[Bug c/34115] New: atomic builtins not supported on i686?

[scovich at gmail dot com]

Linking fails for the program below, with the error:

undefined reference to `___sync_val_compare_and_swap_4'

// gcc -Wall atomic.c
int main() {
  int *a, b, c;
  return __sync_val_compare_and_swap(a, b, c);
}

According to the atomic builtins docs (), "Not all operations are supported by
all target processors. If a particular operation cannot be implemented on the
target processor, a warning will be generated and a call an external function
will be generated. The external function will carry the same name as the
builtin, with an additional suffix `_n' where n is the size of the data type."

If CAS is not supported, how come I don't get a warning? Why would i686 *not*
support compare and swap? The cmpxchg instruction has been around since 80486,
according to the intel IA-32 processor manual. 

Also, does an unsupported builtin mean the user is responsible to write that
function, or simply that the compiler must make a function call to synthesize
its behavior?

FWIW, my x86_64 cross-compile gcc 4.2.0 handles it fine, emitting a
"lock"+"cmpxchg" pair.


-- 
           Summary: atomic builtins not supported on i686?
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: scovich at gmail dot com
  GCC host triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115