From: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
To: Richard Henderson <richard.henderson@linaro.org>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
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: Mon, 16 Sep 2019 11:59:00 -0000 [thread overview]
Message-ID: <VI1PR0801MB2127EAFC3FFF12EEB1A15BAD838C0@VI1PR0801MB2127.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <5c4cbe49-8613-b9e4-9011-df26ced40a35@linaro.org>
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'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
next prev parent reply other threads:[~2019-09-16 11:59 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 [this message]
2019-09-17 8:40 ` Kyrill Tkachov
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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