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Date: Wed, 20 Apr 2016 14:11:32 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160420181132.GA24862@flamenco>
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Subject: Re: [Qemu-devel] [PATCH v3 05/11] qemu-thread: add simple
 test-and-set spinlock
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To: Richard Henderson <rth@twiddle.net>
Cc: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>

On Wed, Apr 20, 2016 at 10:55:45 -0700, Richard Henderson wrote:
> On 04/20/2016 10:17 AM, Emilio G. Cota wrote:
> >I've tried to find a GCC intrinsic for test-and-set, and I've only found
> >lock_test_and_set, which is what we use for atomic_xchg (except on ppc)
> >because it really is an atomic exchange:
> >  "This builtin, as described by Intel, is not a traditional test-and-set
> >   operation, but rather an atomic exchange operation."
> >  https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html
> 
> Please read the entire documentation, not just the first sentence.

I did read it and I'm aware of the limitations of using xchg.

My comment was related to this:

> [...] do note that there are compiler primitives for test-and-set that
> (can be) simpler for a cpu to implement than xchg.

What compiler (I assume gcc) primitives are these? I couldn't find them.

Thanks,

		Emilio