Optimize interfaces in Dfg (#6332)

Interfaces that have no corresponding virtual interface references can
be safely optimized by Dfg after V3Scope. Enabling it to do so.

src/V3AstNodeOther.h

@@ -2531,7 +2531,8 @@ public:
     void classp(AstClass* classp) { m_classp = classp; }
 };
 class AstIface final : public AstNodeModule {
-    // A module declaration
+    // An interface declaration
+    bool m_hasVirtualRef = false;  // There exists a virtual interface reference for this interface
 public:
     AstIface(FileLine* fl, const string& name, const string& libname)
         : ASTGEN_SUPER_Iface(fl, name, libname) {}
@@ -2540,6 +2541,8 @@ public:
     // get false warnings if we enable this
     string verilogKwd() const override { return "interface"; }
     bool timescaleMatters() const override { return false; }
+    bool hasVirtualRef() const { return m_hasVirtualRef; }
+    void setHasVirtualRef() { m_hasVirtualRef = true; }
 };
 class AstModule final : public AstNodeModule {
     // A module declaration

src/V3Dfg.h

@@ -798,10 +798,16 @@ public:
 
     // Returns true if variable can be represented in the graph
     static bool isSupported(const AstVarScope* vscp) {
-        // If the variable is not in a regular module, then we do not support it.
-        // This is especially needed for variabels in interfaces which might be
-        // referenced via virtual intefaces, which cannot be resovled statically.
-        if (!VN_IS(vscp->scopep()->modp(), Module)) return false;
+        AstNodeModule* const modp = vscp->scopep()->modp();
+        if (VN_IS(modp, Module)) {
+            // Regular module supported
+        } else if (AstIface* const ifacep = VN_CAST(modp, Iface)) {
+            // Interfaces supported if there are no virtual interfaces for
+            // them, otherwise they cannot be resovled statically.
+            if (ifacep->hasVirtualRef()) return false;
+        } else {
+            return false;  // Anything else (package, class, etc) not supported
+        }
         // Check the AstVar
         return isSupported(vscp->varp());
     }

src/V3DfgOptimizer.cpp

@@ -321,6 +321,15 @@ class DataflowOptimize final {
 
     DataflowOptimize(AstNetlist* netlistp, const string& label)
         : m_ctx{label} {
+
+        // Mark interfaces that might be referenced by a virtual interface
+        if (v3Global.hasVirtIfaces()) {
+            netlistp->typeTablep()->foreach([](AstIfaceRefDType* nodep) {
+                if (!nodep->isVirtual()) return;
+                nodep->ifaceViaCellp()->setHasVirtualRef();
+            });
+        }
+
         if (!netlistp->topScopep()) {
             // Pre V3Scope application. Run on each module separately.
 

test_regress/t/t_interface_gen7.v

@@ -9,7 +9,9 @@
 interface intf
   #(parameter PARAM = 0)
    ();
+   /* verilator lint_off MULTIDRIVEN */
    logic val;
+   /* verilator lint_on MULTIDRIVEN */
    function integer func (); return 5; endfunction
 endinterface