Re: Is gcc thread-unsafe?

Robert Hancock
2007-10-25T23:25:59Z

Arjan van de Ven wrote:
> On Wed, 24 Oct 2007 21:29:56 -0700
> "David Schwartz" <davids@webmaster.com> wrote:
>
>>> Well that's exactly right. For threaded programs (and maybe even
>>> real-world non-threaded ones in general), you don't want to be
>>> even _reading_ global variables if you don't need to. Cache misses
>>> and cacheline bouncing could easily cause performance to completely
>>> tank in some cases while only gaining a cycle or two in
>>> microbenchmarks for doing these funny x86 predication things.
>> For some CPUs, replacing an conditional branch with a conditional
>> move is a *huge* win because it cannot be mispredicted.
>
> please name one...
> Hint: It's not one made by either Intel or AMD in the last 4 years...

It is a win if the branch cannot be effectively predicted, i.e. if the
outcome is essentially random, as may occur with data-dependent
conditionals. I've seen a doubling of performance on one workload using
a predicated instruction instead of a branch on newer Xeons in such a case.

I suspect that if branch prediction fails often, the data dependency
created by the cmov, etc. is less expensive than the pipeline flush
required by mispredicts..

--
Robert Hancock Saskatoon, SK, Canada
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