RFR: 8342692: C2: long counted loop/long range checks: don't create loop-nest for short running loops [v32]

[Emanuel Peter]

On Mon, 26 May 2025 08:20:50 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 incrementally with one additional commit since the last revision:
> 
>   review

Need to hop trains, sending out a first batch :)

src/hotspot/share/opto/c2_globals.hpp line 860:

> 858:   develop(bool, StressShortRunningLongLoop, false,                          \
> 859:           "Speculate all long counted loops are short running when bounds " \
> 860:           "are unknown even if profile data doesn't say so.")               \

Why only when bounds are unknown?

src/hotspot/share/opto/castnode.cpp line 327:

> 325: }
> 326: 
> 327: bool CastLLNode::inner_loop_backedge(Node* proj) {

Suggestion:

bool CastLLNode::is_inner_loop_backedge(Node* proj) {

Optional. It would help me know that it is just a check. Otherwise, I wonder if we might "make" the inner loop backedge.

src/hotspot/share/opto/castnode.cpp line 339:

> 337: }
> 338: 
> 339: bool CastLLNode::cmp_used_at_inner_loop_exit_test(Node* cmp) {

Assert that `cmp` is really a `CmpNode`?

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

PR Review: https://git.openjdk.org/jdk/pull/21630#pullrequestreview-2869017909
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2107745527
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2107747988
PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2107751453