| //===- InstCombineLoadStoreAlloca.cpp -------------------------------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements the visit functions for load, store and alloca. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "InstCombine.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/Loads.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| using namespace llvm; |
| |
| STATISTIC(NumDeadStore, "Number of dead stores eliminated"); |
| STATISTIC(NumGlobalCopies, "Number of allocas copied from constant global"); |
| |
| /// pointsToConstantGlobal - Return true if V (possibly indirectly) points to |
| /// some part of a constant global variable. This intentionally only accepts |
| /// constant expressions because we can't rewrite arbitrary instructions. |
| static bool pointsToConstantGlobal(Value *V) { |
| if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) |
| return GV->isConstant(); |
| if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) |
| if (CE->getOpcode() == Instruction::BitCast || |
| CE->getOpcode() == Instruction::GetElementPtr) |
| return pointsToConstantGlobal(CE->getOperand(0)); |
| return false; |
| } |
| |
| /// isOnlyCopiedFromConstantGlobal - Recursively walk the uses of a (derived) |
| /// pointer to an alloca. Ignore any reads of the pointer, return false if we |
| /// see any stores or other unknown uses. If we see pointer arithmetic, keep |
| /// track of whether it moves the pointer (with IsOffset) but otherwise traverse |
| /// the uses. If we see a memcpy/memmove that targets an unoffseted pointer to |
| /// the alloca, and if the source pointer is a pointer to a constant global, we |
| /// can optimize this. |
| static bool |
| isOnlyCopiedFromConstantGlobal(Value *V, MemTransferInst *&TheCopy, |
| SmallVectorImpl<Instruction *> &ToDelete, |
| bool IsOffset = false) { |
| // We track lifetime intrinsics as we encounter them. If we decide to go |
| // ahead and replace the value with the global, this lets the caller quickly |
| // eliminate the markers. |
| |
| for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI!=E; ++UI) { |
| User *U = cast<Instruction>(*UI); |
| |
| if (LoadInst *LI = dyn_cast<LoadInst>(U)) { |
| // Ignore non-volatile loads, they are always ok. |
| if (!LI->isSimple()) return false; |
| continue; |
| } |
| |
| if (BitCastInst *BCI = dyn_cast<BitCastInst>(U)) { |
| // If uses of the bitcast are ok, we are ok. |
| if (!isOnlyCopiedFromConstantGlobal(BCI, TheCopy, ToDelete, IsOffset)) |
| return false; |
| continue; |
| } |
| if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) { |
| // If the GEP has all zero indices, it doesn't offset the pointer. If it |
| // doesn't, it does. |
| if (!isOnlyCopiedFromConstantGlobal( |
| GEP, TheCopy, ToDelete, IsOffset || !GEP->hasAllZeroIndices())) |
| return false; |
| continue; |
| } |
| |
| if (CallSite CS = U) { |
| // If this is the function being called then we treat it like a load and |
| // ignore it. |
| if (CS.isCallee(UI)) |
| continue; |
| |
| // If this is a readonly/readnone call site, then we know it is just a |
| // load (but one that potentially returns the value itself), so we can |
| // ignore it if we know that the value isn't captured. |
| unsigned ArgNo = CS.getArgumentNo(UI); |
| if (CS.onlyReadsMemory() && |
| (CS.getInstruction()->use_empty() || CS.doesNotCapture(ArgNo))) |
| continue; |
| |
| // If this is being passed as a byval argument, the caller is making a |
| // copy, so it is only a read of the alloca. |
| if (CS.isByValArgument(ArgNo)) |
| continue; |
| } |
| |
| // Lifetime intrinsics can be handled by the caller. |
| if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) { |
| if (II->getIntrinsicID() == Intrinsic::lifetime_start || |
| II->getIntrinsicID() == Intrinsic::lifetime_end) { |
| assert(II->use_empty() && "Lifetime markers have no result to use!"); |
| ToDelete.push_back(II); |
| continue; |
| } |
| } |
| |
| // If this is isn't our memcpy/memmove, reject it as something we can't |
| // handle. |
| MemTransferInst *MI = dyn_cast<MemTransferInst>(U); |
| if (MI == 0) |
| return false; |
| |
| // If the transfer is using the alloca as a source of the transfer, then |
| // ignore it since it is a load (unless the transfer is volatile). |
| if (UI.getOperandNo() == 1) { |
| if (MI->isVolatile()) return false; |
| continue; |
| } |
| |
| // If we already have seen a copy, reject the second one. |
| if (TheCopy) return false; |
| |
| // If the pointer has been offset from the start of the alloca, we can't |
| // safely handle this. |
| if (IsOffset) return false; |
| |
| // If the memintrinsic isn't using the alloca as the dest, reject it. |
| if (UI.getOperandNo() != 0) return false; |
| |
| // If the source of the memcpy/move is not a constant global, reject it. |
| if (!pointsToConstantGlobal(MI->getSource())) |
| return false; |
| |
| // Otherwise, the transform is safe. Remember the copy instruction. |
| TheCopy = MI; |
| } |
| return true; |
| } |
| |
| /// isOnlyCopiedFromConstantGlobal - Return true if the specified alloca is only |
| /// modified by a copy from a constant global. If we can prove this, we can |
| /// replace any uses of the alloca with uses of the global directly. |
| static MemTransferInst * |
| isOnlyCopiedFromConstantGlobal(AllocaInst *AI, |
| SmallVectorImpl<Instruction *> &ToDelete) { |
| MemTransferInst *TheCopy = 0; |
| if (isOnlyCopiedFromConstantGlobal(AI, TheCopy, ToDelete)) |
| return TheCopy; |
| return 0; |
| } |
| |
| Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) { |
| // Ensure that the alloca array size argument has type intptr_t, so that |
| // any casting is exposed early. |
| if (TD) { |
| Type *IntPtrTy = TD->getIntPtrType(AI.getContext()); |
| if (AI.getArraySize()->getType() != IntPtrTy) { |
| Value *V = Builder->CreateIntCast(AI.getArraySize(), |
| IntPtrTy, false); |
| AI.setOperand(0, V); |
| return &AI; |
| } |
| } |
| |
| // Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1 |
| if (AI.isArrayAllocation()) { // Check C != 1 |
| if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) { |
| Type *NewTy = |
| ArrayType::get(AI.getAllocatedType(), C->getZExtValue()); |
| AllocaInst *New = Builder->CreateAlloca(NewTy, 0, AI.getName()); |
| New->setAlignment(AI.getAlignment()); |
| |
| // Scan to the end of the allocation instructions, to skip over a block of |
| // allocas if possible...also skip interleaved debug info |
| // |
| BasicBlock::iterator It = New; |
| while (isa<AllocaInst>(*It) || isa<DbgInfoIntrinsic>(*It)) ++It; |
| |
| // Now that I is pointing to the first non-allocation-inst in the block, |
| // insert our getelementptr instruction... |
| // |
| Value *NullIdx =Constant::getNullValue(Type::getInt32Ty(AI.getContext())); |
| Value *Idx[2]; |
| Idx[0] = NullIdx; |
| Idx[1] = NullIdx; |
| Instruction *GEP = |
| GetElementPtrInst::CreateInBounds(New, Idx, New->getName()+".sub"); |
| InsertNewInstBefore(GEP, *It); |
| |
| // Now make everything use the getelementptr instead of the original |
| // allocation. |
| return ReplaceInstUsesWith(AI, GEP); |
| } else if (isa<UndefValue>(AI.getArraySize())) { |
| return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType())); |
| } |
| } |
| |
| if (TD && AI.getAllocatedType()->isSized()) { |
| // If the alignment is 0 (unspecified), assign it the preferred alignment. |
| if (AI.getAlignment() == 0) |
| AI.setAlignment(TD->getPrefTypeAlignment(AI.getAllocatedType())); |
| |
| // Move all alloca's of zero byte objects to the entry block and merge them |
| // together. Note that we only do this for alloca's, because malloc should |
| // allocate and return a unique pointer, even for a zero byte allocation. |
| if (TD->getTypeAllocSize(AI.getAllocatedType()) == 0) { |
| // For a zero sized alloca there is no point in doing an array allocation. |
| // This is helpful if the array size is a complicated expression not used |
| // elsewhere. |
| if (AI.isArrayAllocation()) { |
| AI.setOperand(0, ConstantInt::get(AI.getArraySize()->getType(), 1)); |
| return &AI; |
| } |
| |
| // Get the first instruction in the entry block. |
| BasicBlock &EntryBlock = AI.getParent()->getParent()->getEntryBlock(); |
| Instruction *FirstInst = EntryBlock.getFirstNonPHIOrDbg(); |
| if (FirstInst != &AI) { |
| // If the entry block doesn't start with a zero-size alloca then move |
| // this one to the start of the entry block. There is no problem with |
| // dominance as the array size was forced to a constant earlier already. |
| AllocaInst *EntryAI = dyn_cast<AllocaInst>(FirstInst); |
| if (!EntryAI || !EntryAI->getAllocatedType()->isSized() || |
| TD->getTypeAllocSize(EntryAI->getAllocatedType()) != 0) { |
| AI.moveBefore(FirstInst); |
| return &AI; |
| } |
| |
| // If the alignment of the entry block alloca is 0 (unspecified), |
| // assign it the preferred alignment. |
| if (EntryAI->getAlignment() == 0) |
| EntryAI->setAlignment( |
| TD->getPrefTypeAlignment(EntryAI->getAllocatedType())); |
| // Replace this zero-sized alloca with the one at the start of the entry |
| // block after ensuring that the address will be aligned enough for both |
| // types. |
| unsigned MaxAlign = std::max(EntryAI->getAlignment(), |
| AI.getAlignment()); |
| EntryAI->setAlignment(MaxAlign); |
| if (AI.getType() != EntryAI->getType()) |
| return new BitCastInst(EntryAI, AI.getType()); |
| return ReplaceInstUsesWith(AI, EntryAI); |
| } |
| } |
| } |
| |
| if (AI.getAlignment()) { |
| // Check to see if this allocation is only modified by a memcpy/memmove from |
| // a constant global whose alignment is equal to or exceeds that of the |
| // allocation. If this is the case, we can change all users to use |
| // the constant global instead. This is commonly produced by the CFE by |
| // constructs like "void foo() { int A[] = {1,2,3,4,5,6,7,8,9...}; }" if 'A' |
| // is only subsequently read. |
| SmallVector<Instruction *, 4> ToDelete; |
| if (MemTransferInst *Copy = isOnlyCopiedFromConstantGlobal(&AI, ToDelete)) { |
| unsigned SourceAlign = getOrEnforceKnownAlignment(Copy->getSource(), |
| AI.getAlignment(), TD); |
| if (AI.getAlignment() <= SourceAlign) { |
| DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n'); |
| DEBUG(dbgs() << " memcpy = " << *Copy << '\n'); |
| for (unsigned i = 0, e = ToDelete.size(); i != e; ++i) |
| EraseInstFromFunction(*ToDelete[i]); |
| Constant *TheSrc = cast<Constant>(Copy->getSource()); |
| Instruction *NewI |
| = ReplaceInstUsesWith(AI, ConstantExpr::getBitCast(TheSrc, |
| AI.getType())); |
| EraseInstFromFunction(*Copy); |
| ++NumGlobalCopies; |
| return NewI; |
| } |
| } |
| } |
| |
| // At last, use the generic allocation site handler to aggressively remove |
| // unused allocas. |
| return visitAllocSite(AI); |
| } |
| |
| |
| /// InstCombineLoadCast - Fold 'load (cast P)' -> cast (load P)' when possible. |
| static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI, |
| const DataLayout *TD) { |
| User *CI = cast<User>(LI.getOperand(0)); |
| Value *CastOp = CI->getOperand(0); |
| |
| PointerType *DestTy = cast<PointerType>(CI->getType()); |
| Type *DestPTy = DestTy->getElementType(); |
| if (PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType())) { |
| |
| // If the address spaces don't match, don't eliminate the cast. |
| if (DestTy->getAddressSpace() != SrcTy->getAddressSpace()) |
| return 0; |
| |
| Type *SrcPTy = SrcTy->getElementType(); |
| |
| if (DestPTy->isIntegerTy() || DestPTy->isPointerTy() || |
| DestPTy->isVectorTy()) { |
| // If the source is an array, the code below will not succeed. Check to |
| // see if a trivial 'gep P, 0, 0' will help matters. Only do this for |
| // constants. |
| if (ArrayType *ASrcTy = dyn_cast<ArrayType>(SrcPTy)) |
| if (Constant *CSrc = dyn_cast<Constant>(CastOp)) |
| if (ASrcTy->getNumElements() != 0) { |
| Value *Idxs[2]; |
| Idxs[0] = Constant::getNullValue(Type::getInt32Ty(LI.getContext())); |
| Idxs[1] = Idxs[0]; |
| CastOp = ConstantExpr::getGetElementPtr(CSrc, Idxs); |
| SrcTy = cast<PointerType>(CastOp->getType()); |
| SrcPTy = SrcTy->getElementType(); |
| } |
| |
| if (IC.getDataLayout() && |
| (SrcPTy->isIntegerTy() || SrcPTy->isPointerTy() || |
| SrcPTy->isVectorTy()) && |
| // Do not allow turning this into a load of an integer, which is then |
| // casted to a pointer, this pessimizes pointer analysis a lot. |
| (SrcPTy->isPointerTy() == LI.getType()->isPointerTy()) && |
| IC.getDataLayout()->getTypeSizeInBits(SrcPTy) == |
| IC.getDataLayout()->getTypeSizeInBits(DestPTy)) { |
| |
| // Okay, we are casting from one integer or pointer type to another of |
| // the same size. Instead of casting the pointer before the load, cast |
| // the result of the loaded value. |
| LoadInst *NewLoad = |
| IC.Builder->CreateLoad(CastOp, LI.isVolatile(), CI->getName()); |
| NewLoad->setAlignment(LI.getAlignment()); |
| NewLoad->setAtomic(LI.getOrdering(), LI.getSynchScope()); |
| // Now cast the result of the load. |
| return new BitCastInst(NewLoad, LI.getType()); |
| } |
| } |
| } |
| return 0; |
| } |
| |
| Instruction *InstCombiner::visitLoadInst(LoadInst &LI) { |
| Value *Op = LI.getOperand(0); |
| |
| // Attempt to improve the alignment. |
| if (TD) { |
| unsigned KnownAlign = |
| getOrEnforceKnownAlignment(Op, TD->getPrefTypeAlignment(LI.getType()),TD); |
| unsigned LoadAlign = LI.getAlignment(); |
| unsigned EffectiveLoadAlign = LoadAlign != 0 ? LoadAlign : |
| TD->getABITypeAlignment(LI.getType()); |
| |
| if (KnownAlign > EffectiveLoadAlign) |
| LI.setAlignment(KnownAlign); |
| else if (LoadAlign == 0) |
| LI.setAlignment(EffectiveLoadAlign); |
| } |
| |
| // load (cast X) --> cast (load X) iff safe. |
| if (isa<CastInst>(Op)) |
| if (Instruction *Res = InstCombineLoadCast(*this, LI, TD)) |
| return Res; |
| |
| // None of the following transforms are legal for volatile/atomic loads. |
| // FIXME: Some of it is okay for atomic loads; needs refactoring. |
| if (!LI.isSimple()) return 0; |
| |
| // Do really simple store-to-load forwarding and load CSE, to catch cases |
| // where there are several consecutive memory accesses to the same location, |
| // separated by a few arithmetic operations. |
| BasicBlock::iterator BBI = &LI; |
| if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,6)) |
| return ReplaceInstUsesWith(LI, AvailableVal); |
| |
| // load(gep null, ...) -> unreachable |
| if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) { |
| const Value *GEPI0 = GEPI->getOperand(0); |
| // TODO: Consider a target hook for valid address spaces for this xform. |
| if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){ |
| // Insert a new store to null instruction before the load to indicate |
| // that this code is not reachable. We do this instead of inserting |
| // an unreachable instruction directly because we cannot modify the |
| // CFG. |
| new StoreInst(UndefValue::get(LI.getType()), |
| Constant::getNullValue(Op->getType()), &LI); |
| return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType())); |
| } |
| } |
| |
| // load null/undef -> unreachable |
| // TODO: Consider a target hook for valid address spaces for this xform. |
| if (isa<UndefValue>(Op) || |
| (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) { |
| // Insert a new store to null instruction before the load to indicate that |
| // this code is not reachable. We do this instead of inserting an |
| // unreachable instruction directly because we cannot modify the CFG. |
| new StoreInst(UndefValue::get(LI.getType()), |
| Constant::getNullValue(Op->getType()), &LI); |
| return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType())); |
| } |
| |
| // Instcombine load (constantexpr_cast global) -> cast (load global) |
| if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Op)) |
| if (CE->isCast()) |
| if (Instruction *Res = InstCombineLoadCast(*this, LI, TD)) |
| return Res; |
| |
| if (Op->hasOneUse()) { |
| // Change select and PHI nodes to select values instead of addresses: this |
| // helps alias analysis out a lot, allows many others simplifications, and |
| // exposes redundancy in the code. |
| // |
| // Note that we cannot do the transformation unless we know that the |
| // introduced loads cannot trap! Something like this is valid as long as |
| // the condition is always false: load (select bool %C, int* null, int* %G), |
| // but it would not be valid if we transformed it to load from null |
| // unconditionally. |
| // |
| if (SelectInst *SI = dyn_cast<SelectInst>(Op)) { |
| // load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2). |
| unsigned Align = LI.getAlignment(); |
| if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, Align, TD) && |
| isSafeToLoadUnconditionally(SI->getOperand(2), SI, Align, TD)) { |
| LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1), |
| SI->getOperand(1)->getName()+".val"); |
| LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2), |
| SI->getOperand(2)->getName()+".val"); |
| V1->setAlignment(Align); |
| V2->setAlignment(Align); |
| return SelectInst::Create(SI->getCondition(), V1, V2); |
| } |
| |
| // load (select (cond, null, P)) -> load P |
| if (Constant *C = dyn_cast<Constant>(SI->getOperand(1))) |
| if (C->isNullValue()) { |
| LI.setOperand(0, SI->getOperand(2)); |
| return &LI; |
| } |
| |
| // load (select (cond, P, null)) -> load P |
| if (Constant *C = dyn_cast<Constant>(SI->getOperand(2))) |
| if (C->isNullValue()) { |
| LI.setOperand(0, SI->getOperand(1)); |
| return &LI; |
| } |
| } |
| } |
| return 0; |
| } |
| |
| /// InstCombineStoreToCast - Fold store V, (cast P) -> store (cast V), P |
| /// when possible. This makes it generally easy to do alias analysis and/or |
| /// SROA/mem2reg of the memory object. |
| static Instruction *InstCombineStoreToCast(InstCombiner &IC, StoreInst &SI) { |
| User *CI = cast<User>(SI.getOperand(1)); |
| Value *CastOp = CI->getOperand(0); |
| |
| Type *DestPTy = cast<PointerType>(CI->getType())->getElementType(); |
| PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType()); |
| if (SrcTy == 0) return 0; |
| |
| Type *SrcPTy = SrcTy->getElementType(); |
| |
| if (!DestPTy->isIntegerTy() && !DestPTy->isPointerTy()) |
| return 0; |
| |
| /// NewGEPIndices - If SrcPTy is an aggregate type, we can emit a "noop gep" |
| /// to its first element. This allows us to handle things like: |
| /// store i32 xxx, (bitcast {foo*, float}* %P to i32*) |
| /// on 32-bit hosts. |
| SmallVector<Value*, 4> NewGEPIndices; |
| |
| // If the source is an array, the code below will not succeed. Check to |
| // see if a trivial 'gep P, 0, 0' will help matters. Only do this for |
| // constants. |
| if (SrcPTy->isArrayTy() || SrcPTy->isStructTy()) { |
| // Index through pointer. |
| Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext())); |
| NewGEPIndices.push_back(Zero); |
| |
| while (1) { |
| if (StructType *STy = dyn_cast<StructType>(SrcPTy)) { |
| if (!STy->getNumElements()) /* Struct can be empty {} */ |
| break; |
| NewGEPIndices.push_back(Zero); |
| SrcPTy = STy->getElementType(0); |
| } else if (ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) { |
| NewGEPIndices.push_back(Zero); |
| SrcPTy = ATy->getElementType(); |
| } else { |
| break; |
| } |
| } |
| |
| SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace()); |
| } |
| |
| if (!SrcPTy->isIntegerTy() && !SrcPTy->isPointerTy()) |
| return 0; |
| |
| // If the pointers point into different address spaces or if they point to |
| // values with different sizes, we can't do the transformation. |
| if (!IC.getDataLayout() || |
| SrcTy->getAddressSpace() != |
| cast<PointerType>(CI->getType())->getAddressSpace() || |
| IC.getDataLayout()->getTypeSizeInBits(SrcPTy) != |
| IC.getDataLayout()->getTypeSizeInBits(DestPTy)) |
| return 0; |
| |
| // Okay, we are casting from one integer or pointer type to another of |
| // the same size. Instead of casting the pointer before |
| // the store, cast the value to be stored. |
| Value *NewCast; |
| Value *SIOp0 = SI.getOperand(0); |
| Instruction::CastOps opcode = Instruction::BitCast; |
| Type* CastSrcTy = SIOp0->getType(); |
| Type* CastDstTy = SrcPTy; |
| if (CastDstTy->isPointerTy()) { |
| if (CastSrcTy->isIntegerTy()) |
| opcode = Instruction::IntToPtr; |
| } else if (CastDstTy->isIntegerTy()) { |
| if (SIOp0->getType()->isPointerTy()) |
| opcode = Instruction::PtrToInt; |
| } |
| |
| // SIOp0 is a pointer to aggregate and this is a store to the first field, |
| // emit a GEP to index into its first field. |
| if (!NewGEPIndices.empty()) |
| CastOp = IC.Builder->CreateInBoundsGEP(CastOp, NewGEPIndices); |
| |
| NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy, |
| SIOp0->getName()+".c"); |
| SI.setOperand(0, NewCast); |
| SI.setOperand(1, CastOp); |
| return &SI; |
| } |
| |
| /// equivalentAddressValues - Test if A and B will obviously have the same |
| /// value. This includes recognizing that %t0 and %t1 will have the same |
| /// value in code like this: |
| /// %t0 = getelementptr \@a, 0, 3 |
| /// store i32 0, i32* %t0 |
| /// %t1 = getelementptr \@a, 0, 3 |
| /// %t2 = load i32* %t1 |
| /// |
| static bool equivalentAddressValues(Value *A, Value *B) { |
| // Test if the values are trivially equivalent. |
| if (A == B) return true; |
| |
| // Test if the values come form identical arithmetic instructions. |
| // This uses isIdenticalToWhenDefined instead of isIdenticalTo because |
| // its only used to compare two uses within the same basic block, which |
| // means that they'll always either have the same value or one of them |
| // will have an undefined value. |
| if (isa<BinaryOperator>(A) || |
| isa<CastInst>(A) || |
| isa<PHINode>(A) || |
| isa<GetElementPtrInst>(A)) |
| if (Instruction *BI = dyn_cast<Instruction>(B)) |
| if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) |
| return true; |
| |
| // Otherwise they may not be equivalent. |
| return false; |
| } |
| |
| Instruction *InstCombiner::visitStoreInst(StoreInst &SI) { |
| Value *Val = SI.getOperand(0); |
| Value *Ptr = SI.getOperand(1); |
| |
| // Attempt to improve the alignment. |
| if (TD) { |
| unsigned KnownAlign = |
| getOrEnforceKnownAlignment(Ptr, TD->getPrefTypeAlignment(Val->getType()), |
| TD); |
| unsigned StoreAlign = SI.getAlignment(); |
| unsigned EffectiveStoreAlign = StoreAlign != 0 ? StoreAlign : |
| TD->getABITypeAlignment(Val->getType()); |
| |
| if (KnownAlign > EffectiveStoreAlign) |
| SI.setAlignment(KnownAlign); |
| else if (StoreAlign == 0) |
| SI.setAlignment(EffectiveStoreAlign); |
| } |
| |
| // Don't hack volatile/atomic stores. |
| // FIXME: Some bits are legal for atomic stores; needs refactoring. |
| if (!SI.isSimple()) return 0; |
| |
| // If the RHS is an alloca with a single use, zapify the store, making the |
| // alloca dead. |
| if (Ptr->hasOneUse()) { |
| if (isa<AllocaInst>(Ptr)) |
| return EraseInstFromFunction(SI); |
| if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) { |
| if (isa<AllocaInst>(GEP->getOperand(0))) { |
| if (GEP->getOperand(0)->hasOneUse()) |
| return EraseInstFromFunction(SI); |
| } |
| } |
| } |
| |
| // Do really simple DSE, to catch cases where there are several consecutive |
| // stores to the same location, separated by a few arithmetic operations. This |
| // situation often occurs with bitfield accesses. |
| BasicBlock::iterator BBI = &SI; |
| for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts; |
| --ScanInsts) { |
| --BBI; |
| // Don't count debug info directives, lest they affect codegen, |
| // and we skip pointer-to-pointer bitcasts, which are NOPs. |
| if (isa<DbgInfoIntrinsic>(BBI) || |
| (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) { |
| ScanInsts++; |
| continue; |
| } |
| |
| if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) { |
| // Prev store isn't volatile, and stores to the same location? |
| if (PrevSI->isSimple() && equivalentAddressValues(PrevSI->getOperand(1), |
| SI.getOperand(1))) { |
| ++NumDeadStore; |
| ++BBI; |
| EraseInstFromFunction(*PrevSI); |
| continue; |
| } |
| break; |
| } |
| |
| // If this is a load, we have to stop. However, if the loaded value is from |
| // the pointer we're loading and is producing the pointer we're storing, |
| // then *this* store is dead (X = load P; store X -> P). |
| if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { |
| if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) && |
| LI->isSimple()) |
| return EraseInstFromFunction(SI); |
| |
| // Otherwise, this is a load from some other location. Stores before it |
| // may not be dead. |
| break; |
| } |
| |
| // Don't skip over loads or things that can modify memory. |
| if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory()) |
| break; |
| } |
| |
| // store X, null -> turns into 'unreachable' in SimplifyCFG |
| if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) { |
| if (!isa<UndefValue>(Val)) { |
| SI.setOperand(0, UndefValue::get(Val->getType())); |
| if (Instruction *U = dyn_cast<Instruction>(Val)) |
| Worklist.Add(U); // Dropped a use. |
| } |
| return 0; // Do not modify these! |
| } |
| |
| // store undef, Ptr -> noop |
| if (isa<UndefValue>(Val)) |
| return EraseInstFromFunction(SI); |
| |
| // If the pointer destination is a cast, see if we can fold the cast into the |
| // source instead. |
| if (isa<CastInst>(Ptr)) |
| if (Instruction *Res = InstCombineStoreToCast(*this, SI)) |
| return Res; |
| if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) |
| if (CE->isCast()) |
| if (Instruction *Res = InstCombineStoreToCast(*this, SI)) |
| return Res; |
| |
| |
| // If this store is the last instruction in the basic block (possibly |
| // excepting debug info instructions), and if the block ends with an |
| // unconditional branch, try to move it to the successor block. |
| BBI = &SI; |
| do { |
| ++BBI; |
| } while (isa<DbgInfoIntrinsic>(BBI) || |
| (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())); |
| if (BranchInst *BI = dyn_cast<BranchInst>(BBI)) |
| if (BI->isUnconditional()) |
| if (SimplifyStoreAtEndOfBlock(SI)) |
| return 0; // xform done! |
| |
| return 0; |
| } |
| |
| /// SimplifyStoreAtEndOfBlock - Turn things like: |
| /// if () { *P = v1; } else { *P = v2 } |
| /// into a phi node with a store in the successor. |
| /// |
| /// Simplify things like: |
| /// *P = v1; if () { *P = v2; } |
| /// into a phi node with a store in the successor. |
| /// |
| bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) { |
| BasicBlock *StoreBB = SI.getParent(); |
| |
| // Check to see if the successor block has exactly two incoming edges. If |
| // so, see if the other predecessor contains a store to the same location. |
| // if so, insert a PHI node (if needed) and move the stores down. |
| BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0); |
| |
| // Determine whether Dest has exactly two predecessors and, if so, compute |
| // the other predecessor. |
| pred_iterator PI = pred_begin(DestBB); |
| BasicBlock *P = *PI; |
| BasicBlock *OtherBB = 0; |
| |
| if (P != StoreBB) |
| OtherBB = P; |
| |
| if (++PI == pred_end(DestBB)) |
| return false; |
| |
| P = *PI; |
| if (P != StoreBB) { |
| if (OtherBB) |
| return false; |
| OtherBB = P; |
| } |
| if (++PI != pred_end(DestBB)) |
| return false; |
| |
| // Bail out if all the relevant blocks aren't distinct (this can happen, |
| // for example, if SI is in an infinite loop) |
| if (StoreBB == DestBB || OtherBB == DestBB) |
| return false; |
| |
| // Verify that the other block ends in a branch and is not otherwise empty. |
| BasicBlock::iterator BBI = OtherBB->getTerminator(); |
| BranchInst *OtherBr = dyn_cast<BranchInst>(BBI); |
| if (!OtherBr || BBI == OtherBB->begin()) |
| return false; |
| |
| // If the other block ends in an unconditional branch, check for the 'if then |
| // else' case. there is an instruction before the branch. |
| StoreInst *OtherStore = 0; |
| if (OtherBr->isUnconditional()) { |
| --BBI; |
| // Skip over debugging info. |
| while (isa<DbgInfoIntrinsic>(BBI) || |
| (isa<BitCastInst>(BBI) && BBI->getType()->isPointerTy())) { |
| if (BBI==OtherBB->begin()) |
| return false; |
| --BBI; |
| } |
| // If this isn't a store, isn't a store to the same location, or is not the |
| // right kind of store, bail out. |
| OtherStore = dyn_cast<StoreInst>(BBI); |
| if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) || |
| !SI.isSameOperationAs(OtherStore)) |
| return false; |
| } else { |
| // Otherwise, the other block ended with a conditional branch. If one of the |
| // destinations is StoreBB, then we have the if/then case. |
| if (OtherBr->getSuccessor(0) != StoreBB && |
| OtherBr->getSuccessor(1) != StoreBB) |
| return false; |
| |
| // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an |
| // if/then triangle. See if there is a store to the same ptr as SI that |
| // lives in OtherBB. |
| for (;; --BBI) { |
| // Check to see if we find the matching store. |
| if ((OtherStore = dyn_cast<StoreInst>(BBI))) { |
| if (OtherStore->getOperand(1) != SI.getOperand(1) || |
| !SI.isSameOperationAs(OtherStore)) |
| return false; |
| break; |
| } |
| // If we find something that may be using or overwriting the stored |
| // value, or if we run out of instructions, we can't do the xform. |
| if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() || |
| BBI == OtherBB->begin()) |
| return false; |
| } |
| |
| // In order to eliminate the store in OtherBr, we have to |
| // make sure nothing reads or overwrites the stored value in |
| // StoreBB. |
| for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) { |
| // FIXME: This should really be AA driven. |
| if (I->mayReadFromMemory() || I->mayWriteToMemory()) |
| return false; |
| } |
| } |
| |
| // Insert a PHI node now if we need it. |
| Value *MergedVal = OtherStore->getOperand(0); |
| if (MergedVal != SI.getOperand(0)) { |
| PHINode *PN = PHINode::Create(MergedVal->getType(), 2, "storemerge"); |
| PN->addIncoming(SI.getOperand(0), SI.getParent()); |
| PN->addIncoming(OtherStore->getOperand(0), OtherBB); |
| MergedVal = InsertNewInstBefore(PN, DestBB->front()); |
| } |
| |
| // Advance to a place where it is safe to insert the new store and |
| // insert it. |
| BBI = DestBB->getFirstInsertionPt(); |
| StoreInst *NewSI = new StoreInst(MergedVal, SI.getOperand(1), |
| SI.isVolatile(), |
| SI.getAlignment(), |
| SI.getOrdering(), |
| SI.getSynchScope()); |
| InsertNewInstBefore(NewSI, *BBI); |
| NewSI->setDebugLoc(OtherStore->getDebugLoc()); |
| |
| // If the two stores had the same TBAA tag, preserve it. |
| if (MDNode *TBAATag = SI.getMetadata(LLVMContext::MD_tbaa)) |
| if ((TBAATag = MDNode::getMostGenericTBAA(TBAATag, |
| OtherStore->getMetadata(LLVMContext::MD_tbaa)))) |
| NewSI->setMetadata(LLVMContext::MD_tbaa, TBAATag); |
| |
| |
| // Nuke the old stores. |
| EraseInstFromFunction(SI); |
| EraseInstFromFunction(*OtherStore); |
| return true; |
| } |
