Messages in this thread | | | Subject | Re: rwsem benchmark [was Re: [PATCH] rw_semaphores, optimisations try #3] | Date | Tue, 24 Apr 2001 11:33:13 +0100 | From | David Howells <> |
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> I see what you meant here and no, I'm not lucky, I thought about that. gcc x > 2.95.* seems smart enough to produce (%%eax) that you hardcoded when the > sem is not a constant (I'm not clobbering another register, if it does it's > stupid and I consider this a compiler mistake).
It is a compiler mistake... the compiler clobbers another register for you. The compiler does not, however, know about timing issues with the contents of the inline assembly... otherwise it'd stick a delay in front of the XADD in my stuff.
> I tried with a variable pointer and gcc as I expected generated the (%%eax) > but instead when it's a constant like in the bench my way it avoids to stall > the pipeline by using the constant address for the locked incl, exactly as > you said and that's probably why I beat you on the down read fast path too. > (I also benchmarked with a variable semaphore and it was running a little > slower)
*grin* Fun ain't it... Try it on a dual athlon or P4 and the answer may come out differently.
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