RE: Why does ia64 not use fetchadd in atomic.h?

From: Chen, Kenneth W <kenneth.w.chen_at_intel.com>
Date: 2006-06-16 06:50:33
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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Received on Fri Jun 16 06:51:55 2006

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