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

[Kangcheng Xu]

On Fri, 29 Nov 2024 01:08:04 GMT, Kangcheng Xu <kxu 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.

C2 changes

Passes `tier1 hotspot_compiler tier1_compiler tier2_compiler tier3_compiler tier1_compiler_not_xcomp` on x86_64 Linux. Will open this PR for formal review once GHA jcheck confirms passing. Thank you very much!

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PR Comment: https://git.openjdk.org/jdk/pull/22449#issuecomment-2506910167
PR Comment: https://git.openjdk.org/jdk/pull/22449#issuecomment-2513040792