[Bug c++/57241] New: GCC still issues -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[Bug c++/57241] New: GCC still issues -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[scottbaldwin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57241

            Bug ID: 57241
           Summary: GCC still issues -Wmultichar warnings despite a
                    #pragma diagnostic ignored -Wmultichar directive
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: scottbaldwin at gmail dot com

In GCC 4.7.2 and 4.8.x, the #pragma ignore directive for -Wmultichar has no
effect. The code below demonstrates a -Wmultichar which is still emitted as a
warning despite the #pragma directive to ignore it. Specifying -Wno-multichar
on the command line works fine, so this is just an issue with trying to
suppress it with a #pragma directive (which I need for finer-grained control).

Note: I didn't test the other diagnostic options to see if there are others
besides -Wmultichar that have the same issue, so you might want to do a full
check on all diagnostic options.

Here's the demo code and unsuppressed compiler warnings:

int main() {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wmultichar"
    typedef boost::mpl::string<'0123', '4567', '89ab', 'cdef'> hex_chars; //
-Wmultichar (gcc still issues warnings, despite #pragma ignore directive)
#pragma GCC diagnostic pop
    return 0;
}

warnings in gcc 4.7.2:
    sandbox_cpp11.cpp:148:29: warning: multi-character character constant
[-Wmultichar]
    sandbox_cpp11.cpp:148:37: warning: multi-character character constant
[-Wmultichar]
    sandbox_cpp11.cpp:148:45: warning: multi-character character constant
[-Wmultichar]
    sandbox_cpp11.cpp:148:53: warning: multi-character character constant
[-Wmultichar]

warnings in gcc 4.8.x:
    sandbox_cpp11.cpp:148:29: warning: multi-character character constant
[-Wmultichar]
      typedef boost::mpl::string<'0123', '4567', '89ab', 'cdef'> hex_chars;
                                 ^
    sandbox_cpp11.cpp:148:37: warning: multi-character character constant
[-Wmultichar]
      typedef boost::mpl::string<'0123', '4567', '89ab', 'cdef'> hex_chars;
                                         ^
    sandbox_cpp11.cpp:148:45: warning: multi-character character constant
[-Wmultichar]
      typedef boost::mpl::string<'0123', '4567', '89ab', 'cdef'> hex_chars;
                                                 ^
    sandbox_cpp11.cpp:148:53: warning: multi-character character constant
[-Wmultichar]
      typedef boost::mpl::string<'0123', '4567', '89ab', 'cdef'> hex_chars;

[Bug c++/57241] GCC still issues -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57241

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> ---
In general, it's safe to say that #pragma diagnostic ignored is very buggy (in
C++ at least), we have got many long standing PRs.

[Bug c++/57241] GCC still issues -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57241

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Paolo Carlini from comment #1)
> In general, it's safe to say that #pragma diagnostic ignored is very buggy
> (in C++ at least), we have got many long standing PRs.

Well the whole token ahead of time for C++ front-end causes this and another
issue with respect of #pragma's.

[Bug c++/57241] GCC still issues -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[scottbaldwin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57241

--- Comment #3 from etherice <scottbaldwin at gmail dot com> ---
(In reply to Andrew Pinski from comment #2)
> (In reply to Paolo Carlini from comment #1)
> > In general, it's safe to say that #pragma diagnostic ignored is very buggy
> > (in C++ at least), we have got many long standing PRs.
> 
> Well the whole token ahead of time for C++ front-end causes this and another
> issue with respect of #pragma's.

Fixing it would certainly help those of us using -Wall and -Werror and wanting
fine-grained control of when to allow specific cases (such as -Wmultichar when
defining a boost::mpl::string, or making a signed/unsigned comparison, etc.).

[Bug c++/57241] -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57241

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #2)
> (In reply to Paolo Carlini from comment #1)
> > In general, it's safe to say that #pragma diagnostic ignored is very buggy
> > (in C++ at least), we have got many long standing PRs.
> 
> Well the whole token ahead of time for C++ front-end causes this and another
> issue with respect of #pragma's.

Doesn't CPP diagnostics pass through the pragma classify code in diagnostic.c
even when tokenizing ahead of time? If so, I don't understand why the warning
is not suppressed. I don't have time to investigate it myself, but I don't see
why that would be the problem here.
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From: "nszabolcs at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/57258] New: unused variable warning is emitted for volatile variables
Date: Sun, 12 May 2013 20:32:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW258

            Bug ID: 57258
           Summary: unused variable warning is emitted for volatile
                    variables
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nszabolcs at gmail dot com

unused variable warnings are incorrectly issued for
volatile variables

a volatile variable is only unused if it is not
accessed, otherwise code will be generated for
using it following the semantics of the abstract
machine

in the following example the compiler gives a
warning eventhough the generated code uses x:

$ cat example.c
void f()
{
        volatile int x = 0;
}

$ gcc-4.8 -c -Wunused-variable example.c
example.c: In function 'f':
example.c:3:15: warning: unused variable 'x' [-Wunused-variable]
  volatile int x = 0;
               ^

[Bug c++/57241] -Wmultichar warnings despite a #pragma diagnostic ignored -Wmultichar directive

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57241

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Manuel López-Ibáñez from comment #4)
> 
> Doesn't CPP diagnostics pass through the pragma classify code in
> diagnostic.c even when tokenizing ahead of time? If so, I don't understand
> why the warning is not suppressed. I don't have time to investigate it
> myself, but I don't see why that would be the problem here.

So answering to myself: No, it doesn't (see PR53431). I think it must.

This affects all the controllable warnings from CPP when using g++.

*** This bug has been marked as a duplicate of bug 53431 ***
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Subject: [Bug c/59156] code miscompiled with -O2 -fpeel-loops
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY156

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
-fno-strict-aliasing fixes it.