Should it be reported as a bug? (-O2 and cmpxchg instruction)

[Martin Sustrik <sustrik@imatix.com>]

Hi all,

I've encountered a problem with gcc inline assembly.

Following code, when optimised with -O2 gives following machine code:

            __asm__ volatile (
                "lock; cmpxchgl %1, %3\n\t"
                "jz 1f\n\t"
                "mov %2, %%eax\n\t"
                "lock; xchgl %%eax, %3\n\t"
                "1:\n\t"
                : "=a" (oldval)
                : "r" (thenval_), "r" (elseval_), "m" (value), "0" (0)
                : "memory", "cc");

 4031a0:   31 c0                   xor    %eax,%eax
  4031a2:   f0 0f b1 55 40          lock cmpxchg %edx,0x40(%rbp)
  4031a7:   74 06                   je     4031af
  4031a9:   89 c0                   mov    %eax,%eax
  4031ab:   f0 87 45 40             lock xchg %eax,0x40(%rbp)

Note that %2 maps to %%eax (mov %2, %%eax --> mov %eax,%eax). This 
shouldn't happen given that cmpxchg modifies the value of %%eax.

Any idea whether this should be considered a bug and reported as such?

GCC version info:

Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v 
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr 
--enable-shared --with-system-zlib --libexecdir=/usr/lib 
--without-included-gettext --enable-threads=posix --enable-nls 
--program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu 
--enable-libstdcxx-debug --enable-mpfr --enable-checking=release 
x86_64-linux-gnu
Thread model: posix
gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)

Martin