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

[Quan Anh Mai]

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.

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))`.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/22449#discussion_r1863429887