RFR: 8307513: C2: intrinsify Math.max(long,long) and Math.min(long,long) [v12]

[Emanuel Peter]

On Fri, 7 Mar 2025 12:25:51 GMT, Galder Zamarreño <galder at openjdk.org> wrote:

>> @galderz Thanks for the summary of regressions! Yes, there are plenty of speedups, I assume primarily because of `Long.min/max` vectorization, but possibly also because the operation can now "float" out of a loop for example.
>> 
>> All your Regressions 1-3 are cases with "extreme" probabilitiy (close to 100% / 0%), you listed none else. That matches with my intuition, that branching code is usually better than cmove in extreme probability cases.
>> 
>> As for possible solutions. In all Regression 1-3 cases, it seems the issue is scalar cmove. So actually in all cases a possible solution is using branching code (i.e. `cmp+mov`). So to me, these are the follow-up RFE's:
>> - Detect "extreme" probability scalar cmove, and replace them with branching code. This should take care of all regressions here. This one has high priority, as it fixes the regression caused by this patch here. But it would also help to improve performance for the `Integer.min/max` cases, which have the same issue.
>> - Additional performance improvement: make SuperWord recognize more cases as profitble (see Regression 1). Optional.
>> - Additional performance improvement: extend backend capabilities for vectorization (see Regression 2 + 3). Optional.
>> 
>> Does that make sense, or am I missing something?
>
>> As for possible solutions. In all Regression 1-3 cases, it seems the issue is scalar cmove. So actually in all cases a possible solution is using branching code (i.e. `cmp+mov`). So to me, these are the follow-up RFE's:
>> 
>> * Detect "extreme" probability scalar cmove, and replace them with branching code. This should take care of all regressions here. This one has high priority, as it fixes the regression caused by this patch here. But it would also help to improve performance for the `Integer.min/max` cases, which have the same issue.
> 
> I've created [JDK-8351409](https://bugs.openjdk.org/browse/JDK-8351409) to address this.

@galderz Excellent. Testing looks all good on our side. Yes I think what you saw was unrelated.
@rwestrel Could give this a last quick scan and then I think you can integrate :)

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PR Comment: https://git.openjdk.org/jdk/pull/20098#issuecomment-2706434983