[Bug preprocessor/103355] New: libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[Bug preprocessor/103355] New: libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

            Bug ID: 103355
           Summary: libcpp/lex.c:1289:9: warning: use of the 'likely'
                    attribute is a C++20 extension [-Wc++20-extensions]
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marxin at gcc dot gnu.org
                CC: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Right now, we require for building of GCC ISO C++11 compiler. However,
[[likely]] and [[unlikely]] come from C++20, or?

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The question becomes unknown attributes don't do anything right? If so the
warning on an attribute like this seems over the top.

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-11-22
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Martin Liška from comment #0)
> [[likely]] and [[unlikely]] come from C++20, or?

Yes. It should really be guarded by #if __has_cpp_attribute(likely)

Or use [[__likely__]] which recent versions of GCC will accept in C++11 mode,
but non-GCC compilers will warn about.


Or both:

--- a/libcpp/lex.c
+++ b/libcpp/lex.c
@@ -1286,7 +1286,14 @@ namespace bidi {
       case kind::RTL:
        /* These aren't popped by a PDF/PDI.  */
        break;
-      [[likely]] case kind::NONE:
+#ifdef __has_cpp_attribute
+# if __has_cpp_attribute(likely)
+      [[likely]]
+# elif __has_cpp_attribute(__likely__)
+      [[__likely__]]
+# endif
+#endif
+      case kind::NONE:
        break;
       default:
        abort ();

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Mine, but I'll just use Jon's patch.

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #3)
> Mine, but I'll just use Jon's patch.

That's a reasonable solution for me ;)

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:630686f93f0018fa1ef128aa673fddd302cc83e1

commit r12-5459-g630686f93f0018fa1ef128aa673fddd302cc83e1
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Nov 22 11:29:40 2021 -0500

    libcpp: Use [[likely]] conditionally

    Let's hide [[likely]] behind a macro, to suppress warnings if the
    compiler doesn't support it.

    Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

            PR preprocessor/103355

    libcpp/ChangeLog:

            * lex.c: Use ATTR_LIKELY instead of [[likely]].
            * system.h (ATTR_LIKELY): Define.

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.

[Bug preprocessor/103355] libcpp/lex.c:1289:9: warning: use of the 'likely' attribute is a C++20 extension [-Wc++20-extensions]

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103355

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Christophe Lyon <clyon@gcc.gnu.org>:

https://gcc.gnu.org/g:46d3cfd29dc701e1e0d18b0380d6299fce7944f2

commit r12-5472-g46d3cfd29dc701e1e0d18b0380d6299fce7944f2
Author: Christophe Lyon <christophe.lyon@foss.st.com>
Date:   Tue Nov 23 16:06:42 2021 +0000

    libcpp: Fix ATTR_LIKELY definition PR preprocessor/103355

    Fix the definition of ATTR_LIKELY when __has_cpp_attribute is not
    defined, as it is the case with old compilers such as gcc-4.8.5.

            libcpp/:
            PR preprocessor/103355
            * system.h (ATTR_LIKELY): Fix definition.