RFR: 8324655: Identify integer minimum and maximum patterns created with if statements [v2]

[Aleksey Shipilev]

On Sun, 11 Feb 2024 07:16:38 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Jasmine Karthikeyan has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Don't transform highly predictable branches
>
> In general, I don't see why this proposal is only applied to `Min`/`Max` nodes.
> 
>> While this transform doesn't generally improve code generation it's own, it simplifies control flow and creates new opportunities for vectorization.
> 
> From your description and the benchmark results, I think this issue is applied to a general `CMove`. While a `CMove` would simplify the control flow, enable more optimisations, especially vectorisation; a `CMove` may deteriorate the performance if the corresponding branch is predictable. As a result, I don't think only applying the transformations to min/max patterns is a sound solution.
> 
> Thanks.

> @merykitty @shipilev @jaskarth it seems to me that attacking `CMove` is much more tricky. The branch-predictor is very good on some platforms, and not so good on others. In some cases, misprediction has quite a penalty.

I did this exercise back in the days:
 https://shipilev.net/jvm/anatomy-quarks/30-conditional-moves/

What I saw is that branch wins over cmov for very predicted branches (~97%?). But there are also secondary effects like computing _both_ sides of cmov instead of _one_ side of the branch, that would be hard to catch in benchmarks.

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PR Comment: https://git.openjdk.org/jdk/pull/17574#issuecomment-1963861757