Fix unused parameterized class causing internal error (#4013).

Changes

@@ -36,6 +36,7 @@ Verilator 5.041 devel
 * Optimize dead functions in more cases (#6380) (#6430). [Artur Bieniek, Antmicro Ltd.]
 * Optimize constant folding in wide expression expansion (#6381). [Geza Lore]
 * Fix missing BLKSEQ when connecting module port to array (#2973).
+* Fix unused parameterized class causing internal error (#4013). [Alberto Del Rio]
 * Fix false CONSTVAR error on initializers (#4992).
 * Fix interface exposure with `--public-depth` or `--trace-depth` (#5758).
 * Fix cell scoping performance (#6059). [Jerry Tianchen]

src/V3Ast.h

@@ -2165,6 +2165,9 @@ public:
     bool brokeExists() const { return V3Broken::isLinkable(this); }
     bool brokeExistsAbove() const { return brokeExists() && (m_brokenState >> 7); }
 
+    // Perform a function on every link in a node
+    virtual void foreachLink(std::function<void(AstNode** linkpp, const char* namep)> f) = 0;
+
 #ifdef VL_LEAK_CHECKS
     static void* operator new(size_t size);
     static void operator delete(void* obj, size_t size);

src/V3Param.cpp

@@ -1110,7 +1110,7 @@ class ParamProcessor final {
             UINFO(8, "     Done with " << modInfop->m_modp);
             newModp = modInfop->m_modp;
         }
-        if (defaultsResolved) { srcModp->user4p(newModp); }
+        if (defaultsResolved) srcModp->user4p(newModp);
 
         for (auto* stmtp = newModp->stmtsp(); stmtp; stmtp = stmtp->nextp()) {
             if (AstParamTypeDType* dtypep = VN_CAST(stmtp, ParamTypeDType)) {
@@ -1843,12 +1843,15 @@ public:
         resortNetlistModules(netlistp);
 
         // Remove defaulted classes
+        // Unlike modules, which we keep around and mark dead() for later V3Dead
+        std::unordered_set<AstClass*> removedClassps;
         for (AstClass* const classp : m_state.m_paramClasses) {
             if (!classp->user3p()) {
                 // If there was an error, don't remove classes as they might
                 // have remained referenced, and  will crash in V3Broken or
                 // other locations. This is fine, we will abort imminently.
                 if (V3Error::errorCount()) continue;
+                removedClassps.emplace(classp);
                 VL_DO_DANGLING(pushDeletep(classp->unlinkFrBack()), classp);
             } else {
                 // Referenced. classp became a specialized class with the default
@@ -1856,6 +1859,18 @@ public:
                 classp->hasGParam(false);
             }
         }
+        // Remove references to defaulted classes
+        // Reuse user3 to mark all nodes being deleted
+        AstNode::user3ClearTree();
+        for (AstClass* classp : removedClassps) {
+            classp->foreach([](AstNode* const nodep) { nodep->user3(true); });
+        }
+        // Set all links pointing to a user3 (deleting) node as null
+        netlistp->foreach([](AstNode* const nodep) {
+            nodep->foreachLink([](AstNode** const linkpp, const char*) {
+                if (*linkpp && (*linkpp)->user3()) *linkpp = nullptr;
+            });
+        });
     }
     ~ParamTop() = default;
     VL_UNCOPYABLE(ParamTop);

src/astgen

@@ -1027,6 +1027,16 @@ def write_ast_impl(filename):
 
                 emitBlock("}}\n")
 
+            emitBlock("void Ast{t}::foreachLink(std::function<void(" +
+                      "AstNode** linkpp, const char* namep)> f) {{\n",
+                      t=node.name)
+            if node.superClass.name != 'Node':
+                emitBlock("    Ast{base}::foreachLink(f);\n", base=node.superClass.name)
+            for ptr in node.ptrs:
+                emitBlock("    f(reinterpret_cast<AstNode**>(&m_{name}), \"{name}\");",
+                          name=ptr['name'])
+            emitBlock("}}\n")
+
             emitBlock("void Ast{t}::dumpJsonGen(std::ostream& str) const {{\n", t=node.name)
             if node.superClass.name != 'Node':
                 emitBlock("    Ast{base}::dumpJson(str);\n", base=node.superClass.name)
@@ -1088,6 +1098,7 @@ def write_ast_macros(filename):
             Ast{t}* clonep() const {{ return static_cast<Ast{t}*>(AstNode::clonep()); }}
             Ast{t}* addNext(Ast{t}* nodep) {{ return static_cast<Ast{t}*>(AstNode::addNext(this, nodep)); }}
             const char* brokenGen() const override;
+            void foreachLink(std::function<void(AstNode** linkpp, const char* namep)> f) override;
             bool wouldBreakGen(const AstNode* const oldp, const AstNode* const newp) const override;
             void dumpTreeJsonOpGen(std::ostream& str, const string& indent) const override;
             void dumpJsonGen(std::ostream& str) const;

test_regress/t/t_class_param_mailbox.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2024 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_class_param_mailbox.v

@@ -0,0 +1,29 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module sub #(
+    parameter type T = logic
+);
+  mailbox #(T) mbox;
+endmodule
+
+module try #(
+    parameter I = 1
+);
+endmodule
+
+module t;
+  typedef struct packed {
+    logic a;
+    logic b;
+  } my_struct_t;
+
+  sub #(my_struct_t) u_sub ();
+  try #(2) u_try2 ();
+
+  initial $finish;
+
+endmodule