Messages in this thread | | | Date | Wed, 24 Aug 2011 15:03:32 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 07/12] x86: use cmpxchg_flag() where applicable |
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On 08/24/2011 03:01 PM, H. Peter Anvin wrote: > On 08/24/2011 02:56 PM, Linus Torvalds wrote: >> Ok, I see nothing horrible in this series. >> >> The one reaction I have is that the cmpxchg_flag() thing returns an >> 8-bit value, but then a lot of the users end up having to extend it to >> a full "int" purely for calling convention reasons (eg I think >> 'down_write_trylock()' will have 'sete + movzl' - not a new problem, >> but since the whole point was to remove extraneous instructions and we >> no longer have the silly 'testl', it now annoys me more). >> >> So it seems a bit sad. But I guess it doesn't really matter. >> > I think it is a net lose. The most common case is probably going to be > to use it immediately, in which case we have: > > cmpxchg -> sete -> compare -> conditional > > versus > > cmpxchg -> compare -> conditional > > For doubleword cmpxchg it's another matter entirely, because doubleword > comparisons are significantly more expensive that sete + test. > > So unless there is actual data showing this is better, I would like to > see this dropped for now.
Well, we could keep the API (since it is convenient), but just implement it with a compare.
J
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