There is no hardware support for 8 bytes atomic operations on 32-bit MIPS targets.  Unfortunately, programmers still want to use them.  Ex: please check this webkit bug: https://bugs.webkit.org/show_bug.cgi?id=106739 and the following example.

Ex:
fu@debian6:~/dev/test$ cat sync.c
long long K;
void main()
{
   __sync_fetch_and_add(&K, 1);
}

fu@debian6:~/dev/test$ gcc -o sync sync.c
/tmp/ccUoSg9M.o: In function `main':
sync.c:(.text+0x28): undefined reference to `__sync_fetch_and_add_8'
collect2: ld returned 1 exit status

That will be great, if we can add missing atomic functions (via pthread implementation or other methods) to libgcc for 32-bit MIPS targets.

Thanks a lot!