RFR: 8336759: C2: int counted loop with long limit not recognized as counted loop

[Roland Westrelin]

On Fri, 29 Nov 2024 17:05:53 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> This patch implements [JDK-8336759](https://bugs.openjdk.org/browse/JDK-8336759) that recognizes int counted loops with long limits.
>> 
>> Currently, patterns like `for ( int i =...; i < long_limit; ...)` where int `i` is implicitly promoted to long (i.e., `(long) i < long_limit`) is not recognized as (int) counted loop. This patch speculatively and optimistically converts long limits to ints and deoptimize if the limit is outside int range, allowing more optimization opportunities. 
>> 
>> In other words, it transforms 
>> 
>> 
>> for (int i = 0; (long) i < long_limit; i++) {...}
>> 
>> 
>> to 
>> 
>> 
>> if (int_min <= long_limit && long_limit <= int_max ) {
>>     for (int i = 0;  i < (int) long_limit; i++) {...}
>> } else {
>>     trap: loop_limit_check
>> }
>> 
>> 
>> This could benefit calls to APIs like `long MemorySegment#byteSize()` when iterating over a long limit.
>
> src/hotspot/share/opto/loopnode.cpp line 1668:
> 
>> 1666: 
>> 1667:   _igvn.rehash_node_delayed(bol);
>> 1668:   bol->replace_edge(cmp, new_cmp, &_igvn);
> 
> You should use: `PhaseIdealLoop::insert_loop_limit_check_predicate()`. `do_is_counted_loop()` may need to add more loop limit checks and the way you've implemented it, I think you're replacing the place holder predicate that we use to create other ones.

Actually you're not checking that it's a loop limit check so you could be replacing something else. You need to follow the pattern used elsewhere i.e.: https://github.com/openjdk/jdk/blob/2beb2b602bf20f1ec36e6244eca1a2eb50baccb4/src/hotspot/share/opto/loopnode.cpp#L2011

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PR Review Comment: https://git.openjdk.org/jdk/pull/22449#discussion_r1863789968