[Bug other/27417] New: wrong code or aliasing violation with missed diagnostic?

[Bug other/27417] New: wrong code or aliasing violation with missed diagnostic?

[pluto at agmk dot net]

inline float quickBinaryToFloat( unsigned const& in )
{
        return reinterpret_cast< float const& >( in ) ;
}

float foo( unsigned x )
{
        unsigned y = ( x * 2 ) + 1;
        return quickBinaryToFloat( y );
}


[ wrong-code generated ]

$ i486-gnu-linux-g++ bin2float.cpp -Wall -O2 -c -fomit-frame-pointer -m32

00000000 <foo(unsigned int)>:
   0:   83 ec 10                sub    $0x10,%esp
   3:   d9 44 24 0c             flds   0xc(%esp)
   7:   83 c4 10                add    $0x10,%esp
   a:   c3                      ret

[ correct code generated ]

$ i486-gnu-linux-g++ bin2float.cpp -Wall -O2 -c -fomit-frame-pointer -m32 \
                                   -fno-strict-aliasing

00000000 <foo(unsigned int)>:
   0:   83 ec 10                sub    $0x10,%esp
   3:   8b 44 24 14             mov    0x14(%esp),%eax
   7:   8d 44 00 01             lea    0x1(%eax,%eax,1),%eax
   b:   89 44 24 0c             mov    %eax,0xc(%esp)
   f:   d9 44 24 0c             flds   0xc(%esp)
  13:   83 c4 10                add    $0x10,%esp
  16:   c3                      ret


-- 
           Summary: wrong code or aliasing violation with missed diagnostic?
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pluto at agmk dot net
 GCC build triplet: i486-linux
  GCC host triplet: i486-linux
GCC target triplet: i486-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27417

[Bug other/27417] wrong code or aliasing violation with missed diagnostic?

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2006-05-04 11:58 -------
This missed diagnostic is known, as enabling a warning here would cause too
much "false" positives.  But yes, you are violating strict-aliasing rules here.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27417

[Bug other/27417] wrong code or aliasing violation with missed diagnostic?

[pluto at agmk dot net]

------- Comment #2 from pluto at agmk dot net  2006-05-04 12:08 -------
(In reply to comment #1)
> This missed diagnostic is known, as enabling a warning here would cause too
> much "false" positives.

but what about -Wstrict-aliasing=2?
it doesn't report anything, so how can i check possible
violations with accepted false-positive level?


-- 

pluto at agmk dot net changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27417

[Bug other/27417] wrong code or aliasing violation with missed diagnostic?

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2006-05-04 14:00 -------
You cannot.  Though you can try extending c-common.c:strict_aliasing_warning
and
cp/typeck.c:build_reinterpret_cast_1 to warn in this case.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27417

[Bug other/27417] wrong code or aliasing violation with missed diagnostic?

[pluto at agmk dot net]

------- Comment #4 from pluto at agmk dot net  2006-05-05 15:10 -------
(In reply to comment #3)
> You cannot.  Though you can try extending c-common.c:strict_aliasing_warning
> and
> cp/typeck.c:build_reinterpret_cast_1 to warn in this case.

sorry, but i don't know compiler internals to do this.
btw, maybe we should collect in one PR examples of undetected
aliasing violations for future work on better detection?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27417