#include "llvm/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/STLExtras.h"
using namespace llvm;
STATISTIC(NumFastStores, "Number of stores deleted");
struct DSE : public FunctionPass {
AliasAnalysis *AA;
MemoryDependenceAnalysis *MD;
+ DominatorTree *DT;
static char ID; // Pass identification, replacement for typeid
- DSE() : FunctionPass(ID), AA(0), MD(0) {
+ DSE() : FunctionPass(ID), AA(0), MD(0), DT(0) {
initializeDSEPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F) {
AA = &getAnalysis<AliasAnalysis>();
MD = &getAnalysis<MemoryDependenceAnalysis>();
- DominatorTree &DT = getAnalysis<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))
+ if (DT->isReachableFromEntry(I))
Changed |= runOnBasicBlock(*I);
- AA = 0; MD = 0;
+ AA = 0; MD = 0; DT = 0;
return Changed;
}
}
}
+
+/// 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;
+
+ IntrinsicInst *II = cast<IntrinsicInst>(I);
+ switch (II->getIntrinsicID()) {
+ default: return false;
+ case Intrinsic::memset:
+ case Intrinsic::memcpy:
+ // Do shorten memory intrinsics.
+ return true;
+ }
+}
+
/// getStoredPointerOperand - Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
if (StoreInst *SI = dyn_cast<StoreInst>(I))
static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
const TargetData *TD = AA.getTargetData();
+
+ if (CallInst *CI = dyn_cast<CallInst>(V)) {
+ assert(isMalloc(CI) && "Expected Malloc call!");
+ if (ConstantInt *C = dyn_cast<ConstantInt>(CI->getArgOperand(0)))
+ return C->getZExtValue();
+ return AliasAnalysis::UnknownSize;
+ }
+
if (TD == 0)
return AliasAnalysis::UnknownSize;
return AliasAnalysis::UnknownSize;
}
- assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
+ assert(isa<Argument>(V) && "Expected AllocaInst, malloc call or Argument!");
PointerType *PT = cast<PointerType>(V->getType());
return TD->getTypeAllocSize(PT->getElementType());
}
return !GV->mayBeOverridden();
if (const Argument *A = dyn_cast<Argument>(V))
return A->hasByValAttr();
+ if (isMalloc(V))
+ return true;
return false;
}
-/// isCompleteOverwrite - Return true if a store to the 'Later' location
+namespace {
+ enum OverwriteResult
+ {
+ OverwriteComplete,
+ OverwriteEnd,
+ OverwriteUnknown
+ };
+}
+
+/// isOverwrite - Return 'OverwriteComplete' 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) {
+/// '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 Value *P1 = Earlier.Ptr->stripPointerCasts();
const Value *P2 = Later.Ptr->stripPointerCasts();
// 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;
+ if (AA.getTargetData() == 0 &&
+ Later.Ptr->getType() == Earlier.Ptr->getType())
+ return OverwriteComplete;
+
+ return OverwriteUnknown;
}
// Make sure that the Later size is >= the Earlier size.
- if (Later.Size < Earlier.Size)
- return false;
- return true;
+ 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 ||
- Later.Size <= Earlier.Size || AA.getTargetData() == 0)
- return false;
+ AA.getTargetData() == 0)
+ return OverwriteUnknown;
// 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
// 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;
+ return OverwriteUnknown;
// If the "Later" store is to a recognizable object, get its size.
if (isObjectPointerWithTrustworthySize(UO2)) {
uint64_t ObjectSize =
TD.getTypeAllocSize(cast<PointerType>(UO2->getType())->getElementType());
if (ObjectSize == Later.Size)
- return true;
+ 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.
- int64_t EarlierOff = 0, LaterOff = 0;
+ 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;
+ return OverwriteUnknown;
// The later store completely overlaps the earlier store if:
//
//
// 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 true;
+ 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) &&
+ LaterOff + Later.Size >= EarlierOff + Earlier.Size)
+ return OverwriteEnd;
// Otherwise, they don't completely overlap.
- return false;
+ return OverwriteUnknown;
}
/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
// Ignore any store where we can't find a local dependence.
// FIXME: cross-block DSE would be fun. :)
- if (InstDep.isNonLocal() || InstDep.isUnknown())
+ if (!InstDep.isDef() && !InstDep.isClobber())
continue;
// If we're storing the same value back to a pointer that we just
if (Loc.Ptr == 0)
continue;
- while (!InstDep.isNonLocal() && !InstDep.isUnknown()) {
+ 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
// 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) &&
+ if (isRemovable(DepWrite) &&
!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);
- ++NumFastStores;
- MadeChange = true;
-
- // DeleteDeadInstruction can delete the current instruction in loop
- // cases, reset BBI.
- BBI = Inst;
- if (BBI != BB.begin())
- --BBI;
- break;
+ 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);
+ ++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
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;
+ 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;
- MemDepResult Dep = MD->getDependency(F);
+ AliasAnalysis::Location Loc = AliasAnalysis::Location(F->getOperand(0));
+ SmallVector<BasicBlock *, 16> Blocks;
+ Blocks.push_back(F->getParent());
- while (!Dep.isNonLocal() && !Dep.isUnknown()) {
- Instruction *Dependency = Dep.getInst();
- if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
- return MadeChange;
+ while (!Blocks.empty()) {
+ BasicBlock *BB = Blocks.pop_back_val();
+ Instruction *InstPt = BB->getTerminator();
+ if (BB == F->getParent()) InstPt = F;
- Value *DepPointer =
- GetUnderlyingObject(getStoredPointerOperand(Dependency));
+ MemDepResult Dep = MD->getPointerDependencyFrom(Loc, false, InstPt, BB);
+ while (Dep.isDef() || Dep.isClobber()) {
+ Instruction *Dependency = Dep.getInst();
+ if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
+ break;
- // Check for aliasing.
- if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
- return MadeChange;
+ Value *DepPointer =
+ GetUnderlyingObject(getStoredPointerOperand(Dependency));
- // DCE instructions only used to calculate that store
- DeleteDeadInstruction(Dependency, *MD);
- ++NumFastStores;
- MadeChange = true;
+ // Check for aliasing.
+ if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
+ break;
- // 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);
- };
+ Instruction *Next = llvm::next(BasicBlock::iterator(Dependency));
+
+ // DCE instructions only used to calculate that store
+ DeleteDeadInstruction(Dependency, *MD);
+ ++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;
}
// 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)
+ for (BasicBlock::iterator I = Entry->begin(), E = Entry->end(); I != E; ++I) {
if (AllocaInst *AI = dyn_cast<AllocaInst>(I))
DeadStackObjects.insert(AI);
+ // Okay, so these are dead heap objects, but if the pointer never escapes
+ // then it's leaked by this function anyways.
+ if (CallInst *CI = extractMallocCall(I))
+ if (!PointerMayBeCaptured(CI, true, true))
+ DeadStackObjects.insert(CI);
+ }
+
// 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(),
continue;
}
+ if (CallInst *CI = extractMallocCall(BBI)) {
+ DeadStackObjects.erase(CI);
+ continue;
+ }
+
if (CallSite CS = cast<Value>(BBI)) {
// If this call does not access memory, it can't be loading any of our
// pointers.
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