[BasicAA] Extend inbound GEP negative offset logic to GlobalVariables

r270777 improved the precision of alloca vs. inbounbds GEP alias queries: if
we have (a) an inbounds GEP and (b) a pointer based on an alloca, and the
beginning of the object the GEP points to would have a negative offset with
respect to the alloca, then the GEP can not alias pointer (b).

This makes the same logic fire when (b) is based on a GlobalVariable instead
of an alloca.

Differential Revision: http://reviews.llvm.org/D20652

llvm-svn: 270893
This commit is contained in:
Michael Kuperstein 2016-05-26 19:30:49 +00:00
parent d99068d26d
commit ae21491819
3 changed files with 31 additions and 12 deletions

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@ -157,8 +157,8 @@ private:
const DataLayout &DL, AssumptionCache *AC, DominatorTree *DT);
static bool isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompAlloca,
uint64_t AllocaAccessSize);
const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject,
uint64_t ObjectAccessSize);
/// \brief A Heuristic for aliasGEP that searches for a constant offset
/// between the variables.

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@ -955,6 +955,11 @@ static AliasResult aliasSameBasePointerGEPs(const GEPOperator *GEP1,
// repsect to the alloca, that means the GEP can not alias pointer (b).
// Note that the pointer based on the alloca may not be a GEP. For
// example, it may be the alloca itself.
// The same applies if (b) is based on a GlobalVariable. Note that just being
// based on isIdentifiedObject() is not enough - we need an identified object
// that does not permit access to negative offsets. For example, a negative
// offset from a noalias argument or call can be inbounds w.r.t the actual
// underlying object.
//
// For example, consider:
//
@ -977,19 +982,22 @@ static AliasResult aliasSameBasePointerGEPs(const GEPOperator *GEP1,
// the highest %f1 can be is (%alloca + 3). This means %random can not be higher
// than (%alloca - 1), and so is not inbounds, a contradiction.
bool BasicAAResult::isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompAlloca,
uint64_t AllocaAccessSize) {
// If the alloca access size is unknown, or the GEP isn't inbounds, bail.
if (AllocaAccessSize == MemoryLocation::UnknownSize || !GEPOp->isInBounds())
const DecomposedGEP &DecompGEP, const DecomposedGEP &DecompObject,
uint64_t ObjectAccessSize) {
// If the object access size is unknown, or the GEP isn't inbounds, bail.
if (ObjectAccessSize == MemoryLocation::UnknownSize || !GEPOp->isInBounds())
return false;
// We need an alloca, and want to know the offset of the pointer
// from the alloca precisely, so no variable indices are allowed.
if (!isa<AllocaInst>(DecompAlloca.Base) || !DecompAlloca.VarIndices.empty())
// We need the object to be an alloca or a globalvariable, and want to know
// the offset of the pointer from the object precisely, so no variable
// indices are allowed.
if (!(isa<AllocaInst>(DecompObject.Base) ||
isa<GlobalVariable>(DecompObject.Base)) ||
!DecompObject.VarIndices.empty())
return false;
int64_t AllocaBaseOffset = DecompAlloca.StructOffset +
DecompAlloca.OtherOffset;
int64_t ObjectBaseOffset = DecompObject.StructOffset +
DecompObject.OtherOffset;
// If the GEP has no variable indices, we know the precise offset
// from the base, then use it. If the GEP has variable indices, we're in
@ -1000,7 +1008,7 @@ bool BasicAAResult::isGEPBaseAtNegativeOffset(const GEPOperator *GEPOp,
if (DecompGEP.VarIndices.empty())
GEPBaseOffset += DecompGEP.OtherOffset;
return (GEPBaseOffset >= AllocaBaseOffset + (int64_t)AllocaAccessSize);
return (GEPBaseOffset >= ObjectBaseOffset + (int64_t)ObjectAccessSize);
}
/// Provides a bunch of ad-hoc rules to disambiguate a GEP instruction against

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@ -26,6 +26,17 @@ define void @arg(i32* %arg) {
ret void
}
@gv = global i32 1
; CHECK-LABEL: Function: global:
; CHECK-DAG: MayAlias: i32* %p0, i32* @gv
; CHECK-DAG: NoAlias: i32* %p1, i32* @gv
define void @global() {
%random = call i32* @random.i32(i32* @gv)
%p0 = getelementptr inbounds i32, i32* %random, i32 0
%p1 = getelementptr inbounds i32, i32* %random, i32 1
ret void
}
; CHECK-LABEL: Function: struct:
; CHECK-DAG: MayAlias: i32* %f0, i32* %p0
; CHECK-DAG: MayAlias: i32* %f1, i32* %p0