Christoph Lameter wrote on Thursday, June 15, 2006 1:36 PM
> I just looked at at the asm generated by some of the use of atomic for
> statistics and saw that there were complicated compxchg constructs where I
> would have expected a simple fetchadd. Why is this?
>
> F.e.
>
> static __inline__ int
> ia64_atomic_add (int i, atomic_t *v)
> {
> __s32 old, new;
> CMPXCHG_BUGCHECK_DECL
>
> do {
> CMPXCHG_BUGCHECK(v);
> old = atomic_read(v);
> new = old + i;
> } while (ia64_cmpxchg(acq, v, old, new, sizeof(atomic_t)) != old);
> return new;
> }
>
> Why not
>
> #define ia64_atomic_add(__i, __v) ia64_fetchadd(__i, &v->counter, acq)
The immediate operand of fetchadd only takes limited value, so it would have
to be a combination of fetchadd and cmpxchg. If the constant is known at
compile time, it can be optimized for those constants that fetchadd can
operate on.
Back to your original question, I guess whoever implements ia64_atomic_add
didn't bother to optimize it like it's close cousin of atomic_add().
- Ken
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