A simple optimization proposal (original) (raw)

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!):

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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