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Allow isDereferenceablePointer to look through some bitcasts

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isDereferenceablePointer should not give up upon encountering any bitcast. If
we're casting from a pointer to a larger type to a pointer to a small type, we
can continue by examining the bitcast's operand. This missing capability
was noted in a comment in the function.

In order for this to work, isDereferenceablePointer now takes an optional
DataLayout pointer (essentially all callers already had such a pointer
available). Most code uses isDereferenceablePointer though
isSafeToSpeculativelyExecute (which already took an optional DataLayout
pointer), and to enable the LICM test case, LICM needs to actually provide its DL
pointer to isSafeToSpeculativelyExecute (which it was not doing previously).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212686 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel
2014-07-10 05:27:53 +00:00
parent 344517923c
commit a739834446
8 changed files with 202 additions and 21 deletions
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31
lib/IR/Value.cpp
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@@ -17,6 +17,7 @@
#include "llvm/ADT/SmallString.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/InstrTypes.h"
@@ -472,18 +473,32 @@ Value *Value::stripInBoundsOffsets() {
/// isDereferenceablePointer - Test if this value is always a pointer to
/// allocated and suitably aligned memory for a simple load or store.
static bool isDereferenceablePointer(const Value *V,
static bool isDereferenceablePointer(const Value *V, const DataLayout *DL,
SmallPtrSet<const Value *, 32> &Visited) {
// Note that it is not safe to speculate into a malloc'd region because
// malloc may return null.
// It's also not always safe to follow a bitcast, for example:
// bitcast i8* (alloca i8) to i32*
// would result in a 4-byte load from a 1-byte alloca. Some cases could
// be handled using DataLayout to check sizes and alignments though.
// These are obviously ok.
if (isa<AllocaInst>(V)) return true;
// It's not always safe to follow a bitcast, for example:
// bitcast i8* (alloca i8) to i32*
// would result in a 4-byte load from a 1-byte alloca. However,
// if we're casting from a pointer from a type of larger size
// to a type of smaller size (or the same size), and the alignment
// is at least as large as for the resulting pointer type, then
// we can look through the bitcast.
if (DL)
if (const BitCastInst* BC = dyn_cast<BitCastInst>(V)) {
Type *STy = BC->getSrcTy()->getPointerElementType(),
*DTy = BC->getDestTy()->getPointerElementType();
if ((DL->getTypeStoreSize(STy) >=
DL->getTypeStoreSize(DTy)) &&
(DL->getABITypeAlignment(STy) >=
DL->getABITypeAlignment(DTy)))
return isDereferenceablePointer(BC->getOperand(0), DL, Visited);
}
// Global variables which can't collapse to null are ok.
if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
return !GV->hasExternalWeakLinkage();
@@ -497,7 +512,7 @@ static bool isDereferenceablePointer(const Value *V,
// Conservatively require that the base pointer be fully dereferenceable.
if (!Visited.insert(GEP->getOperand(0)))
return false;
if (!isDereferenceablePointer(GEP->getOperand(0), Visited))
if (!isDereferenceablePointer(GEP->getOperand(0), DL, Visited))
return false;
// Check the indices.
gep_type_iterator GTI = gep_type_begin(GEP);
@@ -533,9 +548,9 @@ static bool isDereferenceablePointer(const Value *V,
/// isDereferenceablePointer - Test if this value is always a pointer to
/// allocated and suitably aligned memory for a simple load or store.
bool Value::isDereferenceablePointer() const {
bool Value::isDereferenceablePointer(const DataLayout *DL) const {
SmallPtrSet<const Value *, 32> Visited;
return ::isDereferenceablePointer(this, Visited);
return ::isDereferenceablePointer(this, DL, Visited);
}
/// DoPHITranslation - If this value is a PHI node with CurBB as its parent,