//===- 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/GlobalsModRef.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/TargetLibraryInfo.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/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "dse"
STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
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<AAResultsWrapperPass>().getAAResults();
MD = &getAnalysis<MemoryDependenceAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
bool Changed = false;
for (BasicBlock &I : F)
// 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 MemoryIsNotModifiedBetween(Instruction *FirstI, Instruction *SecondI);
bool HandleFree(CallInst *F);
bool handleEndBlock(BasicBlock &BB);
void RemoveAccessedObjects(const MemoryLocation &LoadedLoc,
SmallSetVector<Value *, 16> &DeadStackObjects,
const DataLayout &DL);
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<MemoryDependenceAnalysis>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addPreserved<MemoryDependenceAnalysis>();
}
};
}
char DSE::ID = 0;
INITIALIZE_PASS_BEGIN(DSE, "dse", "Dead Store Elimination", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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 (auto CS = CallSite(I)) {
if (Function *F = CS.getCalledFunction()) {
if (TLI.has(LibFunc::strcpy) &&
F->getName() == TLI.getName(LibFunc::strcpy)) {
return true;
}
if (TLI.has(LibFunc::strncpy) &&
F->getName() == TLI.getName(LibFunc::strncpy)) {
return true;
}
if (TLI.has(LibFunc::strcat) &&
F->getName() == TLI.getName(LibFunc::strcat)) {
return true;
}
if (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 MemoryLocation getLocForWrite(Instruction *Inst, AliasAnalysis &AA) {
if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
return MemoryLocation::get(SI);
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(Inst)) {
// memcpy/memmove/memset.
MemoryLocation Loc = MemoryLocation::getForDest(MI);
return Loc;
}
IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst);
if (!II)
return MemoryLocation();
switch (II->getIntrinsicID()) {
default:
return MemoryLocation(); // Unhandled intrinsic.
case Intrinsic::init_trampoline:
// FIXME: We don't know the size of the trampoline, so we can't really
// handle it here.
return MemoryLocation(II->getArgOperand(0));
case Intrinsic::lifetime_end: {
uint64_t Len = cast<ConstantInt>(II->getArgOperand(0))->getZExtValue();
return MemoryLocation(II->getArgOperand(1), Len);
}
}
}
/// getLocForRead - Return the location read by the specified "hasMemoryWrite"
/// instruction if any.
static MemoryLocation getLocForRead(Instruction *Inst,
const TargetLibraryInfo &TLI) {
assert(hasMemoryWrite(Inst, TLI) && "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 MemoryLocation::getForSource(MTI);
return MemoryLocation();
}
/// 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 (auto CS = CallSite(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, const DataLayout &DL,
const TargetLibraryInfo &TLI) {
uint64_t Size;
if (getObjectSize(V, Size, DL, &TLI))
return Size;
return MemoryLocation::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 MemoryLocation &Later,
const MemoryLocation &Earlier,
const DataLayout &DL,
const TargetLibraryInfo &TLI,
int64_t &EarlierOff, int64_t &LaterOff) {
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 == MemoryLocation::UnknownSize ||
Earlier.Size == MemoryLocation::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 == MemoryLocation::UnknownSize ||
Earlier.Size == MemoryLocation::UnknownSize)
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, DL, TLI);
if (ObjectSize != MemoryLocation::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 MemoryLocation &InstStoreLoc,
Instruction *DepWrite,
const TargetLibraryInfo &TLI,
AliasAnalysis &AA) {
// Self reads can only happen for instructions that read memory. Get the
// location read.
MemoryLocation InstReadLoc = getLocForRead(Inst, TLI);
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.
MemoryLocation DepReadLoc = getLocForRead(DepWrite, TLI);
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) {
const DataLayout &DL = BB.getModule()->getDataLayout();
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;
// 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)) {
auto RemoveDeadInstAndUpdateBBI = [&](Instruction *DeadInst) {
// DeleteDeadInstruction can delete the current instruction. Save BBI
// in case we need it.
WeakVH NextInst(&*BBI);
DeleteDeadInstruction(DeadInst, *MD, *TLI);
if (!NextInst) // Next instruction deleted.
BBI = BB.begin();
else if (BBI != BB.begin()) // Revisit this instruction if possible.
--BBI;
++NumRedundantStores;
MadeChange = true;
};
if (LoadInst *DepLoad = dyn_cast<LoadInst>(SI->getValueOperand())) {
if (SI->getPointerOperand() == DepLoad->getPointerOperand() &&
isRemovable(SI) &&
MemoryIsNotModifiedBetween(DepLoad, SI)) {
DEBUG(dbgs() << "DSE: Remove Store Of Load from same pointer:\n "
<< "LOAD: " << *DepLoad << "\n STORE: " << *SI << '\n');
RemoveDeadInstAndUpdateBBI(SI);
continue;
}
}
// Remove null stores into the calloc'ed objects
Constant *StoredConstant = dyn_cast<Constant>(SI->getValueOperand());
if (StoredConstant && StoredConstant->isNullValue() &&
isRemovable(SI)) {
Instruction *UnderlyingPointer = dyn_cast<Instruction>(
GetUnderlyingObject(SI->getPointerOperand(), DL));
if (UnderlyingPointer && isCallocLikeFn(UnderlyingPointer, TLI) &&
MemoryIsNotModifiedBetween(UnderlyingPointer, SI)) {
DEBUG(dbgs()
<< "DSE: Remove null store to the calloc'ed object:\n DEAD: "
<< *Inst << "\n OBJECT: " << *UnderlyingPointer << '\n');
RemoveDeadInstAndUpdateBBI(SI);
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;
// Figure out what location is being stored to.
MemoryLocation 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();
MemoryLocation 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, *TLI, *AA)) {
int64_t InstWriteOffset, DepWriteOffset;
OverwriteResult OR =
isOverwrite(Loc, DepLoc, DL, *TLI, 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->getIterator();
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) & MRI_Ref)
break;
InstDep = MD->getPointerDependencyFrom(Loc, false,
DepWrite->getIterator(), &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;
}
/// Returns true if the memory which is accessed by the second instruction is not
/// modified between the first and the second instruction.
/// Precondition: Second instruction must be dominated by the first
/// instruction.
bool DSE::MemoryIsNotModifiedBetween(Instruction *FirstI,
Instruction *SecondI) {
SmallVector<BasicBlock *, 16> WorkList;
SmallPtrSet<BasicBlock *, 8> Visited;
BasicBlock::iterator FirstBBI(FirstI);
++FirstBBI;
BasicBlock::iterator SecondBBI(SecondI);
BasicBlock *FirstBB = FirstI->getParent();
BasicBlock *SecondBB = SecondI->getParent();
MemoryLocation MemLoc = MemoryLocation::get(SecondI);
// Start checking the store-block.
WorkList.push_back(SecondBB);
bool isFirstBlock = true;
// Check all blocks going backward until we reach the load-block.
while (!WorkList.empty()) {
BasicBlock *B = WorkList.pop_back_val();
// Ignore instructions before LI if this is the FirstBB.
BasicBlock::iterator BI = (B == FirstBB ? FirstBBI : B->begin());
BasicBlock::iterator EI;
if (isFirstBlock) {
// Ignore instructions after SI if this is the first visit of SecondBB.
assert(B == SecondBB && "first block is not the store block");
EI = SecondBBI;
isFirstBlock = false;
} else {
// It's not SecondBB or (in case of a loop) the second visit of SecondBB.
// In this case we also have to look at instructions after SI.
EI = B->end();
}
for (; BI != EI; ++BI) {
Instruction *I = &*BI;
if (I->mayWriteToMemory() && I != SecondI) {
auto Res = AA->getModRefInfo(I, MemLoc);
if (Res != MRI_NoModRef)
return false;
}
}
if (B != FirstBB) {
assert(B != &FirstBB->getParent()->getEntryBlock() &&
"Should not hit the entry block because SI must be dominated by LI");
for (auto PredI = pred_begin(B), PE = pred_end(B); PredI != PE; ++PredI) {
if (!Visited.insert(*PredI).second)
continue;
WorkList.push_back(*PredI);
}
}
}
return true;
}
/// 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;
MemoryLocation Loc = MemoryLocation(F->getOperand(0));
SmallVector<BasicBlock *, 16> Blocks;
Blocks.push_back(F->getParent());
const DataLayout &DL = F->getModule()->getDataLayout();
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->getIterator(), BB);
while (Dep.isDef() || Dep.isClobber()) {
Instruction *Dependency = Dep.getInst();
if (!hasMemoryWrite(Dependency, *TLI) || !isRemovable(Dependency))
break;
Value *DepPointer =
GetUnderlyingObject(getStoredPointerOperand(Dependency), DL);
// Check for aliasing.
if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
break;
auto Next = ++Dependency->getIterator();
// 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()->front();
for (Instruction &I : Entry) {
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 (Argument &AI : BB.getParent()->args())
if (AI.hasByValOrInAllocaAttr())
DeadStackObjects.insert(&AI);
const DataLayout &DL = BB.getModule()->getDataLayout();
// 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, DL);
// 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 (auto CS = CallSite(&*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.
ModRefInfo A = AA->getModRefInfo(CS, I, getPointerSize(I, DL, *TLI));
return A == MRI_ModRef || A == MRI_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;
}
MemoryLocation 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 = MemoryLocation::get(L);
} else if (VAArgInst *V = dyn_cast<VAArgInst>(BBI)) {
LoadedLoc = MemoryLocation::get(V);
} else if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(BBI)) {
LoadedLoc = MemoryLocation::getForSource(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, DL);
// 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 MemoryLocation &LoadedLoc,
SmallSetVector<Value *, 16> &DeadStackObjects,
const DataLayout &DL) {
const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr, DL);
// 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.
MemoryLocation StackLoc(I, getPointerSize(I, DL, *TLI));
return !AA->isNoAlias(StackLoc, LoadedLoc);
});
}