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//===- Loads.cpp - Local load analysis ------------------------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file defines simple local analyses for load instructions.
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Target/TargetData.h"
#include "llvm/GlobalAlias.h"
#include "llvm/GlobalVariable.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Operator.h"
using namespace llvm;
/// AreEquivalentAddressValues - 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 AreEquivalentAddressValues(const Value *A, const Value *B) {
// Test if the values are trivially equivalent.
if (A == B) return true;
// Test if the values come from identical arithmetic instructions.
// Use isIdenticalToWhenDefined instead of isIdenticalTo because
// this function is only used when one address use dominates the
// other, 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 (const Instruction *BI = dyn_cast<Instruction>(B))
if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
return true;
// Otherwise they may not be equivalent.
return false;
/// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and
/// bitcasts to get back to the underlying object being addressed, keeping
/// track of the offset in bytes from the GEPs relative to the result.
/// This is closely related to GetUnderlyingObject but is located
/// here to avoid making VMCore depend on TargetData.
static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD,
uint64_t &ByteOffset,
unsigned MaxLookup = 6) {
if (!V->getType()->isPointerTy())
return V;
for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) {
if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
if (!GEP->hasAllConstantIndices())
return V;
SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end());
ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(),
V = GEP->getPointerOperand();
} else if (Operator::getOpcode(V) == Instruction::BitCast) {
V = cast<Operator>(V)->getOperand(0);
} else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
if (GA->mayBeOverridden())
return V;
V = GA->getAliasee();
} else {
return V;
assert(V->getType()->isPointerTy() && "Unexpected operand type!");
return V;
/// isSafeToLoadUnconditionally - Return true if we know that executing a load
/// from this value cannot trap. If it is not obviously safe to load from the
/// specified pointer, we do a quick local scan of the basic block containing
/// ScanFrom, to determine if the address is already accessed.
bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
unsigned Align, const TargetData *TD) {
uint64_t ByteOffset = 0;
Value *Base = V;
if (TD)
Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset);
Type *BaseType = 0;
unsigned BaseAlign = 0;
if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
// An alloca is safe to load from as load as it is suitably aligned.
BaseType = AI->getAllocatedType();
BaseAlign = AI->getAlignment();
} else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) {
// Global variables are safe to load from but their size cannot be
// guaranteed if they are overridden.
if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) {
BaseType = GV->getType()->getElementType();
BaseAlign = GV->getAlignment();
if (BaseType && BaseType->isSized()) {
if (TD && BaseAlign == 0)
BaseAlign = TD->getPrefTypeAlignment(BaseType);
if (Align <= BaseAlign) {
if (!TD)
return true; // Loading directly from an alloca or global is OK.
// Check if the load is within the bounds of the underlying object.
PointerType *AddrTy = cast<PointerType>(V->getType());
uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
(Align == 0 || (ByteOffset % Align) == 0))
return true;
// Otherwise, be a little bit aggressive by scanning the local block where we
// want to check to see if the pointer is already being loaded or stored
// from/to. If so, the previous load or store would have already trapped,
// so there is no harm doing an extra load (also, CSE will later eliminate
// the load entirely).
BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
while (BBI != E) {
// If we see a free or a call which may write to memory (i.e. which might do
// a free) the pointer could be marked invalid.
if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
return false;
if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true;
} else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true;
return false;
/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
/// instruction before ScanFrom) checking to see if we have the value at the
/// memory address *Ptr locally available within a small number of instructions.
/// If the value is available, return it.
/// If not, return the iterator for the last validated instruction that the
/// value would be live through. If we scanned the entire block and didn't find
/// something that invalidates *Ptr or provides it, ScanFrom would be left at
/// begin() and this returns null. ScanFrom could also be left
/// MaxInstsToScan specifies the maximum instructions to scan in the block. If
/// it is set to 0, it will scan the whole block. You can also optionally
/// specify an alias analysis implementation, which makes this more precise.
/// If TBAATag is non-null and a load or store is found, the TBAA tag from the
/// load or store is recorded there. If there is no TBAA tag or if no access
/// is found, it is left unmodified.
Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
BasicBlock::iterator &ScanFrom,
unsigned MaxInstsToScan,
AliasAnalysis *AA,
MDNode **TBAATag) {
if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
// If we're using alias analysis to disambiguate get the size of *Ptr.
uint64_t AccessSize = 0;
if (AA) {
Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
AccessSize = AA->getTypeStoreSize(AccessTy);
while (ScanFrom != ScanBB->begin()) {
// We must ignore debug info directives when counting (otherwise they
// would affect codegen).
Instruction *Inst = --ScanFrom;
if (isa<DbgInfoIntrinsic>(Inst))
// Restore ScanFrom to expected value in case next test succeeds
// Don't scan huge blocks.
if (MaxInstsToScan-- == 0) return 0;
// If this is a load of Ptr, the loaded value is available.
// (This is true even if the load is volatile or atomic, although
// those cases are unlikely.)
if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) {
if (TBAATag) *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
return LI;
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
// If this is a store through Ptr, the value is available!
// (This is true even if the store is volatile or atomic, although
// those cases are unlikely.)
if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) {
if (TBAATag) *TBAATag = SI->getMetadata(LLVMContext::MD_tbaa);
return SI->getOperand(0);
// If Ptr is an alloca and this is a store to a different alloca, ignore
// the store. This is a trivial form of alias analysis that is important
// for reg2mem'd code.
if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
(isa<AllocaInst>(SI->getOperand(1)) ||
// If we have alias analysis and it says the store won't modify the loaded
// value, ignore the store.
if (AA &&
(AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
// Otherwise the store that may or may not alias the pointer, bail out.
return 0;
// If this is some other instruction that may clobber Ptr, bail out.
if (Inst->mayWriteToMemory()) {
// If alias analysis claims that it really won't modify the load,
// ignore it.
if (AA &&
(AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
// May modify the pointer, bail out.
return 0;
// Got to the start of the block, we didn't find it, but are done for this
// block.
return 0;