A simple optimization proposal

[Krystal Mok]

Hi Vladimir,

Thanks for looking at it. I added the other cases and added a missing
condition check.
The patch is updated in place: https://gist.github.com/rednaxelafx/8964030

Ran a few small cases on case 1 and 3 manually and the resulting IR graphs
were right. I wasn't able to check the case 2 ("Change ((x & m) u<= m) to
always true") though, I don't know what Java code could be compiled into
that pattern.

Thanks,
Kris


On Wed, Feb 12, 2014 at 2:00 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com
> wrote:

> 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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