More atomic functions please

[Jakub Jelinek]

On Thu, May 12, 2011 at 06:11:59PM +0200, Piotr Wyderski wrote:
> Hello,
> 
> I'm not sure if it should be better handled as missed optimization,
> but there is a certain lack of functionality in the GCC's __sync_*
> function family.
> 
> When implementing a reference counting smart pointer, two operations
> are of crucial importance:
> 
>     void __sync_increment(T* p);
>     bool __sync_decrement_iszero(T* p);
> 
> The former only increments the location pointed to by p, the latter decrements
> it and returns true if and only if the result was zero.
> 
> Both can be implemented in terms of existing __sync functions (and what
> can't? -- since there is __sync_bool_compare_and_swap()), e.g.:
> 
>    void __sync_increment(T* p) {
> 
>       __sync_fetch_and_add(p, 1);
>    }
> 
>   bool __sync_decrement(T* p) {
> 
>      return __sync_fetch_and_add(p, -1) == 1;
>   }

And that's the right thing to do.  If the generated code is not optimal, we should
just improve __sync_fetch_and_add code generation.
It isn't hard to special case addition of 1 or -1, and we already care whether
the result of the builtin is used or ignored.  For == 1 we could add some pattern that
combiner would merge.

Please file an enhancement request in gcc bugzilla.

	Jakub