patch applied (packages/array): Improve the index checking for array accesses; fixes #2120 #2669

[Ian Lynagh]

On Mon, Jul 20, 2009 at 09:07:14AM +0100, Simon Marlow wrote:
> On 19/07/2009 16:59, Ian Lynagh wrote:
>> Sun Jul 19 08:29:31 PDT 2009  Ian Lynagh<igloo at earth.li>
>>    * Improve the index checking for array accesses; fixes #2120 #2669
>>    As well as checking that offset we are reading is actually inside the
>>    array, we now also check that it is "in range" as defined by the Ix
>>    instance. This fixes confusing behaviour (#2120) and improves some error
>>    messages (#2669).
>
> Does the extra test get optimised away in the common case?

No.

For example, for an Int-indexed array:

We call GHC.Arr.$fIxInt_$windex which does the
    lowerBound <= index
    index <= upperBound
tests, and returns (index - lowerBound).

Then there are inlined tests for
    0 <= result
    result < size


Thanks
Ian