From: Matthias Kretz <m.kretz@gsi.de>
To: Jason Merrill <jason@redhat.com>, Jonathan Wakely <jwakely@redhat.com>
Cc: "Richard Earnshaw \(lists\)" <Richard.Earnshaw@arm.com>,
libstdc++ <libstdc++@gcc.gnu.org>,
gcc-patches List <gcc-patches@gcc.gnu.org>,
GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH] c++: implement C++17 hardware interference size
Date: Fri, 16 Jul 2021 21:26:11 +0200 [thread overview]
Message-ID: <2948804.xd1mhZDcFd@excalibur> (raw)
In-Reply-To: <CACb0b4kp-X+t7+Yuf9VwaX6cBL-jaRoVBE-EsZCyv+S0LQAKCw@mail.gmail.com>
On Friday, 16 July 2021 18:54:30 CEST Jonathan Wakely wrote:
> On Fri, 16 Jul 2021 at 16:33, Jason Merrill wrote:
> > Adjusting them based on tuning would certainly simplify a significant use
> > case, perhaps the only reasonable use. Cases more concerned with ABI
> > stability probably shouldn't use them at all. And that would mean not
> > needing to worry about the impossible task of finding the right values for
> > an entire architecture.
>
> But it would be quite a significant change in behaviour if -mtune
> started affecting ABI, wouldn't it?
For existing code -mtune still doesn't affect ABI. The users who write
struct keep_apart {
alignas(std::hardware_destructive_interference_size) std::atomic<int> cat;
alignas(std::hardware_destructive_interference_size) std::atomic<int> dog;
};
*want* to have different sizeof(keep_apart) depending on the CPU the code is
compiled for. I.e. they *ask* for getting their ABI broken. If they wanted to
specify the value themselves on the command line they'd written:
struct keep_apart {
alignas(SOME_MACRO) std::atomic<int> cat;
alignas(SOME_MACRO) std::atomic<int> dog;
};
I would be very disappointed if std::hardware_destructive_interference_size
and std::hardware_constructive_interference_size turn into a glorified macro.
--
──────────────────────────────────────────────────────────────────────────
Dr. Matthias Kretz https://mattkretz.github.io
GSI Helmholtz Centre for Heavy Ion Research https://gsi.de
std::experimental::simd https://github.com/VcDevel/std-simd
──────────────────────────────────────────────────────────────────────────
next prev parent reply other threads:[~2021-07-16 19:26 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20210716023656.670004-1-jason@redhat.com>
2021-07-16 2:41 ` [PATCH] c++: implement C++17 hardware interference size Jason Merrill via Libc-alpha
2021-07-16 2:48 ` Noah Goldstein via Libc-alpha
2021-07-16 11:17 ` Jonathan Wakely via Libc-alpha
2021-07-16 13:27 ` Richard Earnshaw via Libc-alpha
2021-07-16 13:26 ` Jonathan Wakely via Libc-alpha
2021-07-16 15:12 ` Matthias Kretz
2021-07-16 15:30 ` Jason Merrill via Libc-alpha
2021-07-16 16:54 ` Jonathan Wakely via Libc-alpha
2021-07-16 18:43 ` Jason Merrill via Libc-alpha
2021-07-16 19:26 ` Matthias Kretz [this message]
2021-07-16 19:58 ` Jonathan Wakely via Libc-alpha
2021-07-17 8:14 ` Matthias Kretz
2021-07-17 13:32 ` Jonathan Wakely via Libc-alpha
2021-07-17 13:54 ` Matthias Kretz
2021-07-17 21:37 ` Jason Merrill via Libc-alpha
2021-07-19 9:41 ` Richard Earnshaw via Libc-alpha
2021-07-20 16:43 ` Jason Merrill via Libc-alpha
2021-07-20 18:05 ` Thomas Rodgers
2021-07-16 17:20 ` Noah Goldstein via Libc-alpha
2021-07-16 19:37 ` Matthias Kretz
2021-07-16 21:23 ` Noah Goldstein via Libc-alpha
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