[Bug c/67661] New: Wrong warning when declare VLAs: operation on 'b' may be undefined [-Wsequence-point]

[Bug c/67661] New: Wrong warning when declare VLAs: operation on 'b' may be undefined [-Wsequence-point]

[leechung at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661

            Bug ID: 67661
           Summary: Wrong warning when declare VLAs: operation on 'b' may
                    be undefined [-Wsequence-point]
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leechung at 126 dot com
  Target Milestone: ---

The following code produces a warning: operation on 'b' may be
undefined.[-Wsequence-point]

int x = 0, y [++ x], z [++ x];

But N1570 6.8 says:
A block allows a set of declarations and statements to be grouped into one
syntactic unit. The initializers of objects that have automatic storage
duration, and the variable length array declarators of ordinary identifiers
with block scope, are evaluated and the values are stored in the objects
(including storing an indeterminate value in objects without an initializer)
each time the declaration is reached in the order of execution, as if it were a
statement, and within each declaration in the order that declarators appear.

and 6.7.6 says:
A full declarator is a declarator that is not part of another declarator. The
end of a full declarator is a sequence point.

[Bug c/67661] Wrong warning when declare VLAs: operation on 'x' may be undefined [-Wsequence-point]

[leechung at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661

--- Comment #2 from leechung <leechung at 126 dot com> ---
(In reply to joseph@codesourcery.com from comment #1)
> You'll need to give a full testcase (complete compilable file and options 
> used to compile it).  What you gave isn't a compilable testcase; it gives 
> "error: variably modified 'y' at file scope".  Put inside a function, it 
> gives "warning: unused variable 'y' [-Wunused-variable]", but does not 
> give the warning you mention.  And there's no variable 'b' in your example 
> at all.

Sorry, I am less experience.
The following is the complete code:

#include <stdio.h>

int main (void)
{
    int x = 0, y [++ x], z [++ x];
    printf ("%d, %d, %d\n", sizeof x, sizeof y, sizeof z);
    return 0;
}

and are compiled with option '-Wall'.for example:
gcc xx.c -Wall

then produce a warning 'operation on 'x' may be undefined [-Wsequence-point]'

[Bug c/67661] Wrong warning when declare VLAs: operation on 'x' may be undefined [-Wsequence-point]

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661

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

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

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
int main (void)
{
    int x = 0, y [++ x], z [++ x];
    __builtin_printf ("%d, %d, %d\n", sizeof x, sizeof y, sizeof z);
    return 0;
}

$ cc1 -Wsequence-point test.c
test.c:3:27: warning: operation on ‘x’ may be undefined [-Wsequence-point]
   int x = 0, y [++ x], z [++ x];
                           ^

I'll let someone else with more knowledge of the standard to decide if the
warning is correct or not.
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Subject: [Bug c/67661] Wrong warning when declare VLAs: operation on 'x' may be undefined [-Wsequence-point]
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idg661

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

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

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I think the warning is bogus and the code looks valid.

[Bug c/67661] Wrong warning when declare VLAs: operation on 'x' may be undefined [-Wsequence-point]

[frankhb1989 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661

frankhb1989 at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |frankhb1989 at gmail dot com

--- Comment #5 from frankhb1989 at gmail dot com ---
"%d" in the case should be replaced by "%zu".
Confirmed that clang 3.7 does not complain about it.

[Bug c/67661] Wrong warning when declare VLAs: operation on 'x' may be undefined [-Wsequence-point]

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661

--- Comment #6 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
The warning is bogus.  It appears even if separate declarations are used 
for the three variables; maybe it's something to do with the printf call?

[Bug c/67661] Wrong warning when declare VLAs: operation on 'x' may be undefined [-Wsequence-point]

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67661

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-09-24
     Ever confirmed|0                           |1