Re: Pragma Volatile

["AG" <ang@xtra.co.nz>]

"Robert Dewar" <dewar@gnat.com> wrote in message
news:5ee5b646.0109300510.71255128@posting.google.com...

>
> I often find people making this kind of assumption, and
> in the case of reads I saw one awful bug caused by this:
>
>
>     type R is array (natural range <>) of Boolean;
>     pragma Pack (R);
>
>     Register : R (1 .. 32);
>     for Register'Address use ...
>
>     ...
>
>     B := Register (3);
>
> one compiler generated a word load, and extracted bit 3.
> another compiler generated a byte load, and extracted bit 3 from the
> byte.
>
> Both code sequences are perfectly valid, and just as
> efficient as one another. But the hardware had been
> build to require word loads, and byte loads were don't
> care and put the hardware into some obscure state.
>

Isn't it true that a compiler that puts it's target machine into
some obscure state can't be called perfectly valid? After all,
it is the job of the compiler to either produce a valid code or
reject what isn't possible. Can you really call a code sequence
(generated by a compiler with no control by the programmer)
"perfectly valid" if it sends the target machine from here to
"some obscure state"?

> But of course the real bug is in the implicit assumption
> in the code.

Well, that may be a bug in the assumptions in the code, but
it also looks like a compiler bug in so far that it accepted and
(mis)implemented those assumptions.