Optimize constify within Expand and Subst (#6111)

These passes blow up the Ast size on some designs, so delaying running V3Const
until after the whole pass can notably increase peak memory usage. In this
patch we apply V3Const per CFunc within these passes, which saves on memory.
Added -fno-const-eager to disable the intra-pass V3Const application, for
debugging.

docs/guide/exe_verilator.rst

@@ -590,6 +590,8 @@ Summary:
 
 .. option:: -fno-const-bit-op-tree
 
+.. option:: -fno-const-eager
+
 .. option:: -fno-dedup
 
 .. option:: -fno-dfg

src/V3Expand.cpp

@@ -357,6 +357,16 @@ class ExpandVisitor final : public VNVisitor {
     }
 
     // VISITORS
+    void visit(AstCFunc* nodep) override {
+        iterateChildren(nodep);
+
+        // Constant fold here, as Ast size can likely be reduced
+        if (v3Global.opt.fConstEager()) {
+            AstNode* const editedp = V3Const::constifyEditCpp(nodep);
+            UASSERT_OBJ(editedp == nodep, editedp, "Should not have replaced CFunc");
+        }
+    }
+
     void visit(AstExtend* nodep) override {
         if (nodep->user1SetOnce()) return;  // Process once
         iterateChildren(nodep);

src/V3Options.cpp

@@ -1320,6 +1320,7 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc,
     DECL_OPTION("-fconst", FOnOff, &m_fConst);
     DECL_OPTION("-fconst-before-dfg", FOnOff, &m_fConstBeforeDfg);
     DECL_OPTION("-fconst-bit-op-tree", FOnOff, &m_fConstBitOpTree);
+    DECL_OPTION("-fconst-eager", FOnOff, &m_fConstEager);
     DECL_OPTION("-fdead-assigns", FOnOff, &m_fDeadAssigns);
     DECL_OPTION("-fdead-cells", FOnOff, &m_fDeadCells);
     DECL_OPTION("-fdedup", FOnOff, &m_fDedupe);

src/V3Options.h

@@ -389,6 +389,7 @@ private:
     bool m_fConst;       // main switch: -fno-const: constant folding
     bool m_fConstBeforeDfg = true;  // main switch: -fno-const-before-dfg for testing only!
     bool m_fConstBitOpTree;  // main switch: -fno-const-bit-op-tree constant bit op tree
+    bool m_fConstEager = true;  // main switch: -fno-const-eagerly run V3Const during passes
     bool m_fDedupe;      // main switch: -fno-dedupe: logic deduplication
     bool m_fDfgPeephole = true; // main switch: -fno-dfg-peephole
     bool m_fDfgPreInline;    // main switch: -fno-dfg-pre-inline and -fno-dfg
@@ -701,6 +702,7 @@ public:
     bool fConst() const { return m_fConst; }
     bool fConstBeforeDfg() const { return m_fConstBeforeDfg; }
     bool fConstBitOpTree() const { return m_fConstBitOpTree; }
+    bool fConstEager() const { return m_fConstEager; }
     bool fDedupe() const { return m_fDedupe; }
     bool fDfgPeephole() const { return m_fDfgPeephole; }
     bool fDfgPreInline() const { return m_fDfgPreInline; }

src/V3Subst.cpp

@@ -26,6 +26,7 @@
 
 #include "V3Subst.h"
 
+#include "V3Const.h"
 #include "V3Stats.h"
 
 #include <algorithm>
@@ -361,6 +362,12 @@ class SubstVisitor final : public VNVisitor {
         iterateChildren(nodep);
         for (SubstVarEntry& ip : m_entries) ip.deleteUnusedAssign();
         m_entries.clear();
+
+        // Constant fold here, as Ast size can likely be reduced
+        if (v3Global.opt.fConstEager()) {
+            AstNode* const editedp = V3Const::constifyEditCpp(nodep);
+            UASSERT_OBJ(editedp == nodep, editedp, "Should not have replaced CFunc");
+        }
     }
 
     void visit(AstNode* nodep) override {

test_regress/t/t_case_66bits_no_const_eager.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 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('vlt')
+test.top_filename = "t/t_case_66bits.v"
+
+test.compile(verilator_flags2=['-fno-const-eager'])
+
+test.execute()
+
+test.passes()