Fix constant propagation making upper bits Xs (#5969).

Changes

@@ -16,6 +16,7 @@ Verilator 5.037 devel
 * Add BADVLTPRAGMA on unknown Verilator pragmas (#5945). [Shou-Li Hsu]
 * Fix filename backslash escapes in C code (#5947).
 * Fix sign extension of signed compared with unsigned case items (#5968).
+* Fix constant propagation making upper bits Xs (#5969).
 
 
 Verilator 5.036 2025-04-27

src/V3Number.cpp

@@ -516,6 +516,12 @@ V3Number& V3Number::setValue1() {
     return *this;
 }
 
+void V3Number::setBitX0(int bit) {
+    // Selection beyond bounds after V3Premit needs to have 0s
+    // in upper bits.  Contrast to setAllBitsXRemoved which honors xAssign
+    setBit(bit, v3Global.constRemoveXs() ? 0 : 'x');
+}
+
 V3Number& V3Number::setMask(int nbits, int lsb) {
     setZero();
     for (int bit = lsb; bit < lsb + nbits; bit++) setBit(bit, 1);
@@ -2324,7 +2330,7 @@ V3Number& V3Number::opSel(const V3Number& lhs, uint32_t msbval, uint32_t lsbval)
         if (ibit >= 0 && ibit < lhs.width() && ibit <= static_cast<int>(msbval)) {
             setBit(bit, lhs.bitIs(ibit));
         } else {
-            setBit(bit, 'x');
+            setBitX0(bit);
         }
         ++ibit;
     }
@@ -2345,7 +2351,7 @@ V3Number& V3Number::opSelInto(const V3Number& lhs, int lsbval, int width) {
         if (ibit >= 0 && ibit < lhs.width()) {
             setBit(bit, lhs.bitIs(ibit));
         } else {
-            setBit(bit, 'x');
+            setBitX0(bit);
         }
         ibit++;
     }

src/V3Number.h

@@ -356,6 +356,7 @@ public:
     V3Number& setLong(uint32_t value);
     V3Number& setLongS(int32_t value);
     V3Number& setDouble(double value);
+    void setBitX0(int bit);
     void setBit(int bit, char value) {  // Note: must be initialized as number and pre-zeroed!
         if (bit >= m_data.width()) return;
         const uint32_t mask = (1UL << (bit & 31));
@@ -654,8 +655,7 @@ public:
     uint32_t countBits(const V3Number& ctrl) const;
     uint32_t countBits(const V3Number& ctrl1, const V3Number& ctrl2, const V3Number& ctrl3) const;
     uint32_t countOnes() const;
-    uint32_t
-    mostSetBitP1() const;  // Highest bit set plus one, IE for 16 return 5, for 0 return 0.
+    uint32_t mostSetBitP1() const;  // Highest bit set + 1, e.g. for 16 return 5, for 0 return 0
 
     // Operators
     bool operator<(const V3Number& rhs) const { return isLtXZ(rhs); }

test_regress/t/t_select_c.py

@@ -0,0 +1,18 @@
+#!/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('simulator_st')
+
+test.compile(verilator_flags2=['--binary --fno-expand'])
+
+test.execute()
+
+test.passes()

test_regress/t/t_select_c.v

@@ -0,0 +1,27 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t;
+   // verilator lint_off WIDTH
+   // verilator lint_off IMPLICIT
+   wire [22:0] w274;
+   wire w412;
+   wire w413;
+   wire w509;
+
+   assign w104 = ! w509;
+   assign w201 = w258 > 12'hab7;
+   assign w204 = 7'h7f <= w104;
+   wire [11:0] w258 = 3'h3 || w274;
+   assign w538 = w412 ? out21 : w201;
+   wire [16:0] w539 = w413 ? w538 : 17'h00570;
+   wire [21:5] out21 = w204;
+   assign out51 = w539[0];
+
+   initial begin
+      $display("%0d", out51);
+   end
+endmodule