Fix slow unsized number parsing (#5577)

Try to avoid allocating and deallocating a full --max-num-width
buffer on parsing every single unsized number literal.

src/V3Number.cpp

@@ -198,7 +198,12 @@ void V3Number::create(const char* sourcep) {
     }
     // Otherwise...
     else if (!sized()) {
-        width(v3Global.opt.maxNumWidth(), false);  // Will change width below
+        // We don't use v3Global.opt.maxNumWidth() here, as it can be arbitrarily large,
+        // and cause extremely slow parsing. We will resize the value at the end anyway.
+        // We just need a width big enough to fit the constant, so we use a conservative
+        // upper bound to start from. Should never need more than 4 bits per digit.
+        const int widthBound = std::max<int>(32, std::strlen(value_startp) * 4);
+        width(widthBound, false);  // Will change width below
         if (unbased) isSigned(true);  // Also says the spec.
     }
 

test_regress/t/t_const_number_unsized_parse.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2024 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 signal
+import vltest_bootstrap
+
+test.scenarios('vlt')
+
+test.top_filename = f"{test.obj_dir}/in.v"
+
+with open(test.top_filename, "w", encoding="utf8") as f:
+    f.write("module top;\n")
+    for i in range(100000):
+        f.write(f"  int x{i} = 'd{i};\n")
+    f.write("endmodule\n")
+
+signal.alarm(20)  # 20s timeout
+
+test.lint(verilator_flags2=[f"--max-num-width {2**30}"])
+
+test.passes()