A simple optimization proposal

[Krystal Mok]

Hi Martin and John,

I did a quick-and-dirty patch and it seems to work:
https://gist.github.com/rednaxelafx/8964030
If it looks right then I'll refactor that code a little bit and send it in
for official review.

- Kris


On Wed, Feb 12, 2014 at 11:17 AM, John Rose <john.r.rose at oracle.com> wrote:

> It's totally reasonable, and is already filed as an RFE (please comment on
> it!):
>
>   https://bugs.openjdk.java.net/browse/JDK-8003585
>
> -- John
>
> On Feb 12, 2014, at 9:40 AM, Martin Grajcar <maaartinus at gmail.com> wrote:
>
> Most hash tables are power-of-two sized so that they can use masking for
> the access. It looks like the bounds check doesn't get eliminated,
> although it could be.
>
> Based on the equivalence a[x & (a.length - 1)] throws if and only if a.length
> == 0, I'm proposing this simple algorithm:
>
>
>    - For each array access, check if the index has been computed via a
>    bitwise and.
>    - If so, check if either of the operands was computed as length minus
>    one.
>    - If so, replace the bounds check by a zero-length check.
>
> This zero-length check can then be easily moved out of the loop by the
> existing optimizations.
>
> I hope I'm not talking non-sense. For more details see
>
> http://stackoverflow.com/questions/21702939/why-the-bounds-check-doesnt-get-eliminated
>
> Regards,
> Martin.
>
>
>
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