RFR: 8259609: C2: optimize long range checks in long counted loops [v13]

[Tobias Hartmann]

On Tue, 26 Oct 2021 13:26:36 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> JDK-8255150 makes it possible for java code to explicitly perform a
>> range check on long values. JDK-8223051 provides a transformation of
>> long counted loops into loop nests with an inner int counted
>> loop. With this change I propose transforming long range checks that
>> operate on the iv of a long counted loop into range checks that
>> operate on the iv of the int inner loop once it has been
>> created. Existing range check eliminations can then kick in.
>> 
>> Transformation of range checks is piggy backed on the loop nest
>> creation for 2 reasons:
>> 
>> - pattern matching range checks is easier right before the loop nest
>>   is created
>> 
>> - the number of iterations of the inner loop is adjusted so scale *
>>   inner_iv doesn't overflow
>> 
>> C2 has logic to delay some split if transformations so they don't
>> break the scale * iv + offset pattern. I reused that logic for long
>> range checks and had to relax what's considered a range check because
>> initially a range check from Object.checkIndex() may include a test
>> for range > 0 that needs a round of loop opts to be hoisted. I realize
>> there's some code duplication but I didn't see a way to share logic
>> between IdealLoopTree::may_have_range_check()
>> IdealLoopTree::policy_range_check() that would feel right.
>> 
>> I realize the comment in PhaseIdealLoop::transform_long_range_checks()
>> is scary. FWIW, it's not as complicated as it looks. I found drawing
>> the range covered by the entire long loop and the range covered by the
>> inner loop help see how range checks can be transformed. Then the
>> comment helps make sure all cases are covered and verify the generated
>> code actually covers all of them.
>> 
>> One issue is overflow. I think the fact that inner_iv * scale doesn't
>> overflow helps simplify thing. One possible overflow is that of scale
>> * upper + offset which is handled by forcing all range checks in that
>> case to deoptimize. I don't think other case of overflow needs special
>> handling.
>> 
>> This was tested with a Memory Segment micro benchmark (and patched
>> Memory Segment support to take advantage of the new checkIndex
>> intrinsic, both provided by Maurizio). Range checks in the micro
>> benchmark are properly optimized (and performance increases
>> significantly).
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Thanks, that looks good to me.

-------------

Marked as reviewed by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2045