}
bool runOnBasicBlock(BasicBlock &BB);
- bool handleFreeWithNonTrivialDependency(const CallInst *F,
- MemDepResult Dep);
+ bool HandleFree(CallInst *F);
bool handleEndBlock(BasicBlock &BB);
- bool RemoveUndeadPointers(Value *Ptr, unsigned killPointerSize,
+ bool RemoveUndeadPointers(Value *Ptr, uint64_t killPointerSize,
BasicBlock::iterator &BBI,
SmallPtrSet<Value*, 64> &deadPointers);
void DeleteDeadInstruction(Instruction *I,
FunctionPass *llvm::createDeadStoreEliminationPass() { return new DSE(); }
-/// doesClobberMemory - Does this instruction clobber (write without reading)
-/// some memory?
-static bool doesClobberMemory(Instruction *I) {
+/// 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)) {
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;
+/// 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);
+ }
+ }
+}
+
+/// 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();
- return true;
+
+ 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();
+ }
}
-/// getPointerOperand - Return the pointer that is being clobbered.
+/// getPointerOperand - Return the pointer that is being written to.
static Value *getPointerOperand(Instruction *I) {
- assert(doesClobberMemory(I));
+ assert(hasMemoryWrite(I));
if (StoreInst *SI = dyn_cast<StoreInst>(I))
return SI->getPointerOperand();
if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(I))
}
}
-/// 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;
-}
+/// 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 TargetData *TD) {
+ const Value *P1 = Later.Ptr->stripPointerCasts();
+ const Value *P2 = Earlier.Ptr->stripPointerCasts();
+
+ // Make sure that the start pointers are the same.
+ if (P1 != P2)
+ return false;
-/// 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();
+ // 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 (TD == 0)
+ return Later.Ptr->getType() == Earlier.Ptr->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);
+ // Make sure that the Later size is >= the Earlier size.
+ if (Later.Size < Earlier.Size)
+ return false;
- return I1Size != -1 && I2Size != -1 && I1Size >= I2Size;
+ return true;
}
bool DSE::runOnBasicBlock(BasicBlock &BB) {
MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
TD = getAnalysisIfAvailable<TargetData>();
bool MadeChange = false;
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);
+ // Handle 'free' calls specially.
+ if (CallInst *F = isFreeCall(Inst)) {
+ MadeChange |= HandleFree(F);
continue;
}
- // If not a definite must-alias dependency, ignore it.
- if (!InstDep.isDef())
+ // If we find something that writes memory, get its memory dependence.
+ if (!hasMemoryWrite(Inst))
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;
- }
- }
+ MemDepResult InstDep = MD.getDependency(Inst);
- if (!isElidable(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)
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) {
+ SI->getOperand(0) == DepLoad && !SI->isVolatile()) {
// DeleteDeadInstruction can delete the current instruction. Save BBI
// in case we need it.
WeakVH NextInst(BBI);
}
}
- // 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) {
+ // 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)
+ continue;
+
+ 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)
+ break;
+
+ // If we find a removable write that is completely obliterated by the
+ // store to 'Loc' then we can remove it.
+ if (isRemovable(DepWrite) && isCompleteOverwrite(Loc, DepLoc, AA, TD)) {
// 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;
+ DeleteDeadInstruction(DepWrite);
++NumFastStores;
MadeChange = true;
- continue;
+
+ // DeleteDeadInstruction can delete the current instruction in loop
+ // cases, reset BBI.
+ BBI = Inst;
+ if (BBI != BB.begin())
+ --BBI;
+ break;
}
+
+ // 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);
}
}
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) {
+/// HandleFree - Handle frees of entire structures whose dependency is a store
+/// to a field of that structure.
+bool DSE::HandleFree(CallInst *F) {
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+ MemoryDependenceAnalysis &MD = getAnalysis<MemoryDependenceAnalysis>();
+
+ MemDepResult Dep = MD.getDependency(F);
+ do {
+ if (Dep.isNonLocal()) return false;
+
+ Instruction *Dependency = Dep.getInst();
+ if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
+ return false;
- Instruction *Dependency = Dep.getInst();
- if (!Dependency || !doesClobberMemory(Dependency) || !isElidable(Dependency))
- return false;
-
- Value *DepPointer = getPointerOperand(Dependency)->getUnderlyingObject();
+ Value *DepPointer = getPointerOperand(Dependency)->getUnderlyingObject();
- // Check for aliasing.
- if (AA.alias(F->getArgOperand(0), 1, DepPointer, 1) !=
- AliasAnalysis::MustAlias)
- return false;
+ // 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;
+
+ // 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());
- // DCE instructions only used to calculate that store
- DeleteDeadInstruction(Dependency);
- ++NumFastStores;
return true;
}
--BBI;
// If we find a store whose pointer is dead.
- if (doesClobberMemory(BBI)) {
- if (isElidable(BBI)) {
+ if (hasMemoryWrite(BBI)) {
+ if (isRemovable(BBI)) {
// See through pointer-to-pointer bitcasts
Value *pointerOperand = getPointerOperand(BBI)->getUnderlyingObject();
/// RemoveUndeadPointers - check for uses of a pointer that make it
/// undead when scanning for dead stores to alloca's.
-bool DSE::RemoveUndeadPointers(Value *killPointer, unsigned killPointerSize,
+bool DSE::RemoveUndeadPointers(Value *killPointer, uint64_t killPointerSize,
BasicBlock::iterator &BBI,
SmallPtrSet<Value*, 64> &deadPointers) {
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
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