A simple optimization proposal (original) (raw)

Vladimir Kozlov vladimir.kozlov at oracle.com
Wed Feb 12 14:00:19 PST 2014

Looks reasonable. Kris, you need also look for other patterns listed in JDK-8003585.

Thanks, Vladimir

On 2/12/14 12:39 PM, Krystal Mok wrote:

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_ _<mailto: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_ _<mailto: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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