Messages in this thread | | | Date | Wed, 24 Aug 2011 15:01:39 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 07/12] x86: use cmpxchg_flag() where applicable |
| |
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.
-hpa
| |