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From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Thomas Rodgers <rodgert@appliantology.com>,
	Thomas Rodgers <trodgers@redhat.com>,
	 "libstdc++" <libstdc++@gcc.gnu.org>,
	gcc-patches <gcc-patches@gcc.gnu.org>,
	 Thomas Rodgers <rodgert@twrodgers.com>
Subject: Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange
Date: Thu, 23 Sep 2021 21:15:41 +0100	[thread overview]
Message-ID: <CAH6eHdSKVrMhhBDCWmnBr22dSPs0yx_WEcO-J1CjrH5BMHg0WQ@mail.gmail.com> (raw)
In-Reply-To: <20210923190708.GM304296@tucnak>

On Thu, 23 Sep 2021, 20:07 Jakub Jelinek via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> On Thu, Sep 23, 2021 at 11:08:37AM -0700, Thomas Rodgers wrote:
> > From: Thomas Rodgers <rodgert@twrodgers.com>
> >
> > This change implements P0528 which requires that padding bits not
> > participate in atomic compare exchange operations. All arguments to the
>
> Thanks for working on this.
>
> > generic template are 'sanitized' by the __builtin_clear_padding intrinsic
> > before they are used in atomic compare_exchange. This alrequires that any
> > stores also sanitize the incoming value.
>
> Not a review, just random nit.
> Shouldn't the __builtin_clear_padding calls be guarded with
> #if __has_builtin(__builtin_clear_padding)
> or #ifdef _GLIBCXX_HAVE_BUILTIN_CLEAR_PADDING defined based on that?
>

Yes. It can just use __has_builtin directly. All the compilers we care
about support that now.



I think clang doesn't support it (yet?), and it doesn't support the MSVC
> __builtin_zero_non_value_bits (with very similar, but slightly different,
> behavior).
>
> > Signed-off-by: Thomas Rodgers <trodgers@redhat.com>
> >
> > libstdc++=v3/ChangeLog:
> >
> >       * include/std/atomic (atomic<T>::atomic(_Tp) clear padding for
> >       __cplusplus > 201703L.
> >       (atomic<T>::store()) Clear padding.
> >       (atomic<T>::exchange()) Likewise.
> >       (atomic<T>::compare_exchange_weak()) Likewise.
> >       (atomic<T>::compare_exchange_strong()) Likewise.
> >       * testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
> >       test.
>
>         Jakub
>
>

  parent reply	other threads:[~2021-09-23 20:15 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-23 18:08 Thomas Rodgers
2021-09-23 19:07 ` Jakub Jelinek
2021-09-23 20:15   ` Thomas Rodgers
2021-09-23 20:15   ` Jonathan Wakely [this message]
2021-09-27 14:10 ` Thomas Rodgers
2021-09-29 12:13   ` Jonathan Wakely
2021-09-29 12:18     ` Jonathan Wakely
2021-09-29 12:28     ` Jakub Jelinek
2021-09-29 18:22     ` Thomas Rodgers
2021-09-29 18:29       ` Jakub Jelinek
2021-11-02  1:25     ` Thomas Rodgers
2021-11-02  7:49       ` Jakub Jelinek
2021-11-03  3:06         ` Thomas Rodgers
2021-11-02  8:49       ` Daniel Krügler
2022-01-18 21:48       ` Jonathan Wakely
2022-08-25 10:11         ` Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange) Jakub Jelinek
2022-09-01 22:57           ` Thomas Rodgers
2022-09-07 11:56             ` Jonathan Wakely
2022-09-07 22:06               ` Thomas Rodgers
2022-09-09 18:36               ` Rainer Orth
2022-09-09 18:46                 ` Iain Sandoe
2022-09-09 19:01                   ` Thomas Rodgers
2022-09-09 20:14                     ` Jonathan Wakely

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