RFR: 8342692: C2: long counted loop/long range checks: don't create loop-nest for short running loops [v15]

[Emanuel Peter]

On Wed, 7 May 2025 13:22:30 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> test/hotspot/jtreg/compiler/loopopts/superword/TestMemorySegment.java line 799:
>> 
>>> 797:                   IRNode.ADD_VI,        "> 0",
>>> 798:                   IRNode.STORE_VECTOR,  "> 0"},
>>> 799:         applyIfAnd = { "ShortRunningLongLoop", "true", "AlignVector", "false" },
>> 
>> Can you just copy the IR rule, please, so that we still have a failing rule without `ShortRunningLongLoop`?
>> 
>> The reason I have it here is so that I will catch these cases that are currently not properly vectorized... and it would be a shame if we lost these tests.
>> 
>> Also: can we whitelist `ShortRunningLongLoop` for the IR framework? I think we should make sure that we run all these MemorySegment tests with `ShortRunningLongLoop` enabled and disabled, just to make sure everything is ok with and without.
>> 
>> What do you think?
>> 
>> FYI: I'm making changes to this test again in https://github.com/openjdk/jdk/pull/24278. But I don't want to hold you back here with that.
>> 
>> Still: maybe you can take my approach with `NoSpeculativeAliasingCheck`, and add a run with `ShortRunningLongLoop` enabled or disabled. Just to make sure we have at least something running with both enabled and also with disabled.
>
> Wouldn't I then need to duplicate every `@run` line in the test i.e.:
> 
>  @run driver compiler.loopopts.superword.TestMemorySegment ByteArray
>  @run driver compiler.loopopts.superword.TestMemorySegment ByteArray AlignVector
> 
> 
> would become:
> 
> 
>  @run driver compiler.loopopts.superword.TestMemorySegment ByteArray
>  @run driver compiler.loopopts.superword.TestMemorySegment ByteArray AlignVector
>  @run driver compiler.loopopts.superword.TestMemorySegment ByteArray ShortLoop
>  @run driver compiler.loopopts.superword.TestMemorySegment ByteArray AlignVector ShortLoop
> 
> 
> Same for `CharArray` etc...
> That seems like a lot of extra complexity. Or would it be sufficient to only add it for `ByteArray` to have the non short loop case at least minimally covered?

Yeah, I would only do it for one or two cases. Doing it for all would be a little excessive, and eventually we have too many combinations.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21630#discussion_r2078517077