From: Marc Glisse <marc.glisse@inria.fr>
To: Maged Michael <maged.michael@gmail.com>
Cc: libstdc++@gcc.gnu.org
Subject: Re: Proposed patch to skip the last atomic decrements in _Sp_counted_base::_M_release
Date: Tue, 8 Dec 2020 09:01:23 +0100 (CET) [thread overview]
Message-ID: <64ffb83-460-eb85-108c-df9fc1212654@stedding.saclay.inria.fr> (raw)
In-Reply-To: <CABBFi09ovWwPpUnC2V_vU5iVMLR0vQd77EgrO5XTFPOofmWzdQ@mail.gmail.com>
On Mon, 7 Dec 2020, Maged Michael via Libstdc++ wrote:
> Any pointers on where to file bug reports about the generated code?
https://gcc.gnu.org/bugzilla/
(creating an account may not be as trivial as it used to be, but it still
works)
>> ISTR that libstdc++ uses a surprisingly strong memory barrier when
>> incrementing references (acq_rel, where I would expect acquire to suffice,
>> although I haven't thought about weak references that much). Did you also
>> experiment with that?
(I actually meant "relaxed")
> I applied to x86, where acq_rel doesn't add extra memory fence instructions
> for atomic increment/decrement operations.
Indeed, on x86 just making the increment atomically requires lock add or
lock inc, which already includes a strong barrier. It does make a
difference in generated code on aarch64 or powerpc64el though.
--
Marc Glisse
next prev parent reply other threads:[~2020-12-08 8:01 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-07 12:24 Maged Michael
2020-12-07 15:28 ` Marc Glisse
2020-12-07 15:41 ` Jonathan Wakely
2020-12-08 4:10 ` Maged Michael
2020-12-08 8:01 ` Marc Glisse [this message]
2020-12-08 9:56 ` Jonathan Wakely
2020-12-07 15:37 ` Jonathan Wakely
2020-12-08 4:06 ` Maged Michael
2020-12-07 12:30 Maged Michael
2020-12-07 15:57 ` Jonathan Wakely
2020-12-07 16:23 ` Jonathan Wakely
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