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Support 2**n expressions in constraint randomization (#7422)

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This commit is contained in:
Yilou Wang
2026-04-14 07:17:53 -04:00
committed by GitHub
parent 2770e649ba
commit 9eb2ba4c54
6 changed files with 162 additions and 0 deletions
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src/V3Randomize.cpp
+36
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@@ -1629,6 +1629,42 @@ class ConstraintExprVisitor final : public VNVisitor {
nodep->replaceWith(powerp);
VL_DO_DANGLING(nodep->deleteTree(), nodep);
iterate(powerp);
} else if (const AstConst* const basep = VN_CAST(nodep->lhsp(), Const)) {
if (basep->num().toUInt() == 2) {
// Non-constant exponent, base == 2: transform to shift-based SMT expression.
// AstPow/AstPowSU (unsigned exponent): 2**n -> 1<<n
// AstPowSS/AstPowUS (signed exponent): 2**n -> n>=0 ? 1<<n : 0
// IEEE 1800-2023 ss11.4.3: integer base with negative exponent gives 0,
// which ShiftL cannot represent; use SMT ite (AstCond -> bvsge + bvshl).
FileLine* const fl = nodep->fileline();
AstNodeExpr* const rhsp = nodep->rhsp()->unlinkFrBack();
const int width = nodep->width();
const bool rhsDependent = rhsp->user1();
AstConst* const onep = new AstConst{fl, AstConst::WidthedValue{}, width, 1};
AstShiftL* const shiftp = new AstShiftL{fl, onep, rhsp, width};
shiftp->dtypeFrom(nodep);
shiftp->user1(rhsDependent);
AstNodeExpr* resultp = shiftp;
if (VN_IS(nodep, PowSS) || VN_IS(nodep, PowUS)) {
// Signed exponent: guard 1<<n with n>=0 check
AstNodeExpr* const nClonep = rhsp->cloneTreePure(false);
nClonep->user1(rhsDependent);
AstConst* const zeroNp
= new AstConst{fl, AstConst::WidthedValue{}, rhsp->width(), 0};
AstGteS* const condp = new AstGteS{fl, nClonep, zeroNp};
condp->user1(rhsDependent);
AstConst* const zeroRp = new AstConst{fl, AstConst::WidthedValue{}, width, 0};
resultp = new AstCond{fl, condp, shiftp, zeroRp};
resultp->dtypeFrom(nodep);
resultp->user1(nodep->user1());
}
nodep->replaceWith(resultp);
VL_DO_DANGLING(nodep->deleteTree(), nodep);
iterate(resultp);
} else {
nodep->v3warn(CONSTRAINTIGN,
"Unsupported: Power (**) expression with non-2 base in constraint");
}
} else {
nodep->v3warn(
CONSTRAINTIGN,