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From: "Rolf" <rolf.ebert_nospam_@gmx.net>
Newsgroups: comp.lang.ada
Subject: Re: volatile vs aliased
Date: 6 Oct 2005 04:36:44 -0700
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Date: 2005-10-06T04:36:44-07:00
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Martin Krischik wrote:
> Ludovic Brenta wrote:
>
> > pragma Volatile (Variable) says the compiler must not optimise away
> > any reads or writes to that variable, and that it may not add extra
> > reads or writes beyond those you explicitly request in your program
> > text.  You want to use that for hardware registers, where a "read"
> > operation may have a side effect such as changing the device's state.
>
> Interesting! So with  pragma Volatile (X) the following two statements are
> not the same:
>
> Y := X * X;
> Y := X ** 2;

Do we have a difference to C/C++ increment operator here? consider

pragma Volatile (x);
x := x + 1;   -- (1)
        vs.
x++;          -- (2)

In the Ada case (1) we are forced to have "read from memory",
"increment" and "write to memory" instructions, whereas in C/C++ (2)
you can get a single "increment memory" instruction (presuming the
mentioned assembler instructions exist on a given processor)

    Rolf