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

[Kangcheng Xu]

On Fri, 29 Nov 2024 12:08:41 GMT, Quan Anh Mai <qamai 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 1679:
> 
>> 1677:   }
>> 1678: 
>> 1679:   // Optimistically assume limit in within int range, but add guards and traps to loop_limit_check.
> 
> You can merge these 2 checks into `CmpL(limit, ConvI2L(new_limit))`.

Thanks for reviewing! Good point. Code updated.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22449#discussion_r1866288963