From cc51966ad1bb4acb3ce9e7024dd62b8bae3cf134 Mon Sep 17 00:00:00 2001
From: Geza Lore <gezalore@gmail.com>
Date: Fri, 30 Sep 2022 11:35:03 +0100
Subject: [PATCH] DFG: Remove unconneced variables early

---
 src/V3Dfg.cpp         | 14 ++++++++++----
 src/V3DfgAstToDfg.cpp | 22 ++++++++++++++++++++--
 2 files changed, 30 insertions(+), 6 deletions(-)

diff --git a/src/V3Dfg.cpp b/src/V3Dfg.cpp
index a6bdb2411..fb2c74d44 100644
--- a/src/V3Dfg.cpp
+++ b/src/V3Dfg.cpp
@@ -410,12 +410,18 @@ class ExtractCyclicComponents final {
     static void packSources(DfgGraph& dfg) {
         // Remove undriven variable sources
         dfg.forEachVertex([&](DfgVertex& vtx) {
-            if (DfgVarPacked* const vtxp = vtx.cast<DfgVarPacked>()) {
-                vtxp->packSources();
+            if (DfgVarPacked* const varp = vtx.cast<DfgVarPacked>()) {
+                varp->packSources();
+                if (!varp->hasSinks() && varp->arity() == 0) {
+                    VL_DO_DANGLING(varp->unlinkDelete(dfg), varp);
+                }
                 return;
             }
-            if (DfgVarArray* const vtxp = vtx.cast<DfgVarArray>()) {
-                vtxp->packSources();
+            if (DfgVarArray* const varp = vtx.cast<DfgVarArray>()) {
+                varp->packSources();
+                if (!varp->hasSinks() && varp->arity() == 0) {
+                    VL_DO_DANGLING(varp->unlinkDelete(dfg), varp);
+                }
                 return;
             }
         });
diff --git a/src/V3DfgAstToDfg.cpp b/src/V3DfgAstToDfg.cpp
index 763a1656d..db81c7d11 100644
--- a/src/V3DfgAstToDfg.cpp
+++ b/src/V3DfgAstToDfg.cpp
@@ -110,8 +110,7 @@ class AstToDfgVisitor final : public VNVisitor {
             // Note DfgVertexLValue vertices are not added to m_uncommittedVertices, because we
             // want to hold onto them via AstVar::user1p, and the AstVar might be referenced via
             // multiple AstVarRef instances, so we will never revert a DfgVertexLValue once
-            // created. This means we can end up with DfgVertexLValue vertices in the graph which
-            // have no connections at all (which is fine for later processing).
+            // created. We will delete unconnected variable vertices at the end.
             if (VN_IS(varp->dtypep()->skipRefp(), UnpackArrayDType)) {
                 DfgVarArray* const vtxp = new DfgVarArray{*m_dfgp, varp};
                 varp->user1p();
@@ -264,6 +263,13 @@ class AstToDfgVisitor final : public VNVisitor {
     // Canonicalize packed variables
     void canonicalizePacked() {
         for (DfgVarPacked* const varp : m_varPackedps) {
+            // Delete variables with no sinks nor sources (this can happen due to reverting
+            // uncommitted vertices, which does not remove variables)
+            if (!varp->hasSinks() && varp->arity() == 0) {
+                VL_DO_DANGLING(varp->unlinkDelete(*m_dfgp), varp);
+                continue;
+            }
+
             // Gather (and unlink) all drivers
             struct Driver {
                 FileLine* flp;
@@ -328,6 +334,17 @@ class AstToDfgVisitor final : public VNVisitor {
         }
     }
 
+    // Canonicalize array variables
+    void canonicalizeArray() {
+        for (DfgVarArray* const varp : m_varArrayps) {
+            // Delete variables with no sinks nor sources (this can happen due to reverting
+            // uncommitted vertices, which does not remove variables)
+            if (!varp->hasSinks() && varp->arity() == 0) {
+                VL_DO_DANGLING(varp->unlinkDelete(*m_dfgp), varp);
+            }
+        }
+    }
+
     // VISITORS
     void visit(AstNode* nodep) override {
         // Conservatively treat this node as unhandled
@@ -406,6 +423,7 @@ class AstToDfgVisitor final : public VNVisitor {
 
         // Canonicalize variables
         canonicalizePacked();
+        canonicalizeArray();
     }
 
 public: