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

[Roland Westrelin]

On Tue, 27 May 2025 07:33:05 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   review
>
> src/hotspot/share/opto/loopnode.cpp line 1278:
> 
>> 1276: #endif
>> 1277:     entry_control = head->skip_strip_mined()->in(LoopNode::EntryControl);
>> 1278:   } else if (bt == T_LONG) {
> 
> Suggestion:
> 
>   } else if (bt == T_LONG) {
>     assert(known_short_running_loop, "follows from earlier bailout check.");
> 
> I suppose that is what you mean by `won't need loop limit checks (iters_limit guarantees that)`, right?

The `LongCountedLoop` is turned into a `Loop` here which, on a following pass of loop opts, is turned into a `CountedLoop`. That `CountedLoop` doesn't need loop limit check predicates because of the way its bounds are constructed. But, sometimes, by the time the `CountedLoop` is created, some transformation happened to the bounds and the code that creates the `CountedLoop` can't tell the loop limit checks are not needed. The `Cast` is added to make sure the `CountedLoop` logic sees the bounds are such that the loop limit checks should not be added.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2109517371