Integrated: 8259609: C2: optimize long range checks in long counted loops

[Roland Westrelin]

On Tue, 12 Jan 2021 10:15:01 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).

This pull request has now been integrated.

Changeset: 82f4aacb
Author:    Roland Westrelin <roland at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/82f4aacb42e60e9cd00e199703a869e7ad4465ff
Stats:     975 lines in 13 files changed: 800 ins; 67 del; 108 mod

8259609: C2: optimize long range checks in long counted loops

Co-authored-by: John R Rose <jrose at openjdk.org>
Reviewed-by: thartmann, jrose

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