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

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Date:	Thu, 25 Oct 2007 13:58:54 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kleen, Andi" <ak@...e.de>
Subject: Re: Is gcc thread-unsafe?

On Thursday 25 October 2007 13:46, Arjan van de Ven wrote:
> On Thu, 25 Oct 2007 13:24:49 +1000
>
> Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > Hi,
> >
> > Andi spotted this exchange on the gcc list. I don't think he's
> > brought it up here yet, but it worries me enough that I'd like
> > to discuss it.
> >
> > Starts here
> > http://gcc.gnu.org/ml/gcc/2007-10/msg00266.html
> >
> > Concrete example here
> > http://gcc.gnu.org/ml/gcc/2007-10/msg00275.html
> >
> > Basically, what the gcc developers are saying is that gcc is
> > free to load and store to any memory location, so long as it
> > behaves as if the instructions were executed in sequence.
>
> this optimization btw is a serious mis-optimization, it makes memory
> more dirty and causes cachelines to become unshared.... I'm sure it
> works great on microbenchmarks but it sucks bigtime for anything real

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.

I'm not sure about ia64 -- I _hope_ that for most of their
predication stuff, they also predicate the stores, rather than
just store unconditionally and rely on the source operand not
changing in the case they didn't intend the memory to change.
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