Re: Optimizing Boundary Checks

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

Preben Randhol wrote:

>So to sum up:
>
>If I had done:
>
>   subtype Index_Range is Integer range 1 .. 10;
>   type Items is array (Index_Range) of Natural;
>
>   Boxes : Items;
>   Index : Integer := 11;
>begin
>
>   Boxes (Index) := 1;
>
>then I get an constraint error at run-time as the compiler will accept
>the code.

Yes.

>And in this case range checks on the array must be on.

Of course.

Well, I think there's a catch: it could be possible that even with
range checks turned off, you *could* get an exception here, for
instance if the optimizer can already boil the code down that far.
Suppressing checks doesn't mean you will never get an exception at
runtime, see the note in RM 11.5(29).

>If I
>change to:
>
>   type Index_Range is range 1 .. 10;
>
>Then the range checks on the array *can* be switched off as the range
>checks of Index_Range will do.
>
>Is this right?

Yes, perfectly.

>I'm asking because there was an post some days ago saying that my
>example above could have range checks on the array off IIRC and that
>puzzels me.

I guess, you are refering to
<slrnbdpdan.87p.randhol+abuse@kiuk0152.chembio.ntnu.no>?

Well, the catch here was, that a subtype declaration actually allows
indexing an array with a "larger" type (like plain integer) than the
actual constraints of the indices really are. If range checking is
required or not then, simply depends on the type you use for indexing
in a particular expression.


Vinzent.

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