Re: Is gcc thread-unsafe?

[Robert Hancock]

Arjan van de Ven wrote:
> On Wed, 24 Oct 2007 21:29:56 -0700
> "David Schwartz" <davids@xxxxxxxxxxxxx> 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
To email, remove "nospam" from hancockr@xxxxxxxxxxxxx
Home Page: http://www.roberthancock.com/

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/