Optimize more always blocks in Dfg (#7775)

Optimize always blocks that:
- drive an unused variable (so logic can be pruned)
- write only a single variable (regardless of CFG size)

src/V3DfgPasses.cpp

@@ -78,7 +78,13 @@ void V3DfgPasses::removeUnobservable(DfgGraph& dfg, V3DfgContext& dfgCtx) {
                             && !vVtxp->hasExtWrRefs()  //
                             && !vVtxp->hasModWrRefs();
         VL_DO_DANGLING(vVtxp->unlinkDelete(dfg), vVtxp);
-        if (srcp) VL_DO_DANGLING(srcp->unlinkDelete(dfg), srcp);
+        if (srcp) {
+            srcp->foreachSource([&](DfgVertex& src) {
+                src.as<DfgLogic>()->setDrivesUnusedVars();
+                return false;
+            });
+            VL_DO_DANGLING(srcp->unlinkDelete(dfg), srcp);
+        }
         if (delAst) {
             VL_DO_DANGLING(vscp->unlinkFrBack()->deleteTree(), vscp);
             ++ctx.m_varsDeleted;

src/V3DfgSynthesize.cpp

@@ -2005,6 +2005,11 @@ static void dfgSelectLogicForSynthesis(DfgGraph& dfg) {
     for (DfgVertex& vtx : dfg.opVertices()) {
         DfgLogic* const logicp = vtx.cast<DfgLogic>();
         if (!logicp) continue;
+        // If drives an unused variable, synthesize it so the partial logic can be removed
+        if (logicp->drivesUnusedVars()) {
+            worklist.push_front(*logicp);
+            continue;
+        }
         // Blocks corresponding to continuous assignments
         if (logicp->nodep()->keyword() == VAlwaysKwd::CONT_ASSIGN) {
             worklist.push_front(*logicp);
@@ -2016,10 +2021,8 @@ static void dfgSelectLogicForSynthesis(DfgGraph& dfg) {
             worklist.push_front(*logicp);
             continue;
         }
-        // Simple blocks driving exactly 1 variable, e.g if (rst) a = b else a = c;
-        if (!logicp->hasMultipleSinks() && cfg.nBlocks() <= 4 && cfg.nEdges() <= 4) {
-            worklist.push_front(*logicp);
-        }
+        // Blocks driving exactly 1 variable
+        if (!logicp->hasMultipleSinks()) worklist.push_front(*logicp);
     }
 
     // Now expand to cover all logic driving the same set of variables and mark

src/V3DfgVertices.h

@@ -512,6 +512,7 @@ class DfgLogic final : public DfgVertexVariadic {
     AstScope* const m_scopep;  // The AstScope m_nodep is under, iff scoped
     const std::unique_ptr<CfgGraph> m_cfgp;
     std::vector<DfgVertex*> m_synth;  // Vertices this logic was synthesized into
+    bool m_drivesUnusedVars = false;  // Logic drives unused variables
     bool m_selectedForSynthesis = false;  // Logic selected for synthesis
     bool m_nonSynthesizable = false;  // Logic is not synthesizeable (by DfgSynthesis)
     bool m_reverted = false;  // Logic was synthesized (in part if non-synthesizable) then reverted
@@ -538,6 +539,8 @@ public:
     const CfgGraph& cfg() const { return *m_cfgp; }
     std::vector<DfgVertex*>& synth() { return m_synth; }
     const std::vector<DfgVertex*>& synth() const { return m_synth; }
+    bool drivesUnusedVars() const { return m_drivesUnusedVars; }
+    void setDrivesUnusedVars() { m_drivesUnusedVars = true; }
     bool selectedForSynthesis() const { return m_selectedForSynthesis; }
     void setSelectedForSynthesis() { m_selectedForSynthesis = true; }
     bool nonSynthesizable() const { return m_nonSynthesizable; }