Support selects on arbitrary string expressions (#5773)

src/V3WidthSel.cpp

@@ -305,13 +305,19 @@ class WidthSelVisitor final : public VNVisitor {
             VL_DO_DANGLING(pushDeletep(nodep), nodep);
         } else if (VN_IS(ddtypep, BasicDType) && ddtypep->isString()) {
             // SELBIT(string, index) -> GETC(string, index)
-            const AstNodeVarRef* const varrefp = VN_CAST(fromp, NodeVarRef);
-            if (!varrefp) {
-                nodep->v3warn(E_UNSUPPORTED,
-                              "Unsupported: String array operation on non-variable");
+            AstNodeExpr* exprp = fromp;
+            while (true) {
+                if (AstMemberSel* const memberselp = VN_CAST(exprp, MemberSel)) {
+                    exprp = memberselp->fromp();
+                } else if (AstStructSel* const structselp = VN_CAST(exprp, StructSel)) {
+                    exprp = structselp->fromp();
+                } else {
+                    break;
+                }
             }
+            const AstNodeVarRef* const varrefp = VN_CAST(exprp, NodeVarRef);
             AstNodeExpr* newp;
-            if (varrefp && varrefp->access().isReadOnly()) {
+            if (!varrefp || varrefp->access().isReadOnly()) {
                 newp = new AstGetcN{nodep->fileline(), fromp, rhsp};
             } else {
                 newp = new AstGetcRefN{nodep->fileline(), fromp, rhsp};

test_regress/t/t_string_sel.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('vlt')
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_string_sel.v

@@ -0,0 +1,60 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed into the Public Domain, for any use,
+// without warranty, 2025 by Antmicro.
+// SPDX-License-Identifier: CC0-1.0
+
+typedef struct {
+   string str;
+} str_s;
+
+class c;
+   string str;
+   function new();
+      str = "foo";
+   endfunction
+   function string get_str();
+      return str;
+   endfunction
+endclass
+
+module t (/*AUTOARG*/);
+   string str = "bar";
+
+   function string get_str();
+      return str;
+   endfunction
+
+   initial begin
+      c o = new;
+      str_s st = '{"qux"};
+      string sc = {"foo", "bar"};
+
+      // read
+      if (str[0] != "b") $stop;
+      if (get_str()[1] != "a") $stop;
+      if (str[3] != "\0") $stop;
+      if (st.str[2] != "x") $stop;
+      if (st.str[99] != "\0") $stop;
+      if (o.str[0] != "f") $stop;
+      if (o.get_str()[1] != "o") $stop;
+      if (o.str[-1] != "\0") $stop;
+      if (sc[2] != "o") $stop;
+      if ($sformatf("foo%s", "bar")[3] != "b") $stop;
+      if (sc[-1] != "\0") $stop;
+      if (sc[6] != "\0") $stop;
+      if (sc[99] != "\0") $stop;
+
+      // write
+      sc[5] = "z";
+      if (sc != "foobaz") $stop;
+      o.str[0] = "b";
+      if (o.str != "boo") $stop;
+      st.str[2] = "z";
+      if (st.str != "quz") $stop;
+
+      $write("*-* All Finished *-*\n");
+      $finish;
+   end
+
+endmodule