linux-kernel - Re: [RFC PATCH 03/15] Provide atomic_t functions implemented with ISO-C++11 atomics

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Message-ID: <20160519105000.GV3193@twins.programming.kicks-ass.net>
Date:	Thu, 19 May 2016 12:50:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-arch@...r.kernel.org, x86@...nel.org, will.deacon@....com,
	linux-kernel@...r.kernel.org, ramana.radhakrishnan@....com,
	paulmck@...ux.vnet.ibm.com, dwmw2@...radead.org
Subject: Re: [RFC PATCH 03/15] Provide atomic_t functions implemented with
 ISO-C++11 atomics

On Thu, May 19, 2016 at 10:52:19AM +0100, David Howells wrote:
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Does this generate 'sane' code for LL/SC archs? That is, a single LL/SC
> > loop and not a loop around an LL/SC cmpxchg.

> I think the code it generates should look something like:
> 
> 	test_atomic_add_unless:
> 	.L7:
> 		ldaxr	w1, [x0]		# __atomic_load_n()
> 		cmp	w1, 35			# } if (cur == unless)
> 		beq	.L4			# }     break
> 		add	w2, w1, 86		# new = cur + addend
> 		stlxr	w4, w2, [x0]
> 		cbnz	w4, .L7
> 	.L4:
> 		mov	w1, w0
> 		ret
> 
> but that requires the compiler to split up the LDAXR and STLXR instructions
> and render arbitrary code between.

Exactly.

> I suspect that might be quite a stretch.
> 
> I've opened:
> 
> 	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71191
> 
> to cover this.

Thanks; until such time as this stretch has been made I don't see this
intrinsic stuff being much use on any of the LL/SC archs.