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

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Message-ID: <20071024213533.12115ba0@laptopd505.fenrus.org>
Date:	Wed, 24 Oct 2007 21:35:33 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	davids@...master.com
Cc:	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: Is gcc thread-unsafe?

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...


> In general,
> compilers should optimize for unshared data since that's much more
> common in typical code. Even for shared data, the usual case is that
> you are going to access the data few times, so pulling the cache line
> to the CPU is essentially free since it will happen eventually.

it's not about pulling it to the CPU, it's pulling it *out* of all the
other cpus AS WELL. (and writing it back to memory, taking away memory
bandwidth)


--
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