A simple optimization proposal

John Rose john.r.rose at oracle.com
Wed Feb 12 11:17:29 PST 2014

It's totally reasonable, and is already filed as an RFE (please comment on it!):


— 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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