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SubjectRe: [patch 1/2] atomic_add_unless sadness


On Wed, 18 Jan 2006, Nick Piggin wrote:
>
> For some reason gcc 4 on at least i386 and ppc64 (that I have tested with)
> emit two cmpxchges for atomic_add_unless unless we put branch hints in.
> (it is unlikely for the "unless" case to trigger, and it is unlikely for
> cmpxchg to fail).

Irrelevant. If "atomic_add_unless()" is in a hot path and inlined, we're
doing something else wrong anyway. It's not a good op to use. Just think
of it as being very expensive.

The _only_ user of "atomic_add_unless()" is "dec_and_lock()", which isn't
even inlined. The fact that gcc ends up "unrolling" the loop once is just
fine.

Please keep it that way.

Linus
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