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Fix excessive logic replication in Dfg circular driver tracing (#6561) (#6594) 

Cache already traced vertices and reuse them on subsequent traces. This
avoids a potential combinatorial explosion in the size of the resulting
circuit.
This commit is contained in:
Geza Lore 2025-10-25 16:07:22 +02:00 committed by GitHub
parent e732f656a9
commit 92b490f2f7
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1 changed files with 29 additions and 21 deletions
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50
src/V3DfgBreakCycles.cpp
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@ -73,6 +73,8 @@ class TraceDriver final : public DfgVisitor {
uint32_t m_lsb = 0; // LSB to extract from the currently visited Vertex
uint32_t m_msb = 0; // MSB to extract from the currently visited Vertex
DfgVertex* m_defaultp = nullptr; // When tracing a variable, this is its 'defaultp', if any
// Result cache for reusing already traced vertices
std::unordered_map<Visited, DfgVertex*, Visited::Hash, Visited::Equal> m_cache;
// Result of tracing the currently visited Vertex. Use SET_RESULT below!
DfgVertex* m_resp = nullptr;
std::vector<DfgVertex*> m_newVtxps; // New vertices created during the traversal
@ -155,12 +157,22 @@ class TraceDriver final : public DfgVisitor {
// Push to stack
m_stack.emplace_back(vtxp, msb, lsb);
bool& onStackr = m_visited[m_stack.back()];
// Save onStack before updating
const bool wasOnStack = onStackr;
// We are tracing the vertex now
onStackr = true;
// Check for true combinational cycles
if (onStackr) {
// Pop from stack
m_stack.pop_back();
// The resulting driver that is not part of m_component
DfgVertex* resp = nullptr;
// Retrieve cache entry
const auto& cachePair = m_cache.emplace(m_stack.back(), nullptr);
DfgVertex*& cachedRespr = cachePair.first->second;
// Trace the vertex
if (wasOnStack) {
// If it was already on the stack, then this is a true
// combinational cycles, terminate the trace.
// Note: could issue a "proper combinational cycle" error here,
// but constructing a legible error message is hard as the Vertex
// Filelines can be very rough after optimizations (could consider
@ -168,20 +180,15 @@ class TraceDriver final : public DfgVisitor {
// run mulitple times and report the same error again. There will
// be an UNOPTFLAT issued during scheduling anyway, and the true
// cycle might still settle at run-time.
// Stop trace
return nullptr;
}
// Trace the vertex
onStackr = true;
// The resulting driver that is not part of m_component
DfgVertex* resp = nullptr;
// If the currently traced vertex is in a different component,
// then we found what we were looking for.
if (m_vtx2Scc[vtxp] != m_component) {
resp = nullptr;
} else if (!cachePair.second) {
// If already traced this vtxp/msb/lsb, just use the result.
// This is important to avoid combinatorial explosion when the
// same sub-expression is needed multiple times.
resp = cachedRespr;
} else if (m_vtx2Scc[vtxp] != m_component) {
// If the currently traced vertex is in a different component,
// then we found what we were looking for.
resp = vtxp;
// If the result is a splice, we need to insert a temporary for it
// as a splice cannot be fed into arbitray logic
@ -212,14 +219,15 @@ class TraceDriver final : public DfgVisitor {
}
UASSERT_OBJ(!resp || resp->width() == (msb - lsb + 1), vtxp, "Wrong result width");
// Save to cache
cachedRespr = resp;
// Pop from stack
onStackr = false;
m_stack.pop_back();
// Done
if (!resp) {
UINFO(9, "TraceDriver - Failed to trace vertex of type: " << vtxp->typeName());
}
if (!resp) UINFO(9, "TraceDriver - Failed to trace vertex of type: " << vtxp->typeName());
return resp;
}