Fix constraint_mode()/rand_mode() in constructor being overwritten by init code (#7054)

src/V3Randomize.cpp

@@ -2366,12 +2366,28 @@ class RandomizeVisitor final : public VNVisitor {
             = new AstCMethodHard{fl, new AstVarRef{fl, modeVarModp, modeVarp, VAccess::WRITE},
                                  VCMethod::DYN_RESIZE, new AstConst{fl, modeCount}};
         dynarrayNewp->dtypeSetVoid();
-        AstNodeFTask* const newp = VN_AS(m_memberMap.findMember(classp, "new"), NodeFTask);
-        UASSERT_OBJ(newp, classp, "No new() in class");
-        newp->addStmtsp(dynarrayNewp->makeStmt());
-        newp->addStmtsp(makeModeSetLoop(fl,
-                                        new AstVarRef{fl, modeVarModp, modeVarp, VAccess::WRITE},
-                                        new AstConst{fl, 1}, true));
+        AstNodeFTask* const ctorNewp = VN_AS(m_memberMap.findMember(classp, "new"), NodeFTask);
+        UASSERT_OBJ(ctorNewp, classp, "No new() in class");
+        // Build init chain: resize -> set-all-to-1 loop
+        AstNode* const initFirstp = dynarrayNewp->makeStmt();
+        initFirstp->addNext(
+            makeModeSetLoop(fl, new AstVarRef{fl, modeVarModp, modeVarp, VAccess::WRITE},
+                            new AstConst{fl, 1}, true));
+        // Prepend init code before user statements in constructor body, but after
+        // var declarations and super.new(). This ensures that user's constraint_mode()
+        // or rand_mode() calls in the constructor execute after mode arrays are initialized.
+        // Pattern from V3LinkDot::addImplicitSuperNewCall.
+        for (AstNode* stmtp = ctorNewp->stmtsp(); stmtp; stmtp = stmtp->nextp()) {
+            if (!VN_IS(stmtp, NodeStmt)) continue;  // Skip var declarations
+            if (const AstStmtExpr* const sep = VN_CAST(stmtp, StmtExpr)) {
+                if (VN_IS(sep->exprp(), New)) continue;  // Skip super.new()
+            }
+            // Found first user statement - insert init code before it
+            stmtp->addHereThisAsNext(initFirstp);
+            return;
+        }
+        // No user statements (empty constructor or only var decls/super.new)
+        ctorNewp->addStmtsp(initFirstp);
     }
     void makeStaticModeInit(AstVar* modeVarp, AstClass* classp, uint32_t modeCount) {
         // For static constraint mode, we need lazy initialization since it's shared across

test_regress/t/t_constraint_mode_ctor.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# 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-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+if not test.have_solver:
+    test.skip("No constraint solver installed")
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_constraint_mode_ctor.v

@@ -0,0 +1,47 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 PlanV GmbH
+// SPDX-License-Identifier: CC0-1.0
+
+// Test constraint_mode() called inside class constructor (function new)
+
+class ConstraintModeInCtor;
+  rand bit [7:0] value;
+
+  constraint low_range_c { value < 50; };
+  constraint high_range_c { value >= 50; value < 200; };
+
+  function new;
+    // Disable high_range_c in constructor - only low_range_c should be active
+    high_range_c.constraint_mode(0);
+  endfunction
+endclass
+
+module t;
+  initial begin
+    automatic ConstraintModeInCtor obj = new;
+    automatic int i;
+
+    // Test 1: constraint_mode(0) in constructor should disable constraint
+    for (i = 0; i < 20; i++) begin
+      void'(obj.randomize());
+      if (obj.value >= 50) $stop;
+    end
+
+    // Test 2: Query constraint_mode state set in constructor
+    if (obj.low_range_c.constraint_mode != 1) $stop;
+    if (obj.high_range_c.constraint_mode != 0) $stop;
+
+    // Test 3: Switch constraints at runtime
+    obj.low_range_c.constraint_mode(0);
+    obj.high_range_c.constraint_mode(1);
+    for (i = 0; i < 20; i++) begin
+      void'(obj.randomize());
+      if (obj.value < 50 || obj.value >= 200) $stop;
+    end
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule