Re: volatile vs aliased

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 06 Oct 2005 10:05:38 +0200, 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;
>
> Good to know!

BTW, even without Volatile, they are not necessary same!

For intervals X*X /= X**2!

Example:

[-1, 2] * [-1, 2] = [-1, 4]

[-1, 2]**2 = [2, 4]

X**2 is as twice as more accurate than X*X.

To improve accuracy of * and /, arguments need to be analyzed for
dependency. X + X = 2 * X, no matter what.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de