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Internals: Do not generate redundant masking on word selects (#7819) 

newAstWordSelClone returns a clean word and these And mask out exactly
the bits that are zero after the shift.
This commit is contained in:
Geza Lore 2026-06-22 16:42:22 +01:00 committed by GitHub
parent 9670dabcfe
commit 7765738e11
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GPG Key ID: B5690EEEBB952194
1 changed files with 10 additions and 22 deletions
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32
src/V3Expand.cpp
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@ -206,31 +206,19 @@ class ExpandVisitor final : public VNVisitor {
static AstNodeExpr* newWordGrabShift(FileLine* fl, int word, AstNodeExpr* lhsp, int shift) {
// Extract the expression to grab the value for the specified word, if it's the shift
// of shift bits from lhsp
AstNodeExpr* newp;
// Negative word numbers requested for lhs when it's "before" what we want.
// We get a 0 then.
const int othword = word - shift / VL_EDATASIZE;
AstNodeExpr* const llowp = newAstWordSelClone(lhsp, othword);
if (const int loffset = VL_BITBIT_E(shift)) {
AstNodeExpr* const lhip = newAstWordSelClone(lhsp, othword - 1);
const int nbitsonright = VL_EDATASIZE - loffset; // bits that end up in lword
newp = new AstOr{
fl,
new AstAnd{fl, new AstConst{fl, AstConst::SizedEData{}, VL_MASK_E(loffset)},
new AstShiftR{fl, lhip,
new AstConst{fl, static_cast<uint32_t>(nbitsonright)},
VL_EDATASIZE}},
new AstAnd{fl,
new AstConst{fl, AstConst::SizedEData{},
static_cast<uint32_t>(~VL_MASK_E(loffset))},
new AstShiftL{fl, llowp,
new AstConst{fl, static_cast<uint32_t>(loffset)},
VL_EDATASIZE}}};
newp = V3Const::constifyEditCpp(newp);
} else {
newp = llowp;
}
return newp;
AstNodeExpr* const lop = newAstWordSelClone(lhsp, othword);
const uint32_t loShift = VL_BITBIT_E(shift);
if (!loShift) return lop;
AstNodeExpr* const hip = newAstWordSelClone(lhsp, othword - 1);
const uint32_t hiShift = VL_EDATASIZE - loShift; // Complement offset
AstNodeExpr* const newp
= new AstOr{fl, //
new AstShiftR{fl, hip, new AstConst{fl, hiShift}, VL_EDATASIZE},
new AstShiftL{fl, lop, new AstConst{fl, loShift}, VL_EDATASIZE}};
return V3Const::constifyEditCpp(newp);
}
// Return expression indexing the word that contains 'lsbp' + the given word offset