Re: Bounds Check Overhead

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

On Sat, 27 May 2006 16:56:58 +0200, Ludovic Brenta wrote:

> "robin" writes:
>> "Simon Wright" writes:
>>> In Ada one should where possible use the 'Range attribute:
>>
>> Ideally yes, but in practice, such as in sorting and averaging,
>> the loop is often one or two short of the number of elements in the array.
> 
> No problem:
> 
> type Some_Array_Type is array (Positive range <>) of Something;
> 
> procedure Walk (A : in Some_Array_Type) is
> begin
>    for J in A'First .. A'Last - 1 loop
>       ...
>    end loop;
> end Walk;

I don't think it is same. A range expression of A'First and A'Last is not
statically required to be a subrange of A'Range (or maybe empty). So the
compiler had to prove that, if it would wish to omit checks. In your
example it is trivially provable (if nobody had played with "-"), though I
am not sure if, say, GNAT really does it.

Somebody had mentioned OCaml, I really doubt that inference is the answer.
The programmer knows far more than the compiler (or pier viewer) could
infer. If J has to be in A'Range, I prefer that it could be specified
rather than deduced.

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