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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Oberon and Wirthian languages
Date: Tue, 22 Apr 2014 18:38:32 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2014-04-22T18:38:32-05:00
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"Simon Clubley" <clubley@remove_me.eisner.decus.org-Earth.UFP> wrote in 
message news:lj6kfg$jhs$1@dont-email.me...
...
> Ok, now that's interesting. I would not have assumed the compiler would
> do the read multiple times in this specific case.
>
> I would have assumed the Volatile attribute would be satisfied in this
> specific case by doing the read _once_ at the start of the generated
> code and only generating code to modify the A, B and C bitfields
> before writing everything back in one step by using a single store
> opcode.

C.6(20):

The external effect of a program (see 1.1.3) is defined to include each read 
and update of a volatile or atomic object. The implementation shall not 
generate any memory reads or updates of atomic or volatile objects other 
than those specified by the program.

I see two reads of Rec in the original source code. The above rule means 
that an Ada compiler must NOT combine those reads for convinience; it must 
do EXACTLY what's in the source code. If you only want one read, you have to 
restructure so there is only one explicit read.

There was a long discussion about this some years ago, and the conclusion 
was that combining operations in particular is not allowed for volatile 
objects (and all atomic objects are also volatile).

                                     Randy.