gcc lacks the ability to output condition variables from an asm block which forces you to shove the value of the condition variable into a general purpose register and output that instead.

i didn't expect it to work, but i thought that maybe the gcc optimiser would be smart enough to reorder code of this form:

  if (x)
    y = 1;
  else
    y = 0;

  ...

  if (y)
    a ();
  else
    b ();


to simply:

  if (x)
    a ();
  else
    b ();



it turns out that gcc *is* smart enough to do that, and it even works with 'asm goto'.  fantastic!

bad news is that when things get just a little more complicated (ie: trying to do the same thing twice within a function) then gcc throws this:

at.c:30:1: internal compiler error: in cfg_layout_merge_blocks, at cfgrtl.c:2721

This is Fedora 16.

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.6.2/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre --enable-libgcj-multifile --enable-java-maintainer-mode --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.6.2 20111027 (Red Hat 4.6.2-1) (GCC) 

Here's the code:

#include <stddef.h>

extern void a (void);
extern void b (void);

int
bit_set (int  *address,
         int   bit_address)
{
  asm goto ("lock bts %1, (%0)\n"
            "jc %l[contended]"
            : /* no output */
            : "r" (address), "r" ((size_t) bit_address)
            : "cc", "memory"
            : contended);
  return 0;
contended:
  return 1;
}

void
do_stuff (int *address)
{
  if (bit_set (address, 23))
    a ();
  else
    b ();

  bit_set (address, 2);
}