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Re: [Qemu-devel] [PATCH v3 05/11] qemu-thread: add simple test-and-set s

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [PATCH v3 05/11] qemu-thread: add simple test-and-set spinlock
Date: Wed, 20 Apr 2016 14:11:32 -0400
User-agent: Mutt/1.5.23 (2014-03-12)

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.



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