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[BasicAA] Make non-equal index handling simpler to extend (NFC)

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This commit is contained in:
Nikita Popov 2020-12-12 13:23:49 +01:00
parent 36c749f8f1
commit cf4871bd6a
1 changed files with 21 additions and 18 deletions
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39
lib/Analysis/BasicAliasAnalysis.cpp
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@ -1285,24 +1285,27 @@ AliasResult BasicAAResult::aliasGEP(
(-DecompGEP1.Offset).uge(V1Size.getValue()))
return NoAlias;
// Try to determine whether the variable part of the GEP is non-zero, in
// which case we can add/subtract a minimum scale from the offset.
// TODO: Currently this handles the case of Scale*V0-Scale*V1 where V0!=V1.
// We could also handle Scale*V0 where V0!=0.
if (V1Size.hasValue() && V2Size.hasValue() &&
DecompGEP1.VarIndices.size() == 2) {
const VariableGEPIndex &Var0 = DecompGEP1.VarIndices[0];
const VariableGEPIndex &Var1 = DecompGEP1.VarIndices[1];
// Check that VisitedPhiBBs is empty, to avoid reasoning about inequality
// of values across loop iterations.
if (Var0.Scale == -Var1.Scale && Var0.ZExtBits == Var1.ZExtBits &&
Var0.SExtBits == Var1.SExtBits && VisitedPhiBBs.empty() &&
isKnownNonEqual(Var0.V, Var1.V, DL)) {
// If the indexes are not equal, the actual offset will have at least
// Scale or -Scale added to it.
APInt Scale = Var0.Scale.abs();
APInt OffsetLo = DecompGEP1.Offset - Scale;
APInt OffsetHi = DecompGEP1.Offset + Scale;
if (V1Size.hasValue() && V2Size.hasValue()) {
// Try to determine whether abs(VarIndex) > 0.
Optional<APInt> MinAbsVarIndex;
// TODO: Could handle single non-zero index as well.
if (DecompGEP1.VarIndices.size() == 2) {
// VarIndex = Scale*V0 + (-Scale)*V1.
// If V0 != V1 then abs(VarIndex) >= abs(Scale).
// Check that VisitedPhiBBs is empty, to avoid reasoning about
// inequality of values across loop iterations.
const VariableGEPIndex &Var0 = DecompGEP1.VarIndices[0];
const VariableGEPIndex &Var1 = DecompGEP1.VarIndices[1];
if (Var0.Scale == -Var1.Scale && Var0.ZExtBits == Var1.ZExtBits &&
Var0.SExtBits == Var1.SExtBits && VisitedPhiBBs.empty() &&
isKnownNonEqual(Var0.V, Var1.V, DL))
MinAbsVarIndex = Var0.Scale.abs();
}
if (MinAbsVarIndex) {
// The constant offset will have added at least +/-MinAbsVarIndex to it.
APInt OffsetLo = DecompGEP1.Offset - *MinAbsVarIndex;
APInt OffsetHi = DecompGEP1.Offset + *MinAbsVarIndex;
// Check that an access at OffsetLo or lower, and an access at OffsetHi
// or higher both do not alias.
if (OffsetLo.isNegative() && (-OffsetLo).uge(V1Size.getValue()) &&