Add `$cpure` (#6580)

docs/guide/extensions.rst

@@ -156,7 +156,7 @@ or "`ifdef`"'s may break other tools.
 
    .. t_dist_docs_style restart_sort
 
-.. option:: $c([string], ...);
+.. option:: $c([string], ...);, $cpure([string], ...);
 
    The string will be embedded directly in the output C++ code at the point
    where the surrounding Verilog code is compiled.  It may either be a
@@ -196,6 +196,11 @@ or "`ifdef`"'s may break other tools.
    compatibility with other simulators, which require a differently named
    PLI function name for each different output width.
 
+   `$cpure` is similar to `$c` except that it indicates the
+   expression is pure, versus `$c` which is assumed impure.
+   `$cpure` is for internal use only, and it might change
+   without notice in any future version or Verilator.
+
 .. option:: $display, $write, $fdisplay, $fwrite, $sformat, $swrite
 
    Format arguments may use C fprintf sizes after the % escape.  Per the

src/V3AstNodeExpr.h

@@ -603,23 +603,30 @@ public:
     void add(AstNode* nodep) { addNodesp(nodep); }
 };
 class AstCExprUser final : public AstNodeExpr {
-    // User '$c' statement - Like AstCStmt, with text tracking and optimizations disabled.
+    // User '$c' or '$cpure' expression - Like AstCStmt, with text tracking and optimizations
+    // disabled.
     //
     // Use AstCExpr instead, unless the text is from user input.
     //
     // @astgen op1 := nodesp : List[AstNode<AstNodeExpr|AstText>]
+    const bool m_pure;  // Whether the function is pure
 public:
+    class Pure {};
     AstCExprUser(FileLine* fl)
-        : ASTGEN_SUPER_CExprUser(fl) {}
+        : ASTGEN_SUPER_CExprUser(fl)
+        , m_pure{false} {}
+    AstCExprUser(FileLine* fl, Pure)
+        : ASTGEN_SUPER_CExprUser(fl)
+        , m_pure{true} {}
     ASTGEN_MEMBERS_AstCExprUser;
     // METHODS
     bool cleanOut() const override { return false; }
     std::string emitC() override { V3ERROR_NA_RETURN(""); }
     std::string emitVerilog() override { V3ERROR_NA_RETURN(""); }
-    bool isGateOptimizable() const override { return false; }
+    bool isGateOptimizable() const override { return m_pure; }
     bool isOutputter() override { return true; }
-    bool isPredictOptimizable() const override { return false; }
+    bool isPredictOptimizable() const override { return m_pure; }
-    bool isPure() override { return false; }
+    bool isPure() override { return m_pure; }
     bool sameNode(const AstNode* /*samep*/) const override { return true; }
     // Add some text, or a node to this expression
     void add(const std::string& text) { addNodesp(new AstText{fileline(), text}); }

src/V3EmitCFunc.h

@@ -1376,7 +1376,9 @@ public:
         putnbs(nodep, "");
         ofp()->putsNoTracking("\n");
         if (/* is always from $c */ v3Global.opt.decoration() && !v3Global.opt.protectIds()) {
-            ofp()->putsNoTracking("// $c expression at " + nodep->fileline()->ascii() + "\n");
+            ofp()->putsNoTracking(
+                (nodep->isPure() ? "// $cpure expression at " : "// $c expression at ")
+                + nodep->fileline()->ascii() + "\n");
         }
         emitNodesWithText(nodep->nodesp(), m_useSelfForThis, false, "");
         puts("\n");

src/verilog.l

@@ -175,6 +175,7 @@ vnum    {vnum1}|{vnum2}|{vnum3}|{vnum4}|{vnum5}
   {crnl}                { FL_FWD; FL_BRK; }  /* Count line numbers */
   /*     Extensions to Verilog set, some specified by PSL */
   "$c"[0-9]*            { FL; return yD_C; }      /*Verilator only*/
+  "$cpure"[0-9]*        { FL; return yD_CPURE; }  /*Verilator only*/
   /*     System Tasks */
   "$acos"               { FL; return yD_ACOS; }
   "$acosh"              { FL; return yD_ACOSH; }

src/verilog.y

@@ -611,6 +611,7 @@ BISONPRE_VERSION(3.7,%define api.header.include {"V3ParseBison.h"})
 %token<fl>              yD_BITSTOREAL   "$bitstoreal"
 %token<fl>              yD_BITSTOSHORTREAL "$bitstoshortreal"
 %token<fl>              yD_C            "$c"
+%token<fl>              yD_CPURE        "$cpure"
 %token<fl>              yD_CAST         "$cast"
 %token<fl>              yD_CEIL         "$ceil"
 %token<fl>              yD_CHANGED      "$changed"
@@ -4268,6 +4269,14 @@ system_f_call<nodeExprp>:           // IEEE: system_tf_call (as func)
                     }
                     $$ = cexprp;
                 }
+        |       yD_CPURE '(' cStrList ')' {
+                    AstCExprUser* cexprp = nullptr;
+                    if (!v3Global.opt.ignc()) {
+                        cexprp = new AstCExprUser{$1, AstCExprUser::Pure{}};
+                        cexprp->add($3);
+                    }
+                    $$ = cexprp;
+                }
         |       yD_CAST '(' expr ',' expr ')'           { $$ = new AstCastDynamic{$1, $5, $3}; }
         |       yD_STACKTRACE parenE                    { $$ = new AstStackTraceF{$1}; }
         |       yD_SYSTEM  '(' expr ')'                 { $$ = new AstSystemF{$1, $3}; }

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

test_regress/t/t_cpure.v

@@ -0,0 +1,20 @@
+// DESCRIPTION: Verilator: Verilog Test module for SystemVerilog 'alias'
+//
+// Simple bi-directional transitive alias test.
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Antmicro.
+// SPDX-License-Identifier: CC0-1.0
+
+function int func();
+  static int someVar = 12;
+  return $cpure(someVar, "+ 6");
+endfunction
+
+module t;
+  initial begin
+    if (func() != 18) $stop;
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule