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[CHERIOT] Disallow more ways of forming negative offsets in LSR. #196

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Merged
merged 1 commit into from
Jul 26, 2025
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[CHERIOT] Disallow more ways of forming negative offsets in LSR. #196

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28 changes: 18 additions & 10 deletions llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2404,20 +2404,28 @@ RISCVTTIImpl::getPreferredAddressingMode(const Loop *L,

bool RISCVTTIImpl::isLegalBaseRegForLSR(const SCEV *S) const {
if (ST->hasVendorXCheriot()) {
// Disallow any add-recurrence SCEV where the base offset is negative.
// Disallow any SCEV where the base offset is negative.
// This is needed because CHERIoT can't represent pointers before the
// beginning of an array.
auto IsAddOfNegativeCst = [](const SCEVAddExpr *A) {
if (!A)
return false;
const auto *Offset = dyn_cast<SCEVConstant>(A->getOperand(0));
if (Offset && Offset->getValue()->isNegative())
return true;
Offset = dyn_cast<SCEVConstant>(A->getOperand(1));
if (Offset && Offset->getValue()->isNegative())
return true;
return false;
};

if (const auto *AddRec = dyn_cast<SCEVAddRecExpr>(S)) {
const auto *StartAdd = dyn_cast<SCEVAddExpr>(AddRec->getStart());
if (StartAdd) {
const auto *Offset = dyn_cast<SCEVConstant>(StartAdd->getOperand(0));
if (Offset && Offset->getValue()->isNegative())
return false;
Offset = dyn_cast<SCEVConstant>(StartAdd->getOperand(1));
if (Offset && Offset->getValue()->isNegative())
return false;
}
if (const auto *Cst = dyn_cast<SCEVConstant>(AddRec->getStart()))
return !Cst->getValue()->isNegative();
return !IsAddOfNegativeCst(dyn_cast<SCEVAddExpr>(AddRec->getStart()));
}

return !IsAddOfNegativeCst(dyn_cast<SCEVAddExpr>(S));
}
return BasicTTIImplBase::isLegalBaseRegForLSR(S);
}
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