__sync_bool_compare_and_swap and structures

[Andrew Haley]

Robert Lemmen wrote:
> hi everyone,
> 
> i have written a couple of algorithms using the GCC builtin CAS, which
> all works very nicely. it is a bit ugly and full of casts, and i am
> trying to clean it up. i stumbled over something that i can't really
> resolve however, and was wondering how other people do that:
> 
> 
> // we are on 32bit, so this structure is 64 bits, just like a long long
> struct gptr {
> 	void *ptr;
> 	int gen;
> };
> 
> int main(int argc, char *argv[]) {
> 	long long a, b;
> 	long long *A = &a;
> 
> 	// works as expected
> 	while (!__sync_bool_compare_and_swap(A, a, b)) {};
> 
> 	struct gptr c, d;
> 	struct gptr *C = &c;
> 
> 	// incompatible type for argument 1 of `__sync_bool_compare_and_swap'
> 	while (!__sync_bool_compare_and_swap(C, c, d)) {};
> 
> 	// works but is really ugly
> 	while (!__sync_bool_compare_and_swap((long long*)C, 
> 		*(long long*)&c, *(long long*)&d)) {};
> 
> 	return 0;
> }
> 
> as you can see i can use CAS on 8 byte primitives (long long int) quite
> well, but if i use an equal sized struct instead i get an error. i can
> of course cast it as shown above, but i was hoping that i just miss
> something and that it can be done cleanly.
> 
> how are you guys doing that?

Use a union of an 8 byte struct and a long long.

Andrew.