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From: Kyrill Tkachov <kyrylo.tkachov@foss.arm.com>
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	GCC Patches <gcc-patches@gcc.gnu.org>
Cc: nd <nd@arm.com>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
	"agraf@suse.de" <agraf@suse.de>,
	Marcus Shawcroft <Marcus.Shawcroft@arm.com>,
	James Greenhalgh <James.Greenhalgh@arm.com>
Subject: Re: [PATCH, AArch64, v3 0/6] LSE atomics out-of-line
Date: Tue, 17 Sep 2019 08:40:00 -0000	[thread overview]
Message-ID: <790aabf8-5edc-eb7a-b4ec-cccef74e403a@foss.arm.com> (raw)
In-Reply-To: <VI1PR0801MB2127EAFC3FFF12EEB1A15BAD838C0@VI1PR0801MB2127.eurprd08.prod.outlook.com>


On 9/16/19 12:58 PM, Wilco Dijkstra wrote:
> Hi Richard,
>
> >> So what is the behaviour when you explicitly select a specific CPU?
> >
> > Selecting a specific cpu selects the specific architecture that the cpu
> > supports, does it not?  Thus the architecture example above still 
> applies.
> >
> > Unless I don't understand what distinction that you're making?
>
> When you select a CPU the goal is that we optimize and schedule for that
> specific microarchitecture. That implies using atomics that work best for
> that core rather than outlining them.


I think we want to go ahead with this framework to enable the portable 
deployment of LSE atomics.

More CPU-specific fine-tuning can come later separately.

Thanks,

Kyrill


>
> >> I'd say that by the time GCC10 is released and used in distros, 
> systems without
> >> LSE atomics would be practically non-existent. So we should favour 
> LSE atomics
> >> by default.
> >
> > I suppose.  Does it not continue to be true that an a53 is more 
> impacted by the
> > branch prediction than an a76?
>
> That's hard to say for sure - the cost of taken branches (3 in just a 
> few instructions for
> the outlined atomics) might well affect big/wide cores more. Also note 
> Cortex-A55
> (successor of Cortex-A53) has LSE atomics.
>
> Wilco

  reply	other threads:[~2019-09-17  8:40 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-05 14:36 Wilco Dijkstra
2019-09-14 19:26 ` Richard Henderson
2019-09-16 11:59   ` Wilco Dijkstra
2019-09-17  8:40     ` Kyrill Tkachov [this message]
2019-09-17 10:55       ` Wilco Dijkstra
2019-09-17 21:11         ` Richard Henderson
  -- strict thread matches above, loose matches on Subject: below --
2018-11-01 21:47 Richard Henderson
2018-11-11 12:30 ` Richard Henderson
2019-09-05  9:51 ` Kyrill Tkachov

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