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Optimize temporary insertion for single bit replicates in DFG (#8110) 

A '{N{bit}}' mask expands to a bit vector where every word is the same
value, obtained by negating the replicated bit. Recomputing that at each
use is no more expensive than loading it
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Geza Lore 2026-08-14 14:58:31 +02:00 committed by GitHub
parent 2957939d4b
commit 0a3657517d
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7
src/V3Dfg.h
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@ -829,6 +829,13 @@ bool DfgVertex::isCheaperThanLoad() const {
const uint32_t msb = lsb + selp->width() - 1;
return VL_BITWORD_E(msb) == VL_BITWORD_E(lsb);
}
// Replication of a single cheap bit. Each word of the result is the same
// mask computed by negating that bit, so recomputing it at each use costs
// no more than the load it replaces.
if (const DfgRep* const repp = cast<DfgRep>()) {
const DfgVertex* const srcp = repp->srcp();
return srcp->width() == 1 && srcp->isCheaperThanLoad();
}
// Zero extend of a cheap vertex - Extend(_) was converted to Concat(0, _)
if (const DfgConcat* const catp = cast<DfgConcat>()) {
if (catp->width() > VL_QUADSIZE) return false;