Fix soft constraint relaxation dropping compatible constraints (#7271)

* Fix soft constraint relaxation dropping compatible constraints

* patch changes the soft handle ordering so update the out files

include/verilated_random.cpp

@@ -468,16 +468,36 @@ bool VlRandomizer::next(VlRNG& rngr) {
 
         const size_t nSoft = m_softConstraints.size();
         bool sat = false;
-        for (size_t phase = 0; phase <= nSoft && !sat; ++phase) {
+        if (nSoft > 0) {
-            const bool hasSoft = (phase < nSoft);
+            // Fast path: try all soft constraints at once
-            if (hasSoft) {
+            os << "(push 1)\n";
-                os << "(push 1)\n";
+            for (const auto& s : m_softConstraints) os << "(assert (= #b1 " << s << "))\n";
-                for (size_t i = phase; i < nSoft; ++i)
-                    os << "(assert (= #b1 " << m_softConstraints[i] << "))\n";
-            }
             os << "(check-sat)\n";
-            sat = parseSolution(os, /*log=*/phase == nSoft);
+            sat = parseSolution(os, false);
-            if (!sat && hasSoft) os << "(pop 1)\n";
+            if (!sat) {
+                // Some soft constraints conflict. Incrementally add from back
+                // (highest priority first), keeping only compatible ones.
+                // This preserves the maximum set of compatible soft constraints.
+                os << "(pop 1)\n";
+                for (int i = static_cast<int>(nSoft) - 1; i >= 0; --i) {
+                    os << "(push 1)\n";
+                    os << "(assert (= #b1 " << m_softConstraints[i] << "))\n";
+                    os << "(check-sat)\n";
+                    if (checkSat(os)) {
+                        // Compatible -- keep this push level
+                    } else {
+                        // Incompatible -- remove this soft constraint
+                        os << "(pop 1)\n";
+                    }
+                }
+                // Read solution with remaining compatible soft constraints
+                os << "(check-sat)\n";
+                sat = parseSolution(os, false);
+            }
+        } else {
+            // No soft constraints -- hard-only
+            os << "(check-sat)\n";
+            sat = parseSolution(os, false);
         }
 
         if (!sat) {
@@ -530,6 +550,12 @@ bool VlRandomizer::next(VlRNG& rngr) {
     return false;  // Should not reach here
 }
 
+bool VlRandomizer::checkSat(std::iostream& os) {
+    std::string result;
+    do { std::getline(os, result); } while (result.empty());
+    return result == "sat";
+}
+
 bool VlRandomizer::parseSolution(std::iostream& os, bool log) {
     std::string sat;
     do { std::getline(os, sat); } while (sat == "");

include/verilated_random.h

@@ -230,6 +230,7 @@ class VlRandomizer VL_NOT_FINAL {
     // PRIVATE METHODS
     void randomConstraint(std::ostream& os, VlRNG& rngr, int bits);
     bool parseSolution(std::iostream& file, bool log = false);
+    bool checkSat(std::iostream& os);
     void emitRandcExclusions(std::ostream& os) const;  // Emit randc exclusion constraints
     void recordRandcValues();  // Record solved randc values for future exclusion
     size_t hashConstraints() const;

test_regress/t/t_constraint_unsat.out

@@ -2,8 +2,6 @@
 === Test 1: Valid constraints ===
 
 === Test 2: addr out of range ===
-%Warning-UNSATCONSTR: t/t_constraint_unsat.v:16: Unsatisfied constraint: 'if (!randomize() with { addr == a; data == d; }) begin'
-%Warning-UNSATCONSTR: t/t_constraint_unsat.v:12: Unsatisfied constraint: 'constraint data_range { data > 10 && data < 200; }'
 %Warning-UNSATCONSTR: t/t_constraint_unsat.v:11: Unsatisfied constraint: 'constraint addr_range { addr < 127; }'
 %Warning-UNSATCONSTR: t/t_constraint_unsat.v:16: Unsatisfied constraint: 'if (!randomize() with { addr == a; data == d; }) begin'
 Randomization failed.

test_regress/t/t_constraint_unsat_protect_ids.out

@@ -2,8 +2,6 @@
 === Test 1: Valid constraints ===
 
 === Test 2: addr out of range ===
-%Warning-UNSATCONSTR: PSTByA:16: Unsatisfied constraint
-%Warning-UNSATCONSTR: PSTByA:12: Unsatisfied constraint
 %Warning-UNSATCONSTR: PSTByA:11: Unsatisfied constraint
 %Warning-UNSATCONSTR: PSTByA:16: Unsatisfied constraint
 Randomization failed.

test_regress/t/t_randomize_soft_relaxation.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_randomize_soft_relaxation.v

@@ -0,0 +1,45 @@
+// 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);
+`define check_range(gotv,minv,maxv) do if ((gotv) < (minv) || (gotv) > (maxv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d-%0d\n", `__FILE__,`__LINE__, (gotv), (minv), (maxv)); `stop; end while(0);
+// verilog_format: on
+
+// Soft constraint relaxation must preserve the maximum compatible set.
+// When soft[1] conflicts with soft[2], a correct algorithm should still
+// keep soft[0] if it is compatible with soft[2].
+//
+// soft[0]: b > 100   (low priority, COMPATIBLE with soft[2])
+// soft[1]: a == 30   (mid priority, CONFLICTS with soft[2])
+// soft[2]: a == 80   (high priority, should win over soft[1])
+//
+// Correct result: a == 80, b > 100 (soft[0] and soft[2] both kept)
+// Bug result:     a == 80, b unconstrained (soft[0] wrongly dropped)
+
+class SoftRelax;
+  rand bit [7:0] a;
+  rand bit [7:0] b;
+  constraint c_hard { a < 8'd200; b < 8'd200; }
+  constraint c_soft0 { soft b > 8'd100; }
+  constraint c_soft1 { soft a == 8'd30; }
+  constraint c_soft2 { soft a == 8'd80; }
+endclass
+
+module t;
+  initial begin
+    SoftRelax obj;
+    obj = new;
+    repeat (20) begin
+      `checkd(obj.randomize(), 1)
+      `checkd(obj.a, 8'd80)
+      `check_range(obj.b, 8'd101, 8'd199)
+    end
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule