[Bug c/62024] New: __atomic_always_lock_free is not a constant expression

[Bug c/62024] New: __atomic_always_lock_free is not a constant expression

[amanieu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

            Bug ID: 62024
           Summary: __atomic_always_lock_free is not a constant expression
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amanieu at gmail dot com

This line causes an error despite the documentation saying that
__atomic_always_lock_free is a constant expression:

_Static_assert(__atomic_always_lock_free(sizeof(int), 0), "");

test.c:1:16: error: expression in static assertion is not constant

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-08-13
                 CC|                            |mpolacek at gcc dot gnu.org
   Target Milestone|---                         |4.10.0
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed even for C++.  The issue is the same: fold folds
__atomic_always_lock_free to 1, but that 1 is wrapped in NOP_EXPR - and static
assert code is unhappy.  I think we can just STRIP_TYPE_NOPS - we don't expect
an lvalue in the static assert code.  I'm testing a patch for that.

Not a regression.

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[amanieu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

--- Comment #2 from Amanieu d'Antras <amanieu at gmail dot com> ---
A similar error happens when trying to use the result of
__atomic_always_lock_free as the size of an array:

int array[__atomic_always_lock_free(sizeof(int), 0)];

test.c:1:5: error: variably modified ‘array’ at file scope
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Subject: [Bug c/62024] __atomic_always_lock_free is not a constant expression
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Amanieu d'Antras from comment #2)
> int array[__atomic_always_lock_free(sizeof(int), 0)];
> 
> test.c:1:5: error: variably modified ‘array’ at file scope

I think it should be fine to reject this one - the array size is not an integer
constant expression.  While the first argument of _Static_assert also has to be
an integer constant expression, I thought we could permit some latitude here; 
the _Static_assert (__atomic_always_lock_free (sizeof (int), 0), "foo"); line
looks quite reasonable to me...
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idb059

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-08-13
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Whatever we do for __atomic_always_lock_free, note that we'll probably 
need to find some way for ATOMIC_*_LOCK_FREE (in stdatomic.h) to expand 
to something usable in #if.

http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_458.htm

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to joseph@codesourcery.com from comment #4)
> Whatever we do for __atomic_always_lock_free, note that we'll probably 
> need to find some way for ATOMIC_*_LOCK_FREE (in stdatomic.h) to expand 
> to something usable in #if.
> 
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_458.htm

Couldn't we just map ATOMIC_*_LOCK_FREE macros (in stdatomic.h) to
__GCC_ATOMIC_*_LOCK_FREE macros defined in c-cppbuiltin.c:cpp_atomic_builtins?
FWIW, libsupc++ does exactly that.  If this approach makes sense, I can prepare
a patch with testcase(s).

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Thu, 14 Aug 2014, mpolacek at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024
> 
> --- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
> (In reply to joseph@codesourcery.com from comment #4)
> > Whatever we do for __atomic_always_lock_free, note that we'll probably 
> > need to find some way for ATOMIC_*_LOCK_FREE (in stdatomic.h) to expand 
> > to something usable in #if.
> > 
> > http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_458.htm
> 
> Couldn't we just map ATOMIC_*_LOCK_FREE macros (in stdatomic.h) to
> __GCC_ATOMIC_*_LOCK_FREE macros defined in c-cppbuiltin.c:cpp_atomic_builtins?
> FWIW, libsupc++ does exactly that.  If this approach makes sense, I can prepare
> a patch with testcase(s).

Yes, I think that should work.

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

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 #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I posted a patch.  Let's see what happens.

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Author: mpolacek
Date: Wed Sep  3 11:16:29 2014
New Revision: 214871

URL: https://gcc.gnu.org/viewcvs?rev=214871&root=gcc&view=rev
Log:
    PR c/62024
    * c-parser.c (c_parser_static_assert_declaration_no_semi): Strip no-op
    conversions.

    * g++.dg/cpp0x/pr62024.C: New test.
    * gcc.dg/pr62024.c: New test.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/pr62024.C
    trunk/gcc/testsuite/gcc.dg/pr62024.c
Modified:
    trunk/gcc/c/ChangeLog
    trunk/gcc/c/c-parser.c
    trunk/gcc/testsuite/ChangeLog

[Bug c/62024] __atomic_always_lock_free is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62024

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

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

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fixed.