On Thu, May 19, 2016 at 12:50:00PM +0200, Peter Zijlstra wrote: > > I suspect that might be quite a stretch. > > > > I've opened: > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71191 > > > > to cover this. > > Thanks; until such time as this stretch has been made I don't see this > intrinsic stuff being much use on any of the LL/SC archs. FWIW, Will and me have been discussing a GCC/LLVM language extension that would allow generating the insides of LL/SC loops. But neither has had time to properly write something down yet :/ My latest thinking is something along the lines of: static __always_inline int __load_locked(int *ptr) { int val; __asm__ __volatile__ ("ldaxr %[val], [%[ptr]]" : [val] "r" (val) : [ptr] "m" (*ptr)); return val; } static __always_inline bool __store_conditional(int *ptr, int old, int new) { int ret; __asm__ __volatile__ ("stlxr %[ret], %[new], [%[ptr]]" : [ret] "r" (ret) : [new] "r" (new), [ptr] "m" (*ptr)); return ret != 0; } bool atomic_add_unless(atomic_t *v, int a, int u) { int val, old; do __special_marker__ { old = val = __load_locked(&v->counter); if (val == u) goto fail; val += a; } while (__store_conditional(&v->counter, old, val)); return true; fail: return false; } Where the __special_marker__ marks the whole { } scope as being the inside of LL/SC and all variables must be in registers before we start. If the compiler is not able to guarantee this, it must generate a compile time error etc.. The __sc takes the @old and @new arguments such that we can implement this on CAS archs with a regular load and CAS. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
