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Be more conservative with our symbolic alias analysis. In particular,

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don't assume that A[1][0] and A[0][i] can't alias.  "i" might be out of
range, or even negative.  This fixes a miscompilation of 188.ammp (which
does bad pointer tricks) with the new CFE.

Testcase here: Analysis/BasicAA/2006-03-03-BadArraySubscript.ll


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26515 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-03-04 02:06:34 +00:00
parent 42983657ad
commit 5a3cf8de5d
1 changed files with 35 additions and 6 deletions
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41
lib/Analysis/BasicAliasAnalysis.cpp
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@ -527,6 +527,14 @@ CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
// chain. For example:
// A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
//
// We have to be careful here about array accesses. In particular, consider:
// A[1][0] vs A[0][i]
// In this case, we don't *know* that the array will be accessed in bounds:
// the index could even be negative. Because of this, we have to
// conservatively *give up* and return may alias. We disregard differing
// array subscripts that are followed by a variable index without going
// through a struct.
//
unsigned SizeMax = std::max(G1S, G2S);
if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work.
@ -547,15 +555,36 @@ CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
GEP1Ops[FirstConstantOper] = G1OC;
GEP2Ops[FirstConstantOper] = G2OC;
}
if (G1OC != G2OC) {
// Handle the "be careful" case above: if this is an array
// subscript, scan for a subsequent variable array index.
if (isa<ArrayType>(BasePtr1Ty)) {
const Type *NextTy =cast<ArrayType>(BasePtr1Ty)->getElementType();
bool isBadCase = false;
for (unsigned Idx = FirstConstantOper+1;
Idx != MinOperands && isa<ArrayType>(NextTy); ++Idx) {
const Value *V1 = GEP1Ops[Idx], *V2 = GEP2Ops[Idx];
if (!isa<Constant>(V1) || !isa<Constant>(V2)) {
isBadCase = true;
break;
}
NextTy = cast<ArrayType>(NextTy)->getElementType();
}
if (isBadCase) G1OC = 0;
}
// Make sure they are comparable (ie, not constant expressions), and
// make sure the GEP with the smaller leading constant is GEP1.
Constant *Compare = ConstantExpr::getSetGT(G1OC, G2OC);
if (ConstantBool *CV = dyn_cast<ConstantBool>(Compare)) {
if (CV->getValue()) // If they are comparable and G2 > G1
std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
break;
if (G1OC) {
Constant *Compare = ConstantExpr::getSetGT(G1OC, G2OC);
if (ConstantBool *CV = dyn_cast<ConstantBool>(Compare)) {
if (CV->getValue()) // If they are comparable and G2 > G1
std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
break;
}
}
}
}