Re: Optimizing Boundary Checks

[Vinzent Hoefler <ada.rocks@jlfencey.com>]

Erlo Haugen wrote:

>On Fri, 13 Jun 2003 11:49:03 +0000 (UTC)
>Preben Randhol <randhol+abuse@pvv.org> wrote:
>
>> After reading some posts here lately I have understood that the compiler
>> can optimise out boundary checks on say arrays if you do this:
>> 
>>    type Index_Range is range 1 .. 10;
>>    type Items is array (Index_Range) of Natural;
>> 
>> But if you write:
>> 
>>    type Items is array (1 .. 10) of Natural;
>> 
>> is boundary checks then on?
>
>Yes, but in this case (principally) every acces to the array has to be checked,
>whereas in the former case, the index is guaranteed to be within range.

Well, the compiler *could* see that the type Index_Range always
satisfies the same constraint as for the array. I don't know for sure,
but I'd think, it does, because somehow it must store the constraints
for any type anyway and I strongly doubt that an optimizer would work
on type names instead of actual numbers describing their constraints.

OTOH, I think, using such unnamed indices is a quite bad idea anyway,
because it breaks the type safety. So simply: Don't do it. :-)


Vinzent.

-- 
Parents strongly cautioned  --  this  posting  is  intended for mature
audiences  over  18.  It  may  contain some material that many parents
would not find suitable for children and may include intense violence,
sexual situations, coarse language and suggestive dialogue.