Fix enum variables in constraint solver producing invalid enum values (#7058) (#7065)

src/V3Randomize.cpp

@@ -3338,6 +3338,34 @@ class RandomizeVisitor final : public VNVisitor {
                 }
             }
             randomizep->addStmtsp(implementConstraintsClear(fl, genp));
+
+            // Restrict enum variables in solver to valid members only
+            {
+                AstNodeModule* const genModp = VN_AS(genp->user2p(), NodeModule);
+                nodep->foreachMember([&](AstClass*, AstVar* memberVarp) {
+                    if (!memberVarp->user3()) return;
+                    AstEnumDType* const enumDtp
+                        = VN_CAST(memberVarp->dtypep()->skipRefToEnump(), EnumDType);
+                    if (!enumDtp) return;
+                    const int width = enumDtp->width();
+                    const std::string smtName = memberVarp->name();
+                    std::string constraint = "(__Vbv (or";
+                    for (AstEnumItem* itemp = enumDtp->itemsp(); itemp;
+                         itemp = VN_AS(itemp->nextp(), EnumItem)) {
+                        const AstConst* const vconstp = VN_AS(itemp->valuep(), Const);
+                        constraint += " (= " + smtName + " (_ bv" + cvtToStr(vconstp->toUInt())
+                                      + " " + cvtToStr(width) + "))";
+                    }
+                    constraint += "))";
+                    AstCMethodHard* const callp = new AstCMethodHard{
+                        fl, new AstVarRef{fl, genModp, genp, VAccess::READWRITE},
+                        VCMethod::RANDOMIZER_HARD,
+                        new AstCExpr{fl, AstCExpr::Pure{}, "\"" + constraint + "\""}};
+                    callp->dtypeSetVoid();
+                    randomizep->addStmtsp(callp->makeStmt());
+                });
+            }
+
             AstTask* setupAllTaskp = getCreateConstraintSetupFunc(nodep);
             AstTaskRef* const setupTaskRefp = new AstTaskRef{fl, setupAllTaskp, nullptr};
             randomizep->addStmtsp(setupTaskRefp->makeStmt());

test_regress/t/t_randc_enum_constraint.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_randc_enum_constraint.v

@@ -0,0 +1,82 @@
+// 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
+
+// verilog_format: off
+`define stop $stop
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
+// verilog_format: on
+
+// Test that randc enum variables with user constraints only produce
+// valid enum members (not arbitrary bitvector values).
+
+module t;
+
+  typedef enum bit [2:0] {
+    RED   = 0,
+    GREEN = 1,
+    BLUE  = 2,
+    WHITE = 3,
+    BLACK = 4
+  } color_t;
+
+  class ColorClass;
+    randc color_t color;
+    constraint c_no_dark { color != BLACK; }
+  endclass
+
+  // Test with all enum values allowed (no exclusion constraint)
+  class AllColorsClass;
+    randc color_t color;
+    constraint c_range { color <= WHITE; }
+  endclass
+
+  initial begin
+    ColorClass c;
+    AllColorsClass ac;
+    int color_seen[5];
+    int first_seq, all_same, this_seq;
+
+    // Test 1: randc enum with exclusion constraint
+    // Values must be valid enum members (0-4) and not BLACK (4)
+    c = new;
+    repeat (40) begin
+      `checkd(c.randomize(), 1);
+      `checkd(c.color <= BLACK, 1);
+      `checkd(c.color == BLACK, 0);
+    end
+
+    // Test 2: randc enum with range constraint - run multiple trials,
+    // verify only valid enum members produced and sequence varies
+    all_same = 1;
+    first_seq = 0;
+    for (int trial = 0; trial < 3; trial++) begin
+      ac = new;
+      this_seq = 0;
+      for (int j = 0; j < 4; j++) color_seen[j] = 0;
+      repeat (40) begin
+        `checkd(ac.randomize(), 1);
+        `checkd(ac.color <= WHITE, 1);
+        color_seen[ac.color] = 1;
+      end
+      // Record first 4 values for sequence comparison
+      repeat (4) begin
+        `checkd(ac.randomize(), 1);
+        this_seq = this_seq * 8 + int'(ac.color);
+      end
+      `checkd(color_seen[0], 1);
+      `checkd(color_seen[1], 1);
+      `checkd(color_seen[2], 1);
+      `checkd(color_seen[3], 1);
+      if (trial == 0) first_seq = this_seq;
+      else if (this_seq != first_seq) all_same = 0;
+    end
+    `checkd(all_same, 0);
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+
+endmodule