[Bug c/46073] New: __builtin_choose_expr outputs warnings for unused expression

[Bug c/46073] New: __builtin_choose_expr outputs warnings for unused expression

[kevin.waugh at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46073

           Summary: __builtin_choose_expr outputs warnings for unused
                    expression
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kevin.waugh@gmail.com


$ gcc -std=c99 -Wall -x c - << EOF
void A(int * p) {}
void B(double * p) {}
int main() {
  int * p = 0;
  __builtin_choose_expr(__builtin_types_compatible_p(__typeof__(p), int *),
A(p), B(p));
  return 0;
}
EOF
<stdin>: In function ‘main’:
<stdin>:5: warning: passing argument 1 of ‘B’ from incompatible pointer type

$ gcc -v
Using built-in specs.
Target: i686-apple-darwin9
Configured with: /var/tmp/gcc/gcc-5493~1/src/configure --disable-checking
-enable-werror --prefix=/usr --mandir=/share/man
--enable-languages=c,objc,c++,obj-c++
--program-transform-name=/^[cg][^.-]*$/s/$/-4.0/
--with-gxx-include-dir=/include/c++/4.0.0 --with-slibdir=/usr/lib
--build=i686-apple-darwin9 --with-arch=apple --with-tune=generic
--host=i686-apple-darwin9 --target=i686-apple-darwin9
Thread model: posix
gcc version 4.0.1 (Apple Inc. build 5493)

[Bug c/46073] __builtin_choose_expr outputs warnings for unused expression

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46073

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2010-10-18 19:00:51 UTC ---
I think this is correct as  __builtin_choose_expr still does semantically
checking on the two arguments.  Not to mention  __builtin_choose_expr was added
only to support Altivec intrinsics and that has since been moved away from
that.

[Bug c/46073] __builtin_choose_expr outputs warnings for unused expression

[kevin.waugh at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46073

--- Comment #2 from kevin.waugh at gmail dot com 2010-10-18 19:17:17 UTC ---
(In reply to comment #1)
> I think this is correct as  __builtin_choose_expr still does semantically
> checking on the two arguments.  Not to mention  __builtin_choose_expr was added
> only to support Altivec intrinsics and that has since been moved away from
> that.

>From the documentation:
Note: This construct is only available for C. Furthermore, the unused
expression (exp1 or exp2 depending on the value of const_exp) may still
generate syntax errors. This may change in future revisions.

What are the chances of getting this changed, or having an option to suppress
warnings on the unevaluated argument?  The implementation of tgmath.h is
essentially using __builtin_choose_expr for the same purpose, but it appers to
avoid any warnings/errors because of implicit casts.

[Bug c/46073] __builtin_choose_expr outputs warnings for unused expression

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46073

--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> 2010-10-18 19:55:25 UTC ---
On Mon, 18 Oct 2010, pinskia at gcc dot gnu.org wrote:

> checking on the two arguments.  Not to mention  __builtin_choose_expr was added
> only to support Altivec intrinsics and that has since been moved away from
> that.

When C1X _Generic is implemented that will have similar issues.