llvm-6502/lib/Transforms/Scalar/DeadStoreElimination.cpp
Owen Anderson 2ab36d3502 Begin adding static dependence information to passes, which will allow us to
perform initialization without static constructors AND without explicit initialization
by the client.  For the moment, passes are required to initialize both their
(potential) dependencies and any passes they preserve.  I hope to be able to relax
the latter requirement in the future.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116334 91177308-0d34-0410-b5e6-96231b3b80d8
2010-10-12 19:48:12 +00:00

580 lines
19 KiB
C++

//===- 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(const CallInst *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<MemoryDependenceAnalysis>();
}
unsigned getPointerSize(Value *V) const;
};
}
char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(DSE, "dse", "Dead Store Elimination", false, false)
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->getArgOperand(0);
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: assert(false && "Unexpected intrinsic!");
case Intrinsic::init_trampoline:
return II->getArgOperand(0);
case Intrinsic::lifetime_end:
return II->getArgOperand(1);
}
}
/// 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 {
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: assert(false && "Unexpected intrinsic!");
case Intrinsic::init_trampoline:
return -1u;
case Intrinsic::lifetime_end:
Len = II->getArgOperand(0);
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 (const CallInst *F = isFreeCall(Inst)) {
MadeChange |= handleFreeWithNonTrivialDependency(F, 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(const CallInst *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->getArgOperand(0), 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 CS = cast<Value>(BBI)) {
// If this call does not access memory, it can't
// be undeadifying any of our pointers.
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;
}