Re: variable reusing

On 15-5-2013 0:31, Paul Sokolovsky wrote:
> Hello,
>
> On Wed, 15 May 2013 09:35:14 +1200
> Daniel Beer <[email protected]> wrote:
>
>> On Tue, May 14, 2013 at 09:26:46PM +0300, Paul Sokolovsky wrote:
>>> More interesting question is such support for static (if not global)
>>> variables. When trying to do Elliptic Curve Cryptography on MSP430
>>> value line devices which top up at 0.5K RAM, you really wish that
>>> compiler could "pack" together static buffers of functions which
>>> cannot be active at the same time ;-). If anyone know of that to be
>>> done by gcc, would be nice to get some pointers.
>> If you have static buffers which are never in use simultaneously, why
>> not keep them together in a union { }?
> Yes, the talk was about offloading the work of proving
> non-simultaneous access constraints and creation of that union to the
> compiler ;-). Shouldn't be hard for the case of scalar static variables
> (function-local dataflow analysis and interprocedural control flow
> analysis), so I wondered if someone knows it to be implemented. To be
> really useful this should be applied to structural/array vars though,
> and that's much harder.
>

This would be absolutely impossible because the compiler is generally 
unaware of it's environment. If the application is run on a preemptive 
scheduling operating system, then such schemes could go horribly wrong. 
Of course, you could protect the memory with a mutex, but this would 
defeat the purpose of easy optimization. Anyway, if you really want it I 
think it is best done via an explicit way with unions such that it is 
clear what is being shared, and it compatible amongst compilers.

Arie


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