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Improve MTask coarsening in multi-threaded scheduling (#8120)

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This is a large refactor of the MTask graph and coarsening algorithm, in
prep for fixing the bug described in #7913, it can also improve the
resulting multi-threaded schedule.

Two major changes:

OrderMTaskGraph now maintains the critical paths of the MTasks through
mutation. There are 2 ways to mutate the graph, which are done via
methods on the graph itself: adding an edge (used during construction,
and will be used later during fixing data hazards), or merging an MTask
into another (used during contraction). All critical path measures are
automatically updated and propagated on any mutation, so no external
algorithm needs to maintain them explicitly.

The merge candidate scoreboard used during contraction is simplified to
remove deferral of updated scores. This simplifies the code and results
in a greedily more optimal schedule. (The previous tranched rescore was
an optimization to work around the previous std::set based scoreboard,
however since the algorithm now uses an efficient PairingHeap,
verilation time is not impacted by the more accurate scoring, while
yielding better results).

Combining these two into a single patch as the code is highly
interdependent and any one change without the other would be just a
noisy transit point with unclear performance implications. Together it
should be a clear improvement.

Also added a stronger validation step run with '--debug-partition',
which checks all invariants throughout the algorithms.
This commit is contained in:
Geza Lore
2026-08-17 23:14:52 +02:00
committed by GitHub
parent ace84ef2b3
commit 7dcd4e0b65
10 changed files with 834 additions and 1144 deletions
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src/V3Graph.cpp
-25
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@@ -79,31 +79,6 @@ void V3GraphVertex::rerouteEdges(V3Graph* graphp) {
unlinkEdges(graphp);
}
template <GraphWay::en N_Way>
V3GraphEdge* V3GraphVertex::findConnectingEdgep(V3GraphVertex* waywardp) {
// O(edges) linear search. Searches search both nodes' edge lists in
// parallel. The lists probably aren't _both_ huge, so this is
// unlikely to blow up even on fairly nasty graphs.
constexpr GraphWay way{N_Way};
constexpr GraphWay inv = way.invert();
auto& aEdges = this->edges<way>();
auto aIt = aEdges.begin();
auto aEnd = aEdges.end();
auto& bEdges = waywardp->edges<inv>();
auto bIt = bEdges.begin();
auto bEnd = bEdges.end();
while (aIt != aEnd && bIt != bEnd) {
V3GraphEdge& aedge = *aIt++;
if (aedge.furtherp<way>() == waywardp) return &aedge;
V3GraphEdge& bedge = *bIt++;
if (bedge.furtherp<inv>() == this) return &bedge;
}
return nullptr;
}
template V3GraphEdge* V3GraphVertex::findConnectingEdgep<GraphWay::FORWARD>(V3GraphVertex*);
template V3GraphEdge* V3GraphVertex::findConnectingEdgep<GraphWay::REVERSE>(V3GraphVertex*);
// cppcheck-has-bug-suppress constParameter
void V3GraphVertex::v3errorEnd(const std::ostringstream& str) const // LCOV_EXCL_START
VL_RELEASE(V3Error::s().m_mutex) {