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
