::UnknownSize;
assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
const PointerType *PT = cast<PointerType>(V->getType());
return TD->getTypeAllocSize(PT->getElementType());
/// isObjectPointerWithTrustworthySize - Return true if the specified Value* is
/// pointing to an object with a pointer size we can trust.
static bool isObjectPointerWithTrustworthySize(const Value *V) {
if (const AllocaInst *AI = dyn_cast<AllocaInst>(V))
return !AI->isArrayAllocation();
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
return !GV->mayBeOverridden();
if (const Argument *A = dyn_cast<Argument>(V))
return A->hasByValAttr();
return false;
/// isCompleteOverwrite - Return true if a store to the 'Later' location
/// completely overwrites a store to the 'Earlier' location.
static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
const AliasAnalysis::Location &Earlier,
AliasAnalysis &AA) {
const Value *P1 = Earlier.Ptr->stripPointerCasts();
const Value *P2 = Later.Ptr->stripPointerCasts();
// If the start pointers are the same, we just have to compare sizes to see if
// the later store was larger than the earlier store.
if (P1 == P2) {
// If we don't know the sizes of either access, then we can't do a
// comparison.
if (Later.Size == AliasAnalysis::UnknownSize ||
Earlier.Size == AliasAnalysis::UnknownSize) {
// If we have no TargetData information around, then the size of the store
// is inferrable from the pointee type. If they are the same type, then
// we know that the store is safe.
if (AA.getTargetData() == 0)
return Later.Ptr->getType() == Earlier.Ptr->getType();
return false;
// Make sure that the Later size is >= the Earlier size.
if (Later.Size < Earlier.Size)
return false;
return true;
// Otherwise, we have to have size information, and the later store has to be
// larger than the earlier one.
if (Later.Size == AliasAnalysis::UnknownSize ||
Earlier.Size == AliasAnalysis::UnknownSize ||
Later.Size <= Earlier.Size || AA.getTargetData() == 0)
return false;
// Check to see if the later store is to the entire object (either a global,
// an alloca, or a byval argument). If so, then it clearly overwrites any
// other store to the same object.
const TargetData &TD = *AA.getTargetData();
const Value *UO1 = GetUnderlyingObject(P1, &TD),
*UO2 = GetUnderlyingObject(P2, &TD);
// If we can't resolve the same pointers to the same object, then we can't
// analyze them at all.
if (UO1 != UO2)
return false;
// If the "Later" store is to a recognizable object, get its size.
if (isObjectPointerWithTrustworthySize(UO2)) {
uint64_t ObjectSize =
if (ObjectSize == Later.Size)
return true;
// Okay, we have stores to two completely different pointers. Try to
// decompose the pointer into a "base + constant_offset" form. If the base
// pointers are equal, then we can reason about the two stores.
int64_t EarlierOff = 0, LaterOff = 0;
const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
// If the base pointers still differ, we have two completely different stores.
if (BP1 != BP2)
return false;
// The later store completely overlaps the earlier store if:
// 1. Both start at the same offset and the later one's size is greater than
// or equal to the earlier one's, or
// |--earlier--|
// |-- later --|
// 2. The earlier store has an offset greater than the later offset, but which
// still lies completely within the later store.
// |--earlier--|
// |----- later ------|
// We have to be careful here as *Off is signed while *.Size is unsigned.
if (EarlierOff >= LaterOff &&
uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
return true;
// Otherwise, they don't completely overlap.
return false;
/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
/// memory region into an identical pointer) then it doesn't actually make its
/// input dead in the traditional sense. Consider this case:
/// memcpy(A <- B)
/// memcpy(A <- A)
/// In this case, the second store to A does not make the first store to A dead.
/// The usual situation isn't an explicit A<-A store like this (which can be
/// trivially removed) but a case where two pointers may alias.
/// This function detects when it is unsafe to remove a dependent instruction
/// because the DSE inducing instruction may be a self-read.
static bool isPossibleSelfRead(Instruction *Inst,
const AliasAnalysis::Location &InstStoreLoc,
Instruction *DepWrite, AliasAnalysis &AA) {
// Self reads can only happen for instructions that read memory. Get the
// location read.
AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
// If the read and written loc obviously don't alias, it isn't a read.
if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
// Okay, 'Inst' may copy over itself. However, we can still remove a the
// DepWrite instruction if we can prove that it reads from the same location
// as Inst. This handles useful cases like:
// memcpy(A <- B)
// memcpy(A <- B)
// Here we don't know if A/B may alias, but we do know that B/B are must
// aliases, so removing the first memcpy is safe (assuming it writes <= #
// bytes as the second one.
AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
return false;
// If DepWrite doesn't read memory or if we can't prove it is a must alias,
// then it can't be considered dead.
return true;
// DSE Pass
bool DSE::runOnBasicBlock(BasicBlock &BB) {
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++;
// Handle 'free' calls specially.
if (CallInst *F = isFreeCall(Inst)) {
MadeChange |= HandleFree(F);
// If we find something that writes memory, get its memory dependence.
if (!hasMemoryWrite(Inst))
MemDepResult InstDep = MD->getDependency(Inst);
// Ignore non-local store liveness.
// FIXME: cross-block DSE would be fun. :)
if (InstDep.isNonLocal() ||
// Ignore self dependence, which happens in the entry block of the
// function.
InstDep.getInst() == Inst)
// 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 && !SI->isVolatile()) {
DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
<< "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
// DeleteDeadInstruction can delete the current instruction. Save BBI
// in case we need it.
WeakVH NextInst(BBI);
DeleteDeadInstruction(SI, *MD);
if (NextInst == 0) // Next instruction deleted.
BBI = BB.begin();
else if (BBI != BB.begin()) // Revisit this instruction if possible.
MadeChange = true;
// Figure out what location is being stored to.
AliasAnalysis::Location Loc = getLocForWrite(Inst, *AA);
// If we didn't get a useful location, fail.
if (Loc.Ptr == 0)
while (!InstDep.isNonLocal()) {
// Get the memory clobbered by the instruction we depend on. MemDep will
// skip any instructions that 'Loc' clearly doesn't interact with. If we
// end up depending on a may- or must-aliased load, then we can't optimize
// away the store and we bail out. However, if we depend on on something
// that overwrites the memory location we *can* potentially optimize it.
// Find out what memory location the dependant instruction stores.
Instruction *DepWrite = InstDep.getInst();
AliasAnalysis::Location DepLoc = getLocForWrite(DepWrite, *AA);
// If we didn't get a useful location, or if it isn't a size, bail out.
if (DepLoc.Ptr == 0)
// If we find a write that is a) removable (i.e., non-volatile), b) is
// completely obliterated by the store to 'Loc', and c) which we know that
// 'Inst' doesn't load from, then we can remove it.
if (isRemovable(DepWrite) && isCompleteOverwrite(Loc, DepLoc, *AA) &&
!isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
<< *DepWrite << "\n KILLER: " << *Inst << '\n');
// Delete the store and now-dead instructions that feed it.
DeleteDeadInstruction(DepWrite, *MD);
MadeChange = true;
// DeleteDeadInstruction can delete the current instruction in loop
// cases, reset BBI.
BBI = Inst;
if (BBI != BB.begin())
// If this is a may-aliased store that is clobbering the store value, we
// can keep searching past it for another must-aliased pointer that stores
// to the same location. For example, in:
// store -> P
// store -> Q
// store -> P
// we can remove the first store to P even though we don't know if P and Q
// alias.
if (DepWrite == &BB.front()) break;
// Can't look past this instruction if it might read 'Loc'.
if (AA->getModRefInfo(DepWrite, Loc) & AliasAnalysis::Ref)
InstDep = MD->getPointerDependencyFrom(Loc, false, DepWrite, &BB);
// 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;
/// HandleFree - Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
bool DSE::HandleFree(CallInst *F) {
MemDepResult Dep = MD->getDependency(F);
do {
if (Dep.isNonLocal()) return false;
Instruction *Dependency = Dep.getInst();
if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
return false;
Value *DepPointer =
// Check for aliasing.
if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
return false;
// DCE instructions only used to calculate that store
DeleteDeadInstruction(Dependency, *MD);
// Inst's old Dependency is now deleted. Compute the next dependency,
// which may also be dead, as in
// s[0] = 0;
// s[1] = 0; // This has just been deleted.
// free(s);
Dep = MD->getDependency(F);
} while (!Dep.isNonLocal());
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) {
bool MadeChange = false;
// Keep track of all of the stack objects that are dead at the end of the
// function.
SmallPtrSet<Value*, 16> DeadStackObjects;
// 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))
// 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())
// Scan the basic block backwards
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
// If we find a store, check to see if it points into a dead stack value.
if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
// See through pointer-to-pointer bitcasts
Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));
// Stores to stack values are valid candidates for removal.
if (DeadStackObjects.count(Pointer)) {
Instruction *Dead = BBI++;
DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
<< *Dead << "\n Object: " << *Pointer << '\n');
// DCE instructions only used to calculate that store.
DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
MadeChange = true;
// Remove any dead non-memory-mutating instructions.
if (isInstructionTriviallyDead(BBI)) {
Instruction *Inst = BBI++;
DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
MadeChange = true;
if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
if (CallSite CS = cast<Value>(BBI)) {
// If this call does not access memory, it can't be loading any of our
// pointers.
if (AA->doesNotAccessMemory(CS))
unsigned NumModRef = 0, NumOther = 0;
// If the call might load from any of our allocas, then any store above
// the call is live.
SmallVector<Value*, 8> LiveAllocas;
for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
E = DeadStackObjects.end(); I != E; ++I) {
// 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 out.
if (NumModRef >= 16 && NumOther == 0)
return MadeChange;
// See if the call site touches it.
AliasAnalysis::ModRefResult A =
AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
if (A == AliasAnalysis::ModRef)
if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
E = LiveAllocas.end(); I != E; ++I)
// If all of the allocas were clobbered by the call then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
return MadeChange;
AliasAnalysis::Location LoadedLoc;
// If we encounter a use of the pointer, it is no longer considered dead
if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
LoadedLoc = AA->getLocation(L);
} else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
LoadedLoc = AA->getLocation(V);
} else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
LoadedLoc = AA->getLocationForSource(MTI);
} else {
// Not a loading instruction.
// Remove any allocas from the DeadPointer set that are loaded, as this
// makes any stores above the access live.
RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
// If all of the allocas were clobbered by the access then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
return MadeChange;
/// RemoveAccessedObjects - Check to see if the specified location may alias any
/// of the stack objects in the DeadStackObjects set. If so, they become live
/// because the location is being loaded.
void DSE::RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
SmallPtrSet<Value*, 16> &DeadStackObjects) {
const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
// A constant can't be in the dead pointer set.
if (isa<Constant>(UnderlyingPointer))
// If the kill pointer can be easily reduced to an alloca, don't bother doing
// extraneous AA queries.
if (isa<AllocaInst>(UnderlyingPointer) || isa<Argument>(UnderlyingPointer)) {
SmallVector<Value*, 16> NowLive;
for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
E = DeadStackObjects.end(); I != E; ++I) {
// See if the loaded location could alias the stack location.
AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
if (!AA->isNoAlias(StackLoc, LoadedLoc))
for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
I != E; ++I)