RFR: 8342692: C2: long counted loop/long range checks: don't create loop-nest for short running loops [v34]

Christian Hagedorn chagedorn at openjdk.org
Mon Jul 7 13:38:55 UTC 2025


On Thu, 5 Jun 2025 08:27:47 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> To optimize a long counted loop and long range checks in a long or int
>> counted loop, the loop is turned into a loop nest. When the loop has
>> few iterations, the overhead of having an outer loop whose backedge is
>> never taken, has a measurable cost. Furthermore, creating the loop
>> nest usually causes one iteration of the loop to be peeled so
>> predicates can be set up. If the loop is short running, then it's an
>> extra iteration that's run with range checks (compared to an int
>> counted loop with int range checks).
>> 
>> This change doesn't create a loop nest when:
>> 
>> 1- it can be determined statically at loop nest creation time that the
>>    loop runs for a short enough number of iterations
>>   
>> 2- profiling reports that the loop runs for no more than ShortLoopIter
>>    iterations (1000 by default).
>>   
>> For 2-, a guard is added which is implemented as yet another predicate.
>> 
>> While this change is in principle simple, I ran into a few
>> implementation issues:
>> 
>> - while c2 has a way to compute the number of iterations of an int
>>   counted loop, it doesn't have that for long counted loop. The
>>   existing logic for int counted loops promotes values to long to
>>   avoid overflows. I reworked it so it now works for both long and int
>>   counted loops.
>> 
>> - I added a new deoptimization reason (Reason_short_running_loop) for
>>   the new predicate. Given the number of iterations is narrowed down
>>   by the predicate, the limit of the loop after transformation is a
>>   cast node that's control dependent on the short running loop
>>   predicate. Because once the counted loop is transformed, it is
>>   likely that range check predicates will be inserted and they will
>>   depend on the limit, the short running loop predicate has to be the
>>   one that's further away from the loop entry. Now it is also possible
>>   that the limit before transformation depends on a predicate
>>   (TestShortRunningLongCountedLoopPredicatesClone is an example), we
>>   can have: new predicates inserted after the transformation that
>>   depend on the casted limit that itself depend on old predicates
>>   added before the transformation. To solve this cicular dependency,
>>   parse and assert predicates are cloned between the old predicates
>>   and the loop head. The cloned short running loop parse predicate is
>>   the one that's used to insert the short running loop predicate.
>> 
>> - In the case of a long counted loop, the loop is transformed into a
>>   regular loop with a ...
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 94 commits:
> 
>  - small fix
>  - Merge branch 'master' into JDK-8342692
>  - review
>  - review
>  - Update test/micro/org/openjdk/bench/java/lang/foreign/HeapMismatchManualLoopTest.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/longcountedloops/TestShortRunningLongCountedLoopScaleOverflow.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/longcountedloops/TestShortRunningLongCountedLoopPredicatesClone.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/longcountedloops/TestShortRunningLongCountedLoop.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/longcountedloops/TestShortRunningIntLoopWithLongChecksPredicates.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update src/hotspot/share/opto/loopnode.cpp
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - ... and 84 more: https://git.openjdk.org/jdk/compare/faf19abd...fd19ee84

I quickly ran through Emanuel's review comments. I think they all have been addressed. Added some follow-up suggestions on top but otherwise, it still looks good to me.

I guess since Emanuel is short on time, it would be good to have another review from someone.

src/hotspot/share/opto/c2_globals.hpp line 868:

> 866:   product(bool, ShortRunningLongLoop, true, DIAGNOSTIC,                     \
> 867:           "long counted loop/long range checks: don't create loop nest if " \
> 868:           "loop runs for small enough number of iterations. Long loop is"   \

Suggestion:

          "loop runs for small enough number of iterations. Long loop is "  \

src/hotspot/share/opto/loopnode.cpp line 1125:

> 1123: }
> 1124: 
> 1125: class NodeInSingleLoopBody : public NodeInLoopBody {

After the suggested update by Emanuel to have a more general name, you could also consider moving it to `predicates.hpp` to the other implementing classes of `NodeInLoopBody`.

src/hotspot/share/opto/loopnode.cpp line 1147:

> 1145:   CloneShortLoopPredicateVisitor(LoopNode* target_loop_head,
> 1146:                                   const NodeInSingleLoopBody& node_in_loop_body,
> 1147:                                   PhaseIdealLoop* phase)

Suggestion:

  CloneShortLoopPredicateVisitor(LoopNode* target_loop_head,
                                 const NodeInSingleLoopBody& node_in_loop_body,
                                 PhaseIdealLoop* phase)

test/hotspot/jtreg/compiler/longcountedloops/TestStressShortRunningLongCountedLoop.java line 35:

> 33:  * @build jdk.test.whitebox.WhiteBox
> 34:  * @run driver jdk.test.lib.helpers.ClassFileInstaller jdk.test.whitebox.WhiteBox
> 35:  * @run main/othervm -Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI compiler.longcountedloops.TestStressShortRunningLongCountedLoop

Probably a copy-paste error: You do not seem to be using the WhiteBox API for this test. You can then just use


 * @run driver compiler.longcountedloops.TestStressShortRunningLongCountedLoop

instead.

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

Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21630#pullrequestreview-2993768522
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2190022389
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2190056593
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2190094320
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2190128934


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