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llvm-mirror/lib/Transforms/Scalar/DeadStoreElimination.cpp
Hal Finkel bbd88ec4fc [DSE] Remove no-data-layout-only type-based overlap checking 
DSE's overlap checking contained special logic, used only when no DataLayout
was available, which inferred a complete overwrite when the pointee types were
equal. This logic seems fine for regular loads/stores, but does not work for
memcpy and friends. Instead of fixing this, I'm just removing it.
Philosophically, transformations should not contain enhanced behavior used only
when data layout is lacking (data layout should be strictly additive), and
maintaining these rarely-tested code paths seems not worthwhile at this stage.

Credit to Aliaksei Zasenka for the bug report and the diagnosis. The test case
(slightly reduced from that provided by Aliaksei) replaces the original
contents of test/Transforms/DeadStoreElimination/no-targetdata.ll -- a few
other tests have been updated to have a data layout.

llvm-svn: 220035
2014-10-17 11:56:00 +00:00

874 lines
31 KiB
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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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "dse"
STATISTIC(NumFastStores, "Number of stores deleted");
STATISTIC(NumFastOther , "Number of other instrs removed");
namespace {
struct DSE : public FunctionPass {
AliasAnalysis *AA;
MemoryDependenceAnalysis *MD;
DominatorTree *DT;
const TargetLibraryInfo *TLI;
static char ID; // Pass identification, replacement for typeid
DSE() : FunctionPass(ID), AA(nullptr), MD(nullptr), DT(nullptr) {
initializeDSEPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipOptnoneFunction(F))
return false;
AA = &getAnalysis<AliasAnalysis>();
MD = &getAnalysis<MemoryDependenceAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
TLI = AA->getTargetLibraryInfo();
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 = nullptr; MD = nullptr; DT = nullptr;
return Changed;
}
bool runOnBasicBlock(BasicBlock &BB);
bool HandleFree(CallInst *F);
bool handleEndBlock(BasicBlock &BB);
void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
SmallSetVector<Value*, 16> &DeadStackObjects);
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<MemoryDependenceAnalysis>();
AU.addPreserved<AliasAnalysis>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<MemoryDependenceAnalysis>();
}
};
}
char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
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,
const TargetLibraryInfo *TLI,
SmallSetVector<Value*, 16> *ValueSet = nullptr) {
SmallVector<Instruction*, 32> NowDeadInsts;
NowDeadInsts.push_back(I);
--NumFastOther;
// Before we touch this instruction, remove it from memdep!
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.
MD.removeInstruction(DeadInst);
for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
Value *Op = DeadInst->getOperand(op);
DeadInst->setOperand(op, nullptr);
// 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, TLI))
NowDeadInsts.push_back(OpI);
}
DeadInst->eraseFromParent();
if (ValueSet) ValueSet->remove(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, const TargetLibraryInfo *TLI) {
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;
}
}
if (CallSite CS = I) {
if (Function *F = CS.getCalledFunction()) {
if (TLI && TLI->has(LibFunc::strcpy) &&
F->getName() == TLI->getName(LibFunc::strcpy)) {
return true;
}
if (TLI && TLI->has(LibFunc::strncpy) &&
F->getName() == TLI->getName(LibFunc::strncpy)) {
return true;
}
if (TLI && TLI->has(LibFunc::strcat) &&
F->getName() == TLI->getName(LibFunc::strcat)) {
return true;
}
if (TLI && TLI->has(LibFunc::strncat) &&
F->getName() == TLI->getName(LibFunc::strncat)) {
return true;
}
}
}
return false;
}
/// getLocForWrite - Return a Location stored to by the specified instruction.
/// If isRemovable returns true, this function and getLocForRead completely
/// describe the memory operations for this instruction.
static AliasAnalysis::Location
getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
const DataLayout *DL = AA.getDataLayout();
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 && DL == nullptr)
return AliasAnalysis::Location();
return Loc;
}
IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
if (!II) 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 (!DL) 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, AA.getTargetLibraryInfo()) &&
"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/atomic stores.
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->isUnordered();
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: llvm_unreachable("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();
}
}
if (CallSite CS = I)
return CS.getInstruction()->use_empty();
return false;
}
/// isShortenable - Returns true if this instruction can be safely shortened in
/// length.
static bool isShortenable(Instruction *I) {
// Don't shorten stores for now
if (isa<StoreInst>(I))
return false;
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: return false;
case Intrinsic::memset:
case Intrinsic::memcpy:
// Do shorten memory intrinsics.
return true;
}
}
// Don't shorten libcalls calls for now.
return false;
}
/// 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();
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: llvm_unreachable("Unexpected intrinsic!");
case Intrinsic::init_trampoline:
return II->getArgOperand(0);
}
}
CallSite CS = I;
// All the supported functions so far happen to have dest as their first
// argument.
return CS.getArgument(0);
}
static uint64_t getPointerSize(const Value *V, AliasAnalysis &AA) {
uint64_t Size;
if (getObjectSize(V, Size, AA.getDataLayout(), AA.getTargetLibraryInfo()))
return Size;
return AliasAnalysis::UnknownSize;
}
namespace {
enum OverwriteResult
{
OverwriteComplete,
OverwriteEnd,
OverwriteUnknown
};
}
/// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
/// completely overwrites a store to the 'Earlier' location.
/// 'OverwriteEnd' if the end of the 'Earlier' location is completely
/// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
const AliasAnalysis::Location &Earlier,
AliasAnalysis &AA,
int64_t &EarlierOff,
int64_t &LaterOff) {
const DataLayout *DL = AA.getDataLayout();
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)
return OverwriteUnknown;
// Make sure that the Later size is >= the Earlier size.
if (Later.Size >= Earlier.Size)
return OverwriteComplete;
}
// 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 || DL == nullptr)
return OverwriteUnknown;
// Check to see if the later store is to the entire object (either a global,
// an alloca, or a byval/inalloca argument). If so, then it clearly
// overwrites any other store to the same object.
const Value *UO1 = GetUnderlyingObject(P1, DL),
*UO2 = GetUnderlyingObject(P2, DL);
// If we can't resolve the same pointers to the same object, then we can't
// analyze them at all.
if (UO1 != UO2)
return OverwriteUnknown;
// If the "Later" store is to a recognizable object, get its size.
uint64_t ObjectSize = getPointerSize(UO2, AA);
if (ObjectSize != AliasAnalysis::UnknownSize)
if (ObjectSize == Later.Size && ObjectSize >= Earlier.Size)
return OverwriteComplete;
// 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.
EarlierOff = 0;
LaterOff = 0;
const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, DL);
const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, DL);
// If the base pointers still differ, we have two completely different stores.
if (BP1 != BP2)
return OverwriteUnknown;
// 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 &&
Later.Size >= Earlier.Size &&
uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
return OverwriteComplete;
// The other interesting case is if the later store overwrites the end of
// the earlier store
//
// |--earlier--|
// |-- later --|
//
// In this case we may want to trim the size of earlier to avoid generating
// writes to addresses which will definitely be overwritten later
if (LaterOff > EarlierOff &&
LaterOff < int64_t(EarlierOff + Earlier.Size) &&
int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
return OverwriteEnd;
// Otherwise, they don't completely overlap.
return OverwriteUnknown;
}
/// 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) 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, TLI)) {
MadeChange |= HandleFree(F);
continue;
}
// If we find something that writes memory, get its memory dependence.
if (!hasMemoryWrite(Inst, TLI))
continue;
MemDepResult InstDep = MD->getDependency(Inst);
// Ignore any store where we can't find a local dependence.
// FIXME: cross-block DSE would be fun. :)
if (!InstDep.isDef() && !InstDep.isClobber())
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 && isRemovable(SI)) {
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, TLI);
if (!NextInst) // Next instruction deleted.
BBI = BB.begin();
else if (BBI != BB.begin()) // Revisit this instruction if possible.
--BBI;
++NumFastStores;
MadeChange = true;
continue;
}
}
}
// 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)
continue;
while (InstDep.isDef() || InstDep.isClobber()) {
// 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 dependent 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)
break;
// 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) &&
!isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
int64_t InstWriteOffset, DepWriteOffset;
OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
DepWriteOffset, InstWriteOffset);
if (OR == OverwriteComplete) {
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, TLI);
++NumFastStores;
MadeChange = true;
// DeleteDeadInstruction can delete the current instruction in loop
// cases, reset BBI.
BBI = Inst;
if (BBI != BB.begin())
--BBI;
break;
} else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
// TODO: base this on the target vector size so that if the earlier
// store was too small to get vector writes anyway then its likely
// a good idea to shorten it
// Power of 2 vector writes are probably always a bad idea to optimize
// as any store/memset/memcpy is likely using vector instructions so
// shortening it to not vector size is likely to be slower
MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
unsigned DepWriteAlign = DepIntrinsic->getAlignment();
if (llvm::isPowerOf2_64(InstWriteOffset) ||
((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
<< *DepWrite << "\n KILLER (offset "
<< InstWriteOffset << ", "
<< DepLoc.Size << ")"
<< *Inst << '\n');
Value* DepWriteLength = DepIntrinsic->getLength();
Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
InstWriteOffset -
DepWriteOffset);
DepIntrinsic->setLength(TrimmedLength);
MadeChange = true;
}
}
}
// 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)
break;
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;
}
/// Find all blocks that will unconditionally lead to the block BB and append
/// them to F.
static void FindUnconditionalPreds(SmallVectorImpl<BasicBlock *> &Blocks,
BasicBlock *BB, DominatorTree *DT) {
for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
BasicBlock *Pred = *I;
if (Pred == BB) continue;
TerminatorInst *PredTI = Pred->getTerminator();
if (PredTI->getNumSuccessors() != 1)
continue;
if (DT->isReachableFromEntry(Pred))
Blocks.push_back(Pred);
}
}
/// HandleFree - Handle frees of entire structures whose dependency is a store
/// to a field of that structure.
bool DSE::HandleFree(CallInst *F) {
bool MadeChange = false;
AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
SmallVector<BasicBlock *, 16> Blocks;
Blocks.push_back(F->getParent());
while (!Blocks.empty()) {
BasicBlock *BB = Blocks.pop_back_val();
Instruction *InstPt = BB->getTerminator();
if (BB == F->getParent()) InstPt = F;
MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
while (Dep.isDef() || Dep.isClobber()) {
Instruction *Dependency = Dep.getInst();
if (!hasMemoryWrite(Dependency, TLI) || !isRemovable(Dependency))
break;
Value *DepPointer =
GetUnderlyingObject(getStoredPointerOperand(Dependency));
// Check for aliasing.
if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
break;
Instruction *Next = std::next(BasicBlock::iterator(Dependency));
// DCE instructions only used to calculate that store
DeleteDeadInstruction(Dependency, *MD, TLI);
++NumFastStores;
MadeChange = true;
// 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->getPointerDependencyFrom(Loc, false, Next, BB);
}
if (Dep.isNonLocal())
FindUnconditionalPreds(Blocks, BB, DT);
}
return MadeChange;
}
/// 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.
SmallSetVector<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 (isa<AllocaInst>(I))
DeadStackObjects.insert(I);
// Okay, so these are dead heap objects, but if the pointer never escapes
// then it's leaked by this function anyways.
else if (isAllocLikeFn(I, TLI) && !PointerMayBeCaptured(I, true, true))
DeadStackObjects.insert(I);
}
// Treat byval or inalloca 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->hasByValOrInAllocaAttr())
DeadStackObjects.insert(AI);
// Scan the basic block backwards
for (BasicBlock::iterator BBI = BB.end(); BBI != BB.begin(); ){
--BBI;
// If we find a store, check to see if it points into a dead stack value.
if (hasMemoryWrite(BBI, TLI) && isRemovable(BBI)) {
// See through pointer-to-pointer bitcasts
SmallVector<Value *, 4> Pointers;
GetUnderlyingObjects(getStoredPointerOperand(BBI), Pointers);
// Stores to stack values are valid candidates for removal.
bool AllDead = true;
for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
E = Pointers.end(); I != E; ++I)
if (!DeadStackObjects.count(*I)) {
AllDead = false;
break;
}
if (AllDead) {
Instruction *Dead = BBI++;
DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
<< *Dead << "\n Objects: ";
for (SmallVectorImpl<Value *>::iterator I = Pointers.begin(),
E = Pointers.end(); I != E; ++I) {
dbgs() << **I;
if (std::next(I) != E)
dbgs() << ", ";
}
dbgs() << '\n');
// DCE instructions only used to calculate that store.
DeleteDeadInstruction(Dead, *MD, TLI, &DeadStackObjects);
++NumFastStores;
MadeChange = true;
continue;
}
}
// Remove any dead non-memory-mutating instructions.
if (isInstructionTriviallyDead(BBI, TLI)) {
Instruction *Inst = BBI++;
DeleteDeadInstruction(Inst, *MD, TLI, &DeadStackObjects);
++NumFastOther;
MadeChange = true;
continue;
}
if (isa<AllocaInst>(BBI)) {
// Remove allocas from the list of dead stack objects; there can't be
// any references before the definition.
DeadStackObjects.remove(BBI);
continue;
}
if (CallSite CS = cast<Value>(BBI)) {
// Remove allocation function calls from the list of dead stack objects;
// there can't be any references before the definition.
if (isAllocLikeFn(BBI, TLI))
DeadStackObjects.remove(BBI);
// If this call does not access memory, it can't be loading any of our
// pointers.
if (AA->doesNotAccessMemory(CS))
continue;
// If the call might load from any of our allocas, then any store above
// the call is live.
DeadStackObjects.remove_if([&](Value *I) {
// See if the call site touches the value.
AliasAnalysis::ModRefResult A =
AA->getModRefInfo(CS, I, getPointerSize(I, *AA));
return A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref;
});
// If all of the allocas were clobbered by the call then we're not going
// to find anything else to process.
if (DeadStackObjects.empty())
break;
continue;
}
AliasAnalysis::Location LoadedLoc;
// If we encounter a use of the pointer, it is no longer considered dead
if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
if (!L->isUnordered()) // Be conservative with atomic/volatile load
break;
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 if (!BBI->mayReadFromMemory()) {
// Instruction doesn't read memory. Note that stores that weren't removed
// above will hit this case.
continue;
} else {
// Unknown inst; assume it clobbers everything.
break;
}
// 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())
break;
}
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,
SmallSetVector<Value*, 16> &DeadStackObjects) {
const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
// A constant can't be in the dead pointer set.
if (isa<Constant>(UnderlyingPointer))
return;
// 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)) {
DeadStackObjects.remove(const_cast<Value*>(UnderlyingPointer));
return;
}
// Remove objects that could alias LoadedLoc.
DeadStackObjects.remove_if([&](Value *I) {
// See if the loaded location could alias the stack location.
AliasAnalysis::Location StackLoc(I, getPointerSize(I, *AA));
return !AA->isNoAlias(StackLoc, LoadedLoc);
});
}
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