[cfe-dev] How to disable built-in?

[Dmitri Shubin]

On 24.04.2012 15:48, David Chisnall wrote:
> This builtin is also present in GCC 4.7.  I suspect that the code brackets it in a version check that tests for a GCC version>= 4.7.  Since clang reports the GCC version 4.2, this will fail.
No, in fact it has brackets for (X86 AND GCC):

#if defined(HAVE_ATOMIC_X86_GCC_ASSEMBLY)
/* x86/x86_64 gcc  */
...
/*
  * x86/gcc Compare exchange for shared latches. i486+
  *      Returns 1 for success, 0 for failure
  *
  * GCC 4.1+ has an equivalent  __sync_bool_compare_and_swap() as well as
  * __sync_val_compare_and_swap() which returns the value read from *dest
  * http://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html
  * which configure could be changed to use.
  */
static inline int __atomic_compare_exchange(
         db_atomic_t *p, atomic_value_t oldval, atomic_value_t newval)
{
...
}
#endif

And check for x86/gcc looks like:

if test "$db_cv_atomic" = no; then
         cat confdefs.h - <<_ACEOF >conftest.$ac_ext
/* end confdefs.h.  */

int
main ()
{

         #if ((defined(i386) || defined(__i386__)) && defined(__GNUC__))
                 exit(0);
         #elif ((defined(x86_64) || defined(__x86_64__)) && 
defined(__GNUC__))
                 exit(0);
         #else
                 FAIL TO COMPILE/LINK
         #endif

   ;
   return 0;
}
_ACEOF
if ac_fn_c_try_compile "$LINENO"; then :
   db_cv_atomic="x86/gcc-assembly"
fi
rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
fi


> The correct fix is to wrap the check in !__has_builtin(__atomic_compare_exchange), rather than a specific compiler version.  A more hacky work-around would be to #undef the GCC version macros before including this header and re#define them as gcc 4.7.
I think __has_builtin() itself should be bracketed by ifdef __GNUC__ 
(probably with specific version?).

So looks like the easiest way is to rename the function.

Thanks!