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llvm-mirror/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
Victor Hernandez e4d8ef8fc4 Keep ignoring pointer-to-pointer bitcasts 
llvm-svn: 94194
2010-01-22 19:05:05 +00:00

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//===- 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/IntrinsicInst.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumDeadStore, "Number of dead stores eliminated");
Instruction *InstCombiner::visitAllocaInst(AllocaInst &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())) {
const Type *NewTy =
ArrayType::get(AI.getAllocatedType(), C->getZExtValue());
assert(isa<AllocaInst>(AI) && "Unknown type of allocation inst!");
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;
Value *V = GetElementPtrInst::CreateInBounds(New, Idx, Idx + 2,
New->getName()+".sub", It);
// Now make everything use the getelementptr instead of the original
// allocation.
return ReplaceInstUsesWith(AI, V);
} else if (isa<UndefValue>(AI.getArraySize())) {
return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));
}
}
if (TD && isa<AllocaInst>(AI) && AI.getAllocatedType()->isSized()) {
// If alloca'ing a zero byte object, replace the alloca with a null pointer.
// 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)
return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType()));
// If the alignment is 0 (unspecified), assign it the preferred alignment.
if (AI.getAlignment() == 0)
AI.setAlignment(TD->getPrefTypeAlignment(AI.getAllocatedType()));
}
return 0;
}
/// InstCombineLoadCast - Fold 'load (cast P)' -> cast (load P)' when possible.
static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI,
const TargetData *TD) {
User *CI = cast<User>(LI.getOperand(0));
Value *CastOp = CI->getOperand(0);
const PointerType *DestTy = cast<PointerType>(CI->getType());
const Type *DestPTy = DestTy->getElementType();
if (const 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;
const Type *SrcPTy = SrcTy->getElementType();
if (DestPTy->isInteger() || isa<PointerType>(DestPTy) ||
isa<VectorType>(DestPTy)) {
// 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 (const 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, 2);
SrcTy = cast<PointerType>(CastOp->getType());
SrcPTy = SrcTy->getElementType();
}
if (IC.getTargetData() &&
(SrcPTy->isInteger() || isa<PointerType>(SrcPTy) ||
isa<VectorType>(SrcPTy)) &&
// Do not allow turning this into a load of an integer, which is then
// casted to a pointer, this pessimizes pointer analysis a lot.
(isa<PointerType>(SrcPTy) == isa<PointerType>(LI.getType())) &&
IC.getTargetData()->getTypeSizeInBits(SrcPTy) ==
IC.getTargetData()->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.
Value *NewLoad =
IC.Builder->CreateLoad(CastOp, LI.isVolatile(), CI->getName());
// 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()));
if (KnownAlign >
(LI.getAlignment() == 0 ? TD->getABITypeAlignment(LI.getType()) :
LI.getAlignment()))
LI.setAlignment(KnownAlign);
}
// 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 loads.
if (LI.isVolatile()) return 0;
// Do really simple store-to-load forwarding and load CSE, to catch cases
// where there are several consequtive 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).
if (isSafeToLoadUnconditionally(SI->getOperand(1), SI) &&
isSafeToLoadUnconditionally(SI->getOperand(2), SI)) {
Value *V1 = Builder->CreateLoad(SI->getOperand(1),
SI->getOperand(1)->getName()+".val");
Value *V2 = Builder->CreateLoad(SI->getOperand(2),
SI->getOperand(2)->getName()+".val");
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);
const Type *DestPTy = cast<PointerType>(CI->getType())->getElementType();
const PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType());
if (SrcTy == 0) return 0;
const Type *SrcPTy = SrcTy->getElementType();
if (!DestPTy->isInteger() && !isa<PointerType>(DestPTy))
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 (isa<ArrayType>(SrcPTy) || isa<StructType>(SrcPTy)) {
// Index through pointer.
Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext()));
NewGEPIndices.push_back(Zero);
while (1) {
if (const StructType *STy = dyn_cast<StructType>(SrcPTy)) {
if (!STy->getNumElements()) /* Struct can be empty {} */
break;
NewGEPIndices.push_back(Zero);
SrcPTy = STy->getElementType(0);
} else if (const ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) {
NewGEPIndices.push_back(Zero);
SrcPTy = ATy->getElementType();
} else {
break;
}
}
SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace());
}
if (!SrcPTy->isInteger() && !isa<PointerType>(SrcPTy))
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.getTargetData() ||
SrcTy->getAddressSpace() !=
cast<PointerType>(CI->getType())->getAddressSpace() ||
IC.getTargetData()->getTypeSizeInBits(SrcPTy) !=
IC.getTargetData()->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;
const Type* CastSrcTy = SIOp0->getType();
const Type* CastDstTy = SrcPTy;
if (isa<PointerType>(CastDstTy)) {
if (CastSrcTy->isInteger())
opcode = Instruction::IntToPtr;
} else if (isa<IntegerType>(CastDstTy)) {
if (isa<PointerType>(SIOp0->getType()))
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.begin(),
NewGEPIndices.end());
NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy,
SIOp0->getName()+".c");
return new StoreInst(NewCast, CastOp);
}
/// 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;
}
// If this instruction has two uses, one of which is a llvm.dbg.declare,
// return the llvm.dbg.declare.
DbgDeclareInst *InstCombiner::hasOneUsePlusDeclare(Value *V) {
if (!V->hasNUses(2))
return 0;
for (Value::use_iterator UI = V->use_begin(), E = V->use_end();
UI != E; ++UI) {
if (DbgDeclareInst *DI = dyn_cast<DbgDeclareInst>(UI))
return DI;
if (isa<BitCastInst>(UI) && UI->hasOneUse()) {
if (DbgDeclareInst *DI = dyn_cast<DbgDeclareInst>(UI->use_begin()))
return DI;
}
}
return 0;
}
Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
Value *Val = SI.getOperand(0);
Value *Ptr = SI.getOperand(1);
// If the RHS is an alloca with a single use, zapify the store, making the
// alloca dead.
// If the RHS is an alloca with a two uses, the other one being a
// llvm.dbg.declare, zapify the store and the declare, making the
// alloca dead. We must do this to prevent declares from affecting
// codegen.
if (!SI.isVolatile()) {
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);
if (DbgDeclareInst *DI = hasOneUsePlusDeclare(GEP->getOperand(0))) {
EraseInstFromFunction(*DI);
return EraseInstFromFunction(SI);
}
}
}
}
if (DbgDeclareInst *DI = hasOneUsePlusDeclare(Ptr)) {
EraseInstFromFunction(*DI);
return EraseInstFromFunction(SI);
}
}
// Attempt to improve the alignment.
if (TD) {
unsigned KnownAlign =
GetOrEnforceKnownAlignment(Ptr, TD->getPrefTypeAlignment(Val->getType()));
if (KnownAlign >
(SI.getAlignment() == 0 ? TD->getABITypeAlignment(Val->getType()) :
SI.getAlignment()))
SI.setAlignment(KnownAlign);
}
// 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) && isa<PointerType>(BBI->getType()))) {
ScanInsts++;
continue;
}
if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) {
// Prev store isn't volatile, and stores to the same location?
if (!PrevSI->isVolatile() &&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) &&
!SI.isVolatile())
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;
}
if (SI.isVolatile()) return 0; // Don't hack volatile stores.
// 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) && isa<PointerType>(BBI->getType())));
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 *OtherBB = 0;
if (*PI != StoreBB)
OtherBB = *PI;
++PI;
if (PI == pred_end(DestBB))
return false;
if (*PI != StoreBB) {
if (OtherBB)
return false;
OtherBB = *PI;
}
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) && isa<PointerType>(BBI->getType()))) {
if (BBI==OtherBB->begin())
return false;
--BBI;
}
// If this isn't a store, isn't a store to the same location, or if the
// alignments differ, bail out.
OtherStore = dyn_cast<StoreInst>(BBI);
if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) ||
OtherStore->getAlignment() != SI.getAlignment())
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) ||
OtherStore->getAlignment() != SI.getAlignment())
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(), "storemerge");
PN->reserveOperandSpace(2);
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->getFirstNonPHI();
InsertNewInstBefore(new StoreInst(MergedVal, SI.getOperand(1),
OtherStore->isVolatile(),
SI.getAlignment()), *BBI);
// Nuke the old stores.
EraseInstFromFunction(SI);
EraseInstFromFunction(*OtherStore);
return true;
}
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