From: Michael Matz <matz@suse.de>
To: Richard Henderson <rth@redhat.com>
Cc: Torvald Riegel <triegel@redhat.com>,
Bin Fan <bin.x.fan@oracle.com>,
"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>,
Jakub Jelinek <jakub@redhat.com>
Subject: Re: GCC libatomic ABI specification draft
Date: Fri, 20 Jan 2017 13:42:00 -0000 [thread overview]
Message-ID: <alpine.LSU.2.20.1701201436410.31530@wotan.suse.de> (raw)
In-Reply-To: <2cc60e20-8b70-1be3-e052-b3c0f5b0750c@redhat.com>
Hi,
On Wed, 18 Jan 2017, Richard Henderson wrote:
> > Section 3 Rationale, alternative 1: I'm wondering if the example is
> > correct. For a 4-byte-aligned type of size 3, the implementation
> > cannot simply use 4-byte hardware-backed atomics because this will
> > inevitably touch the 4th byte I think, and the implementation can't
> > know whether this is padding or not. Or do we expect that things like
> > packed structs are disallowed?
>
> If we atomically store an unchanged value into the 4th byte, can we
> tell?
You can't have a 4-aligned type of size 3. Sizes must be multiples of
alignment (otherwise arrays don't work). The type of a 3-sized field in
a packed struct that syntactically might be a 4-aligned type (e.g. by
using attributes on char-array types) is actually a different type having
an alignment of 1. It's easier to simply regard all types inside packed
structs as 1-aligned (which is IMO what we try to do).
That is, the byte after a 4-aligned "3-sized" type is always padding.
Ciao,
Michael.
next prev parent reply other threads:[~2017-01-20 13:42 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cbd2c83a-b50b-b2ac-b62d-b2d26178c2b1@oracle.com>
2016-07-06 17:50 ` Fwd: Re: GCC libatomic questions Richard Henderson
2016-07-06 19:41 ` Richard Henderson
2016-07-07 23:56 ` Bin Fan
[not found] ` <ac2d60ed-a659-f018-1f11-63fa8f5847f5@oracle.com>
[not found] ` <1470412312.14544.4.camel@localhost.localdomain>
[not found] ` <4a182edd-41a8-4ad9-444a-bf0af567ae98@oracle.com>
[not found] ` <8317ec9d-41ad-d806-9144-eac2984cdd38@oracle.com>
2016-11-17 20:12 ` GCC libatomic ABI specification draft Bin Fan
2016-11-29 11:12 ` Szabolcs Nagy
2016-12-01 19:14 ` Bin Fan at Work
2016-12-02 11:13 ` Gabriel Paubert
2016-12-19 16:33 ` Torvald Riegel
2016-12-20 13:27 ` Ulrich Weigand
2016-12-20 13:58 ` Szabolcs Nagy
2016-12-22 14:29 ` Ulrich Weigand
2016-12-22 17:38 ` Segher Boessenkool
2017-01-04 11:25 ` Szabolcs Nagy
2017-01-19 15:18 ` Torvald Riegel
2017-01-17 17:00 ` Torvald Riegel
2017-01-18 22:23 ` Richard Henderson
2017-01-19 15:02 ` Torvald Riegel
2017-01-20 13:42 ` Michael Matz [this message]
2017-01-20 17:17 ` Richard Henderson
2017-01-23 14:00 ` Michael Matz
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