#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
+#include "llvm/GlobalVariable.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/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumFastStores, "Number of stores deleted");
namespace {
struct DSE : public FunctionPass {
- TargetData *TD;
+ AliasAnalysis *AA;
+ MemoryDependenceAnalysis *MD;
static char ID; // Pass identification, replacement for typeid
- DSE() : FunctionPass(&ID) {}
+ DSE() : FunctionPass(ID), AA(0), MD(0) {
+ initializeDSEPass(*PassRegistry::getPassRegistry());
+ }
virtual bool runOnFunction(Function &F) {
+ AA = &getAnalysis<AliasAnalysis>();
+ MD = &getAnalysis<MemoryDependenceAnalysis>();
+ DominatorTree &DT = getAnalysis<DominatorTree>();
+
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
- Changed |= runOnBasicBlock(*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 = 0; MD = 0;
return Changed;
}
bool runOnBasicBlock(BasicBlock &BB);
- bool handleFreeWithNonTrivialDependency(Instruction *F, MemDepResult Dep);
+ bool HandleFree(CallInst *F);
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);
+ void RemoveAccessedObjects(const AliasAnalysis::Location &LoadedLoc,
+ SmallPtrSet<Value*, 16> &DeadStackObjects);
// getAnalysisUsage - We require post dominance frontiers (aka Control
AU.addRequired<DominatorTree>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<MemoryDependenceAnalysis>();
- AU.addPreserved<DominatorTree>();
AU.addPreserved<AliasAnalysis>();
+ AU.addPreserved<DominatorTree>();
AU.addPreserved<MemoryDependenceAnalysis>();
}
};
}
char DSE::ID = 0;
-static RegisterPass<DSE> X("dse", "Dead Store Elimination");
+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(); }
-/// isValueAtLeastAsBigAs - Return true if V1 is greater than or equal to the
-/// stored size of V2. This returns false if we don't know.
+//===----------------------------------------------------------------------===//
+// 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.
///
-static bool isValueAtLeastAsBigAs(Value *V1, Value *V2, const TargetData *TD) {
- const Type *V1Ty = V1->getType(), *V2Ty = V2->getType();
+/// If ValueSet is non-null, remove any deleted instructions from it as well.
+///
+static void DeleteDeadInstruction(Instruction *I,
+ MemoryDependenceAnalysis &MD,
+ SmallPtrSet<Value*, 16> *ValueSet = 0) {
+ SmallVector<Instruction*, 32> NowDeadInsts;
- // Exactly the same type, must have exactly the same size.
- if (V1Ty == V2Ty) return true;
+ NowDeadInsts.push_back(I);
+ --NumFastOther;
- // If we don't have target data, we don't know.
- if (TD == 0) return false;
+ // 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, 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());
+}
+
+
+/// 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)) {
+ 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;
+}
+
+/// 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();
- return TD->getTypeStoreSize(V1Ty) >= TD->getTypeStoreSize(V2Ty);
+ 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);
+ }
+ }
}
-bool DSE::runOnBasicBlock(BasicBlock &BB) {
- MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
- TD = getAnalysisIfAvailable<TargetData>();
+/// getLocForRead - Return the location read by the specified "hasMemoryWrite"
+/// instruction if any.
+static AliasAnalysis::Location
+getLocForRead(Instruction *Inst, AliasAnalysis &AA) {
+ assert(hasMemoryWrite(Inst) && "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 stores.
+ if (StoreInst *SI = dyn_cast<StoreInst>(I))
+ return !SI->isVolatile();
+
+ 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();
+ }
+}
+
+/// 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();
+
+ IntrinsicInst *II = cast<IntrinsicInst>(I);
+ switch (II->getIntrinsicID()) {
+ default: assert(false && "Unexpected intrinsic!");
+ case Intrinsic::init_trampoline:
+ return II->getArgOperand(0);
+ }
+}
+
+static uint64_t getPointerSize(Value *V, AliasAnalysis &AA) {
+ const TargetData *TD = AA.getTargetData();
+ if (TD == 0)
+ return AliasAnalysis::UnknownSize;
+
+ 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());
+ return AliasAnalysis::UnknownSize;
+ }
+
+ assert(isa<Argument>(V) && "Expected AllocaInst or Argument!");
+ const PointerType *PT = cast<PointerType>(V->getType());
+ return TD->getTypeAllocSize(PT->getElementType());
+}
+/// isObjectPointerWithTrustworthySize - Return true if the specified Value* is
+/// pointing to an object with a pointer size we can trust.
+static bool isObjectPointerWithTrustworthySize(const Value *V) {
+ if (const AllocaInst *AI = dyn_cast<AllocaInst>(V))
+ return !AI->isArrayAllocation();
+ if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
+ return !GV->mayBeOverridden();
+ if (const Argument *A = dyn_cast<Argument>(V))
+ return A->hasByValAttr();
+ return false;
+}
+
+/// 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 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) {
+ // 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;
+ }
+
+ // Make sure that the Later size is >= the Earlier size.
+ if (Later.Size < Earlier.Size)
+ return false;
+ return true;
+ }
+
+ // 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;
+
+ // 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
+ // other store to the same object.
+ const TargetData &TD = *AA.getTargetData();
+
+ const Value *UO1 = GetUnderlyingObject(P1), *UO2 = GetUnderlyingObject(P2);
+
+ // 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;
+
+ // 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;
+ }
+
+ // 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 Off1 = 0, Off2 = 0;
+ const Value *BP1 = GetPointerBaseWithConstantOffset(P1, Off1, TD);
+ const Value *BP2 = GetPointerBaseWithConstantOffset(P2, Off2, TD);
+
+ // If the base pointers still differ, we have two completely different stores.
+ if (BP1 != BP2)
+ return false;
+
+ // Otherwise, we might have a situation like:
+ // store i16 -> P + 1 Byte
+ // store i32 -> P
+ // In this case, we see if the later store completely overlaps all bytes
+ // stored by the previous store.
+ if (Off1 < Off2 || // Earlier starts before Later.
+ Off1+Earlier.Size > Off2+Later.Size) // Earlier goes beyond Later.
+ return false;
+ // Otherwise, we have complete overlap.
+ return true;
+}
+
+/// 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 == 0) 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++;
- // If we find a store or a free, get its memory dependence.
- if (!isa<StoreInst>(Inst) && !isFreeCall(Inst))
+ // Handle 'free' calls specially.
+ if (CallInst *F = isFreeCall(Inst)) {
+ MadeChange |= HandleFree(F);
continue;
+ }
- // Don't molest volatile stores or do queries that will return "clobber".
- if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
- if (SI->isVolatile())
- continue;
+ // If we find something that writes memory, get its memory dependence.
+ if (!hasMemoryWrite(Inst))
+ continue;
- MemDepResult InstDep = MD.getDependency(Inst);
+ MemDepResult InstDep = MD->getDependency(Inst);
- // Ignore non-local stores.
+ // Ignore non-local store liveness.
// FIXME: cross-block DSE would be fun. :)
- if (InstDep.isNonLocal()) continue;
-
- // Handle frees whose dependencies are non-trivial.
- if (isFreeCall(Inst)) {
- MadeChange |= handleFreeWithNonTrivialDependency(Inst, InstDep);
+ 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->isVolatile()) {
+ 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);
+
+ if (NextInst == 0) // Next instruction deleted.
+ BBI = BB.begin();
+ else if (BBI != BB.begin()) // Revisit this instruction if possible.
+ --BBI;
+ ++NumFastStores;
+ MadeChange = true;
+ continue;
+ }
+ }
}
- StoreInst *SI = cast<StoreInst>(Inst);
-
- // If not a definite must-alias dependency, ignore it.
- if (!InstDep.isDef())
+ // 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;
- // 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 (StoreInst *DepStore = dyn_cast<StoreInst>(InstDep.getInst()))
- if (isValueAtLeastAsBigAs(SI->getOperand(0), DepStore->getOperand(0),TD)){
+ 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 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) &&
+ !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(DepStore);
- NumFastStores++;
+ DeleteDeadInstruction(DepWrite, *MD);
+ ++NumFastStores;
MadeChange = true;
-
+
// DeleteDeadInstruction can delete the current instruction in loop
// cases, reset BBI.
BBI = Inst;
if (BBI != BB.begin())
--BBI;
- continue;
- }
-
- // If we're storing the same value back to a pointer that we just
- // loaded from, then the store can be removed.
- 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;
+ break;
}
- }
-
- // 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(SI);
+
+ // 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;
- if (NextInst == 0) // Next instruction deleted.
- BBI = BB.begin();
- else if (BBI != BB.begin()) // Revisit this instruction if possible.
- --BBI;
- NumFastStores++;
- MadeChange = true;
- continue;
- }
+ 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(Instruction *F, MemDepResult Dep) {
- AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
-
- StoreInst *Dependency = dyn_cast_or_null<StoreInst>(Dep.getInst());
- if (!Dependency || Dependency->isVolatile())
- return false;
+/// HandleFree - Handle frees of entire structures whose dependency is a store
+/// to a field of that structure.
+bool DSE::HandleFree(CallInst *F) {
+ MemDepResult Dep = MD->getDependency(F);
+ do {
+ if (Dep.isNonLocal()) return false;
+
+ Instruction *Dependency = Dep.getInst();
+ if (!hasMemoryWrite(Dependency) || !isRemovable(Dependency))
+ return false;
- Value *DepPointer = Dependency->getPointerOperand()->getUnderlyingObject();
+ Value *DepPointer =
+ GetUnderlyingObject(getStoredPointerOperand(Dependency));
- // Check for aliasing.
- if (AA.alias(F->getOperand(1), 1, DepPointer, 1) !=
- AliasAnalysis::MustAlias)
- return false;
+ // Check for aliasing.
+ if (!AA->isMustAlias(F->getArgOperand(0), DepPointer))
+ return false;
+
+ // DCE instructions only used to calculate that store
+ DeleteDeadInstruction(Dependency, *MD);
+ ++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;
}
/// 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;
+ // Keep track of all of the stack objects that are dead at the end of the
+ // function.
+ SmallPtrSet<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 (AllocaInst *AI = dyn_cast<AllocaInst>(I))
- deadPointers.insert(AI);
+ DeadStackObjects.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);
+ DeadStackObjects.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 (StoreInst* S = dyn_cast<StoreInst>(BBI)) {
- if (!S->isVolatile()) {
- // See through pointer-to-pointer bitcasts
- Value* pointerOperand = S->getPointerOperand()->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.
- BBI++;
- DeleteDeadInstruction(S, &deadPointers);
- NumFastStores++;
- MadeChange = true;
- }
- }
-
- continue;
- }
-
- // We can also remove memcpy's to local variables at the end of a function.
- if (MemCpyInst *M = dyn_cast<MemCpyInst>(BBI)) {
- Value *dest = M->getDest()->getUnderlyingObject();
-
- if (deadPointers.count(dest)) {
- BBI++;
- DeleteDeadInstruction(M, &deadPointers);
- NumFastOther++;
+ // If we find a store, check to see if it points into a dead stack value.
+ if (hasMemoryWrite(BBI) && isRemovable(BBI)) {
+ // See through pointer-to-pointer bitcasts
+ Value *Pointer = GetUnderlyingObject(getStoredPointerOperand(BBI));
+
+ // Stores to stack values are valid candidates for removal.
+ if (DeadStackObjects.count(Pointer)) {
+ Instruction *Dead = BBI++;
+
+ DEBUG(dbgs() << "DSE: Dead Store at End of Block:\n DEAD: "
+ << *Dead << "\n Object: " << *Pointer << '\n');
+
+ // DCE instructions only used to calculate that store.
+ DeleteDeadInstruction(Dead, *MD, &DeadStackObjects);
+ ++NumFastStores;
MadeChange = true;
continue;
}
-
- // Because a memcpy is also a load, we can't skip it if we didn't remove
- // it.
}
- Value* killPointer = 0;
- uint64_t killPointerSize = ~0UL;
+ // Remove any dead non-memory-mutating instructions.
+ if (isInstructionTriviallyDead(BBI)) {
+ Instruction *Inst = BBI++;
+ DeleteDeadInstruction(Inst, *MD, &DeadStackObjects);
+ ++NumFastOther;
+ MadeChange = true;
+ continue;
+ }
- // 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<MemCpyInst>(BBI) &&
- isa<ConstantInt>(cast<MemCpyInst>(BBI)->getLength())) {
- killPointer = cast<MemCpyInst>(BBI)->getSource();
- killPointerSize = cast<ConstantInt>(
- cast<MemCpyInst>(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;
- }
-
+ if (AllocaInst *A = dyn_cast<AllocaInst>(BBI)) {
+ DeadStackObjects.erase(A);
continue;
- } else if (CallSite::get(BBI).getInstruction() != 0) {
- // If this call does not access memory, it can't
- // be undeadifying any of our pointers.
- CallSite CS = CallSite::get(BBI);
- if (AA.doesNotAccessMemory(CS))
+ }
+
+ if (CallSite CS = cast<Value>(BBI)) {
+ // If this call does not access memory, it can't be loading any of our
+ // pointers.
+ if (AA->doesNotAccessMemory(CS))
continue;
- unsigned modRef = 0;
- unsigned other = 0;
+ unsigned NumModRef = 0, NumOther = 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();
+ // If the call might load from any of our allocas, then any store above
+ // the call is live.
+ SmallVector<Value*, 8> LiveAllocas;
+ for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
+ E = DeadStackObjects.end(); I != E; ++I) {
+ // 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 out.
+ if (NumModRef >= 16 && NumOther == 0)
return MadeChange;
- }
-
- // Get size information for the alloca
- unsigned pointerSize = ~0U;
- if (TD) {
- if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
- if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
- pointerSize = C->getZExtValue() *
- TD->getTypeAllocSize(A->getAllocatedType());
- } else {
- const PointerType* PT = cast<PointerType>(
- cast<Argument>(*I)->getType());
- pointerSize = TD->getTypeAllocSize(PT->getElementType());
- }
- }
- // See if the call site touches it
- AliasAnalysis::ModRefResult A = AA.getModRefInfo(CS, *I, pointerSize);
+ // See if the call site touches it.
+ AliasAnalysis::ModRefResult A =
+ AA->getModRefInfo(CS, *I, getPointerSize(*I, *AA));
if (A == AliasAnalysis::ModRef)
- modRef++;
+ ++NumModRef;
else
- other++;
+ ++NumOther;
if (A == AliasAnalysis::ModRef || A == AliasAnalysis::Ref)
- dead.push_back(*I);
+ LiveAllocas.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;
+ for (SmallVector<Value*, 8>::iterator I = LiveAllocas.begin(),
+ E = LiveAllocas.end(); I != E; ++I)
+ DeadStackObjects.erase(*I);
+
+ // If all of the allocas were clobbered by the call then we're not going
+ // to find anything else to process.
+ if (DeadStackObjects.empty())
+ return MadeChange;
+
continue;
}
- if (!killPointer)
+ AliasAnalysis::Location LoadedLoc;
+
+ // If we encounter a use of the pointer, it is no longer considered dead
+ if (LoadInst *L = dyn_cast<LoadInst>(BBI)) {
+ 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 {
+ // Not a loading instruction.
continue;
+ }
- killPointer = killPointer->getUnderlyingObject();
+ // Remove any allocas from the DeadPointer set that are loaded, as this
+ // makes any stores above the access live.
+ RemoveAccessedObjects(LoadedLoc, DeadStackObjects);
- // Deal with undead pointers
- MadeChange |= RemoveUndeadPointers(killPointer, killPointerSize, BBI,
- deadPointers);
+ // 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;
}
-/// 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;
+/// 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,
+ SmallPtrSet<Value*, 16> &DeadStackObjects) {
+ const Value *UnderlyingPointer = GetUnderlyingObject(LoadedLoc.Ptr);
+
+ // A constant can't be in the dead pointer set.
+ if (isa<Constant>(UnderlyingPointer))
+ return;
- bool MadeChange = false;
+ // 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.erase(const_cast<Value*>(UnderlyingPointer));
+ return;
+ }
- SmallVector<Value*, 16> undead;
-
- for (SmallPtrSet<Value*, 64>::iterator I = deadPointers.begin(),
- E = deadPointers.end(); I != E; ++I) {
- // Get size information for the alloca.
- unsigned pointerSize = ~0U;
- if (TD) {
- if (AllocaInst* A = dyn_cast<AllocaInst>(*I)) {
- if (ConstantInt* C = dyn_cast<ConstantInt>(A->getArraySize()))
- pointerSize = C->getZExtValue() *
- TD->getTypeAllocSize(A->getAllocatedType());
- } else {
- const PointerType* PT = cast<PointerType>(cast<Argument>(*I)->getType());
- pointerSize = TD->getTypeAllocSize(PT->getElementType());
- }
- }
-
- // See if this pointer could alias it
- AliasAnalysis::AliasResult A = AA.alias(*I, pointerSize,
- 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);
+ SmallVector<Value*, 16> NowLive;
+ for (SmallPtrSet<Value*, 16>::iterator I = DeadStackObjects.begin(),
+ E = DeadStackObjects.end(); I != E; ++I) {
+ // See if the loaded location could alias the stack location.
+ AliasAnalysis::Location StackLoc(*I, getPointerSize(*I, *AA));
+ if (!AA->isNoAlias(StackLoc, LoadedLoc))
+ NowLive.push_back(*I);
}
- for (SmallVector<Value*, 16>::iterator I = undead.begin(), E = undead.end();
+ for (SmallVector<Value*, 16>::iterator I = NowLive.begin(), E = NowLive.end();
I != E; ++I)
- deadPointers.erase(*I);
-
- return MadeChange;
+ DeadStackObjects.erase(*I);
}
-/// 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>();
- while (!NowDeadInsts.empty()) {
- Instruction *DeadInst = NowDeadInsts.back();
- NowDeadInsts.pop_back();
-
- ++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);
- }
-}
projects / oota-llvm.git / blobdiff