Fix rand_mode() on nested object variables causes Z3 solver error (#7031) (#7034)

src/V3Randomize.cpp

@@ -890,8 +890,9 @@ class ConstraintExprVisitor final : public VNVisitor {
         nodep->unlinkFrBack(&relinker);
         AstNodeExpr* exprp;
         if (randMode.usesMode) {
-            // Use string literal to avoid double formatting
-            exprp = new AstConst{nodep->fileline(), AstConst::String{}, smtName};
+            // Use AstSFormatF (not AstConst{String}) to prevent editFormat/V3Const
+            // from reformatting the SMT variable name into a hex literal
+            exprp = new AstSFormatF{nodep->fileline(), smtName, false, nullptr};
 
             // Get const format, using membersel if available for correct width/value
             AstNodeExpr* constFormatp

test_regress/t/t_constraint_global_nested_randmode.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_global_nested_randmode.v

@@ -0,0 +1,111 @@
+// 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: rand_mode on nested object's variable should not cause Z3 solver error.
+// Previously, the generated SMT constraint used "#x0" instead of the variable
+// name for nested rand variables when rand_mode was involved, causing a width
+// mismatch in the constraint solver.
+
+class InnerClass;
+  rand bit [7:0] inner_val1;
+  rand bit [7:0] inner_val2;
+
+  constraint inner_con {
+    inner_val1 inside {[8'd10:8'd50]};
+    inner_val2 inside {[8'd100:8'd200]};
+  }
+
+  function new();
+    inner_val1 = 0;
+    inner_val2 = 0;
+  endfunction
+endclass
+
+class OuterClass;
+  rand InnerClass nested_obj;
+  rand bit [7:0] outer_val;
+
+  constraint outer_con {
+    outer_val inside {[8'd1:8'd20]};
+  }
+
+  function new();
+    nested_obj = new();
+    outer_val = 0;
+  endfunction
+endclass
+
+module t_constraint_global_nested_randmode;
+  OuterClass obj;
+  int rand_ok;
+  int change_count;
+  bit [7:0] prev_val;
+
+  initial begin
+    obj = new();
+
+    // Test 1: Normal randomization of nested object with constraints
+    rand_ok = obj.randomize();
+    if (rand_ok == 0) $stop;
+    if (obj.nested_obj.inner_val1 < 8'd10 || obj.nested_obj.inner_val1 > 8'd50) $stop;
+    if (obj.nested_obj.inner_val2 < 8'd100 || obj.nested_obj.inner_val2 > 8'd200) $stop;
+    if (obj.outer_val < 8'd1 || obj.outer_val > 8'd20) $stop;
+
+    // Test 2: Multiple randomizations with constraint checking
+    change_count = 0;
+    prev_val = obj.nested_obj.inner_val1;
+    repeat (10) begin
+      rand_ok = obj.randomize();
+      if (rand_ok == 0) $stop;
+      if (obj.nested_obj.inner_val1 < 8'd10 || obj.nested_obj.inner_val1 > 8'd50) $stop;
+      if (obj.nested_obj.inner_val2 < 8'd100 || obj.nested_obj.inner_val2 > 8'd200) $stop;
+      if (obj.outer_val < 8'd1 || obj.outer_val > 8'd20) $stop;
+      if (obj.nested_obj.inner_val1 != prev_val) change_count++;
+      prev_val = obj.nested_obj.inner_val1;
+    end
+    if (change_count == 0) $stop;
+
+    // Test 3: rand_mode(0) on nested var - must not crash solver
+    void'(obj.nested_obj.inner_val1.rand_mode(0));
+    if (obj.nested_obj.inner_val1.rand_mode() != 0) $stop;
+    // Calling randomize must not cause a solver crash
+    rand_ok = obj.randomize();
+    // (randomize may return 0 due to constraint interaction - separate issue)
+
+    // Test 4: Re-enable rand_mode and verify randomization resumes
+    void'(obj.nested_obj.inner_val1.rand_mode(1));
+    if (obj.nested_obj.inner_val1.rand_mode() != 1) $stop;
+    change_count = 0;
+    prev_val = obj.nested_obj.inner_val1;
+    repeat (10) begin
+      rand_ok = obj.randomize();
+      if (rand_ok == 0) $stop;
+      if (obj.nested_obj.inner_val1 != prev_val) change_count++;
+      prev_val = obj.nested_obj.inner_val1;
+    end
+    if (change_count == 0) $stop;
+
+    // Test 5: rand_mode(0) on entire nested object - must not crash solver
+    void'(obj.nested_obj.rand_mode(0));
+    rand_ok = obj.randomize();
+    // (randomize may return 0 due to constraint interaction - separate issue)
+
+    // Test 6: Re-enable nested object and verify randomization resumes
+    void'(obj.nested_obj.rand_mode(1));
+    change_count = 0;
+    prev_val = obj.nested_obj.inner_val1;
+    repeat (10) begin
+      rand_ok = obj.randomize();
+      if (rand_ok == 0) $stop;
+      if (obj.nested_obj.inner_val1 != prev_val) change_count++;
+      prev_val = obj.nested_obj.inner_val1;
+    end
+    if (change_count == 0) $stop;
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule