RFR: 8324655: Identify integer minimum and maximum patterns created with if statements [v3]

[Emanuel Peter]

On Tue, 27 Feb 2024 17:02:17 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> @jaskarth @jatin-bhateja 
>>> While you are handling this, following Identity transforms can be added to integer and long Max/Min.
>> 
>> Yes, but in a separate RFE. These are simple IGVN optimization patterns.
>
> @eme64 I thought the merged values are the ones used in the comparison, which means that the 2 branches are empty? Maybe there are cases that I am missing here?

@merykitty I am thinking of a case like this:

a = <something very expensive>
b = <something cheap>
x = (a < b) ? a : b;

If in most cases we take `b` (because it is larger), then we might speculatively assign `x = b`, before we have finished computing `a`. That way we can already continue (speculatively) with `x = b`, while `a` is still computing. If the speculation is wrong, then the CPU flushes the pipeline.

If this is converted to `max/min`, then we need to wait for `a` to be computed.

@merykitty @jaskarth does this make sense as an example for a potential performance regression?

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PR Comment: https://git.openjdk.org/jdk/pull/17574#issuecomment-1967233730