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Teach BasicAliasAnalysis to understand constant gep indices that fall 

beyond their associated static array type.

I believe that this fixes a legitimate bug, because BasicAliasAnalysis
already has code to check for this condition that works for non-constant
indices, however it was missing the case of constant indices. With this
change, it checks for both.

This fixes PR4267, and miscompiles of SPEC 188.ammp and 464.h264.href.

llvm-svn: 72451
This commit is contained in:
Dan Gohman 2009-05-27 01:48:27 +00:00
parent ace7215dd4
commit 776e4c8d35
2 changed files with 52 additions and 4 deletions
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29
llvm/lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -611,18 +611,39 @@ BasicAliasAnalysis::CheckGEPInstructions(
if (G1OC != G2OC) {
// Handle the "be careful" case above: if this is an array/vector
// subscript, scan for a subsequent variable array index.
if (isa<SequentialType>(BasePtr1Ty)) {
const Type *NextTy =
cast<SequentialType>(BasePtr1Ty)->getElementType();
if (const SequentialType *STy =
dyn_cast<SequentialType>(BasePtr1Ty)) {
const Type *NextTy = STy;
bool isBadCase = false;
for (unsigned Idx = FirstConstantOper+1;
for (unsigned Idx = FirstConstantOper;
Idx != MinOperands && isa<SequentialType>(NextTy); ++Idx) {
const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
isBadCase = true;
break;
}
// If the array is indexed beyond the bounds of the static type
// at this level, it will also fall into the "be careful" case.
// It would theoretically be possible to analyze these cases,
// but for now just be conservatively correct.
if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
if (cast<ConstantInt>(G1OC)->getZExtValue() >=
ATy->getNumElements() ||
cast<ConstantInt>(G2OC)->getZExtValue() >=
ATy->getNumElements()) {
isBadCase = true;
break;
}
if (const VectorType *VTy = dyn_cast<VectorType>(STy))
if (cast<ConstantInt>(G1OC)->getZExtValue() >=
VTy->getNumElements() ||
cast<ConstantInt>(G2OC)->getZExtValue() >=
VTy->getNumElements()) {
isBadCase = true;
break;
}
STy = cast<SequentialType>(NextTy);
NextTy = cast<SequentialType>(NextTy)->getElementType();
}

27
llvm/test/Analysis/BasicAA/constant-over-index.ll Normal file
View File

@ -0,0 +1,27 @@
; RUN: llvm-as < %s | opt -aa-eval -print-all-alias-modref-info \
; RUN: |& grep {MayAlias: double\\* \[%\]p.0.i.0, double\\* \[%\]p3\$}
; PR4267
; %p3 is equal to %p.0.i.0 on the second iteration of the loop,
; so MayAlias is needed.
define void @foo([3 x [3 x double]]* noalias %p) {
entry:
%p3 = getelementptr [3 x [3 x double]]* %p, i64 0, i64 0, i64 3
br label %loop
loop:
%i = phi i64 [ 0, %entry ], [ %i.next, %loop ]
%p.0.i.0 = getelementptr [3 x [3 x double]]* %p, i64 0, i64 %i, i64 0
volatile store double 0.0, double* %p3
volatile store double 0.1, double* %p.0.i.0
%i.next = add i64 %i, 1
%cmp = icmp slt i64 %i.next, 3
br i1 %cmp, label %loop, label %exit
exit:
ret void
}