From: Richard Henderson <richard.henderson@linaro.org>
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Richard Henderson <richard.henderson@linaro.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: Sat, 14 Sep 2019 19:26:00 -0000 [thread overview]
Message-ID: <5c4cbe49-8613-b9e4-9011-df26ced40a35@linaro.org> (raw)
In-Reply-To: <VI1PR0801MB212764F895C50D4A51E7748183BB0@VI1PR0801MB2127.eurprd08.prod.outlook.com>
On 9/5/19 10:35 AM, Wilco Dijkstra wrote:
> Agreed. I've got a couple of general comments:
>
> * The option name -matomic-ool sounds too abbreviated. I think eg.
> -moutline-atomics is more descriptive and user friendlier.
Changed.
> * Similarly the exported __aa64_have_atomics variable could be named
> __aarch64_have_lse_atomics so it's clear that it is about LSE atomics.
Changed.
> +@item -matomic-ool
> +@itemx -mno-atomic-ool
> +Enable or disable calls to out-of-line helpers to implement atomic operations.
> +These helpers will, at runtime, determine if ARMv8.1-Atomics instructions
> +should be used; if not, they will use the load/store-exclusive instructions
> +that are present in the base ARMv8.0 ISA.
> +
> +This option is only applicable when compiling for the base ARMv8.0
> +instruction set. If using a later revision, e.g. @option{-march=armv8.1-a}
> +or @option{-march=armv8-a+lse}, the ARMv8.1-Atomics instructions will be
> +used directly.
>
> 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?
> +/* Branch to LABEL if LSE is enabled.
> + The branch should be easily predicted, in that it will, after constructors,
> + always branch the same way. The expectation is that systems that implement
> + ARMv8.1-Atomics are "beefier" than those that omit the extension.
> + By arranging for the fall-through path to use load-store-exclusive insns,
> + we aid the branch predictor of the smallest cpus. */
>
> 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?
r~
next prev parent reply other threads:[~2019-09-14 19:26 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 [this message]
2019-09-16 11:59 ` Wilco Dijkstra
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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