Request for Reviews (S): JDK-8003585 strength reduce or eliminate range checks for power-of-two sized arrays (Was: Re: A simple optimization proposal)

[Krystal Mok]

Hi all,

Could I get a couple of reviews for this change, please?

Bug: https://bugs.openjdk.java.net/browse/JDK-8003585
Webrev: http://cr.openjdk.java.net/~kmo/8003585/webrev.00/

Note 1: Cases 2 and 4 are handled by the same code in this change.
Note 2: Martin's concerns seems to hold, so the patch will need to be
changed to handle that:

On Wed, Feb 12, 2014 at 3:45 PM, Martin Grajcar <maaartinus at gmail.com>
wrote:
> I'm afraid, not all equivalences listed in
> https://bugs.openjdk.java.net/browse/JDK-8003585
> are right, namely the first one
>
> (x & m) <= m, if and only if (m >= 0)
>
> Using x = -1 reduces the LHS to m <= m.

Description: (copied from the bug report)

Integer expressions which perform bitwise and can be proven to be less than
or equal to either operand, as long as the compared operand is
non-negative. In other words:
Case 1:
  (x & m) <= m, if and only if (m >= 0)

This means the left-hand test can be replaced by the simpler right-hand
test.

There are also off-by-one versions, such as:
Case 2:
  (x & (m-1)) < m, if and only if (m > 0)

There are also unsigned versions:
Case 3:
  (x & m) u<= m, always
Case 4:
  (x & (m-1)) u< m, if and only if (m > 0)

The optimizer should recognize these patterns. They are common in
implicitly generated range checks for power-of-two sized arrays:

  int hash = ...;
  int bucket = hash & (array.length-1);
  Entry e = array[bucket];

The range check is:
  (hash & (array.length-1)) u< array.length

This check can be strength reduced to:
  array.length != 0

If the array is constant, or if user code has a dominating check for an
empty array, this check will go away completely.

Tests:
Ran some tests manually and checked that Case 1, 2 and 4 does get pattern
matched. Need someone from Oracle to run JPRT and other tests appropriate.

Thanks,
Kris (OpenJDK username: kmo)

On Wed, Feb 12, 2014 at 3:33 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com
> wrote:

> Kris,
>
> Can you submit formal review request as changes for 8003585 with webrev on
> cr.openjdk?
>
> Note, you can't return return phase->intcon(1) from Ideal() because we
> need new node. Return ConINode::make(phase->C, 1) instead.
>
> Thanks,
> Vladimir
>
>
> On 2/12/14 3:05 PM, Krystal Mok wrote:
>
>> 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 <mailto: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
>>
>>         <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>
>>         <mailto: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
>>
>>         <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>
>>              <mailto: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
>>             <http://stackoverflow.com/questions/21702939/why-the-
>> bounds-check-doesnt-get-eliminated>
>>
>>                  Regards,
>>                  Martin.
>>
>>
>>
>>
>>
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