From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 buffer1.nntp.dca1.giganews.com!border1.nntp.dca1.giganews.com!nntp.giganews.com!newspeer1.nac.net!newsfeed.xs4all.nl!newsfeed2.news.xs4all.nl!xs4all!news.stack.nl!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Modified proposals for Ada-Comment
Date: Sat, 12 Jul 2014 22:00:36 +0100
Organization: A noiseless patient Spider
Message-ID: <ly8unyxpfv.fsf@pushface.org>
References: <lprvo8$mcl$1@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: mx05.eternal-september.org;
 posting-host="5cbec41b82bafb9ba55107f02585e18b";
 logging-data="13070"; mail-complaints-to="abuse@eternal-september.org";
 posting-account="U2FsdGVkX18KxlMaaZTfEc5Vz5VoN/YYoM8H2MvxZjs="
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (darwin)
Cancel-Lock: sha1:g7uzShj6oeAmcr6w+MWbAZTfydA=
 sha1:VdlEB5NW4BJFvJhIkbxIJjlK/wo=
Xref: number.nntp.dca.giganews.com comp.lang.ada:187542
Date: 2014-07-12T22:00:36+01:00
List-Id: <comp.lang.ada>

Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> writes:

> was defined as Atomic. However, the generated code showed that when a
> bitfield, instead of the record as a whole, was referenced, GNAT used
> ldrb/strb (byte level access instructions) instead of ldr/str (32 bit
> access instructions) when the bitfield was within the first 8 bits
> of the record.

Likewise when the bitfield was in the MS 8 bits. I haven't tried with
bitfields of length greater than 1.

> C.6(15) states:
>
> 	For an atomic object (including an atomic component) all reads and
> 	updates of the object as a whole are indivisible.
>
> There are conflicting opinions about the above rule in comp.lang.ada.
> The words "as a whole" in C.6(15) were used to justify the position
> that access to this single bitfield is not required to be in units of
> the record size which on the surface seemed reasonable.

I think the problem is with the phrase "as a whole". If it means that
partial access need not be indivisible it probably needs to spell it out
better, because that is completely at odds with what I would expect of
"atomic".

> However, other opinions are that C.6(15) does apply when accessing
> this single bitfield.

I think that the "other opinions" are that "as a whole" is a mistake, so
that C.6(15) should say

   For an atomic object (including an atomic component) all reads and
   updates of the object are indivisible.


Aside from these niggles, a useful pair of proposals.