Re: [RFC PATCH 03/15] Provide atomic_t functions implemented with ISO-C++11 atomics

[Peter Zijlstra]

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.