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Dfg: Use a worklist driven algorithm for unused vertex removal

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This improves verilation speed slightly.
This commit is contained in:
Geza Lore 2022-11-06 14:13:42 +00:00
parent fb9ec03c3f
commit 454efbe3fc
1 changed files with 46 additions and 22 deletions
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64
src/V3DfgPasses.cpp
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@ -209,30 +209,54 @@ void V3DfgPasses::removeVars(DfgGraph& dfg, DfgRemoveVarsContext& ctx) {
}
void V3DfgPasses::removeUnused(DfgGraph& dfg) {
// Iteratively remove operation vertices
while (true) {
// Do one pass over the graph.
bool changed = false;
// DfgVertex::user is the next pointer of the work list elements
const auto userDataInUse = dfg.userDataInUse();
// Head of work list. Note that we want all next pointers in the list to be non-zero (including
// that of the last element). This allows as to do two important things: detect if an element
// is in the list by checking for a non-zero next poitner, and easy prefetching without
// conditionals. The address of the graph is a good sentinel as it is a valid memory address,
// and we can easily check for the end of the list.
DfgVertex* const sentinelp = reinterpret_cast<DfgVertex*>(&dfg);
DfgVertex* workListp = sentinelp;
// Add all unused vertices to the work list. This also allocates all DfgVertex::user.
for (DfgVertex *vtxp = dfg.opVerticesBeginp(), *nextp; vtxp; vtxp = nextp) {
nextp = vtxp->verticesNext();
if (!vtxp->hasSinks()) {
changed = true;
if (VL_LIKELY(nextp)) VL_PREFETCH_RW(nextp);
if (vtxp->hasSinks()) {
// This vertex is used. Allocate user, but don't add to work list.
vtxp->setUser<DfgVertex*>(nullptr);
} else {
// This vertex is unused. Add to work list.
vtxp->setUser<DfgVertex*>(workListp);
workListp = vtxp;
}
}
// Process the work list
while (workListp != sentinelp) {
// Pick up the head
DfgVertex* const vtxp = workListp;
// Detach the head
workListp = vtxp->getUser<DfgVertex*>();
// Prefetch next item
VL_PREFETCH_RW(workListp);
// If used, then nothing to do, so move on
if (vtxp->hasSinks()) continue;
// Add sources of unused vertex to work list
vtxp->forEachSource([&](DfgVertex& src) {
// We only remove actual operation vertices in this loop
if (src.is<DfgConst>() || src.is<DfgVertexVar>()) return;
// If already in work list then nothing to do
if (src.getUser<DfgVertex*>()) return;
// Actually add to work list.
src.setUser<DfgVertex*>(workListp);
workListp = &src;
});
// Remove the unused vertex
vtxp->unlinkDelete(dfg);
}
}
if (!changed) break;
// Do another pass in the opposite direction. Alternating directions reduces
// the pathological complexity with left/right leaning trees.
changed = false;
for (DfgVertex *vtxp = dfg.opVerticesRbeginp(), *nextp; vtxp; vtxp = nextp) {
nextp = vtxp->verticesPrev();
if (!vtxp->hasSinks()) {
changed = true;
vtxp->unlinkDelete(dfg);
}
}
if (!changed) break;
}
// Finally remove unused constants
for (DfgConst *vtxp = dfg.constVerticesBeginp(), *nextp; vtxp; vtxp = nextp) {