Re: Optimizing Boundary Checks

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

Preben Randhol wrote:

>Reason for asking is that I get:
>
>t.adb:10:11: warning: value not in range of subtype of "Standard.integer" defined at line 6
>t.adb:10:11: warning: "constraint_error" will be raised at run time
>
>if I do:
>
>   type Items is array (1 .. 10) of Natural;
>   Boxes : Items;
>
>begin
>   
>   Boxes(11) := 1;

Well, yes. It can be determined at the time the code is compiled,
because

|1 < 11 or 11 > 10

simply results in True here, because everything is constant. Here the
compiler even is allowed to not do anything at all with the array,
instead it is enough to just generate the call to the exception
handler.

But if you'd like to try

|i : Integer;
| ...
|i := 11;
|Boxes (i) := 1;

instead, then I guess, there will be no warning, because at the time
the code is compiled, "i" will *not* be replaced by the constant 11
and the actual value of "i" is probably already out of scope for the
compiler (highly depends on the implementation, I think). Although,
any optimizing stage later probably still only generates a call to the
exception handler if it is smart enough. If it is not, then there must
be a range check here because

|1 <= Integer'First or 10 >= Integer'Last

results in False.

Of course, my understanding of when a compiler generates a range check
might be completely wrong, I am by no means a compiler writer. :)


Vinzent.

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