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lutpack: assertion failure in Lpk_MuxSplit from an approximate cofactor support #540

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lutpack: assertion failure in Lpk_MuxSplit from an approximate cofactor support #540

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18 changes: 18 additions & 0 deletions src/opt/lpk/lpkAbcDec.c
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Original file line number Diff line number Diff line change
Expand Up @@ -202,6 +202,24 @@ pMan->timeEvalMuxAn += Abc_Clock() - clk;
assert( pResMux == NULL || pResDsd == NULL );
if ( pResMux )
{
// Lpk_MuxAnalize() decides feasibility from the cached cofactor supports in
// p->puSupps. Those may have come from Lpk_ComputeSupports(), which derives
// them from two BDDs built in opposite variable orders and stitches the halves
// together, and that estimate can be a strict SUBSET of the true cofactor
// support. When it is, the component retained by the split below ends up with
// no vacant fanin slot for the component that is split off, and Lpk_MuxSplit()
// fails its assertion `iVarVac < (int)p->nVars'. Re-derive the one support the
// split actually depends on and decline the MUX decomposition if it does not fit.
unsigned * pTruthThis = Lpk_FunTruth( p, 0 );
unsigned * pTruthCof = Lpk_FunTruth( p, 1 );
unsigned uSuppExact;
if ( pResMux->Polarity )
Kit_TruthCofactor1New( pTruthCof, pTruthThis, p->nVars, pResMux->Variable );
else
Kit_TruthCofactor0New( pTruthCof, pTruthThis, p->nVars, pResMux->Variable );
uSuppExact = Kit_TruthSupport( pTruthCof, p->nVars ) | ( 1 << pResMux->Variable );
if ( Kit_WordCountOnes( uSuppExact ) >= (int)p->nVars )
return 0;
clk = Abc_Clock();
p2 = Lpk_MuxSplit( pMan, p, pResMux->Variable, pResMux->Polarity );
pMan->timeEvalMuxSp += Abc_Clock() - clk;
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