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

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v3 05/11] qemu-thread: add simple test-and-set spinlock
Date: Wed, 20 Apr 2016 12:39:45 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/20/2016 11:11 AM, Emilio G. Cota wrote:
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."

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.

__sync_lock_test_and_set and __atomic_test_and_set.

Both expand to ldstub on sparcv7, tas on coldfire, tas.b on sh.
None of these are xchg operations.

I had forgotten that there wasn't a __sync_exchange builtin, so __sync_lock_test_and_set plays double-duty as both xchg and test-and-set.

But when __atomic builtins are available, __atomic_exchange does not fall back; you must use __atomic_test_and_set for less capable hosts.

But of course none of this is really relevant for normal hosts.


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