linux-kernel - Re: Is gcc thread-unsafe?

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Message-id: <4721265B.2060504@shaw.ca>
Date:	Thu, 25 Oct 2007 17:27:23 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	davids@...master.com,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: Is gcc thread-unsafe?

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

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