[Bug rtl-optimization/51353] New: GCC incorrectly optimizes away assignment to return address

[Bug rtl-optimization/51353] New: GCC incorrectly optimizes away assignment to return address

[nkeynes at deadcoderemoval dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51353

             Bug #: 51353
           Summary: GCC incorrectly optimizes away assignment to return
                    address
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: nkeynes@deadcoderemoval.net


In the following code, GCC with -O completely removes the if block and
assignment:

int foo( int arg, void *exc )
{
    if( arg < 2 ) {
        *(((void **)__builtin_frame_address(0))+1) = exc;
    }
    return 0;
}

Producing (for i386):
.globl _foo
_foo:
    pushl    %ebp
    movl    %esp, %ebp
    movl    $0, %eax
    popl    %ebp
    ret

The relative merits and portability of the code aside, it doesn't seem correct
to completely remove the assignment. 

The code was compiled correctly with gcc 4.2.4, and fails with 4.4.6, 4.5.3,
and 4.6.1. The generated code is also correct with all optimizations off when
compiled with any of these versions.

[Bug rtl-optimization/51353] GCC incorrectly optimizes away assignment to return address

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51353

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-12-14 23:28:00 UTC ---
I think this should be:
        *(((volatile void **)__builtin_frame_address(0))+1) = exc;

GCC thinks it does not alias thing which is why it is removing it.

[Bug rtl-optimization/51353] GCC incorrectly optimizes away assignment to return address

[nkeynes at deadcoderemoval dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51353

--- Comment #2 from Nathan Keynes <nkeynes at deadcoderemoval dot net> 2011-12-15 02:14:48 UTC ---
Testing this out, 

     *(((volatile void **)__builtin_frame_address(0))+1) = exc;

still does not work in 4.4.6 and 4.5.3, but

     *(((void * volatile *)__builtin_frame_address(0))+1) = exc;

does work correctly.

[Bug rtl-optimization/51353] GCC incorrectly optimizes away assignment to return address

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51353

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-01-04 00:37:33 UTC ---
__builtin_frame_address(0) points to an address on the stack which means at the
end of the function, it can be removed.  The tricks you are doing require the
use of volatile so that the compiler cannot remove it.