RFR: 8307084: C2: Vectorized drain loop is not executed for some small trip counts

[Emanuel Peter]

On Tue, 10 Dec 2024 14:19:46 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/loopTransform.cpp line 1690:
>> 
>>> 1688: 
>>> 1689: //------------------------------insert_atomic_post_loop_impl-------------------------------
>>> 1690: // The main implementation of inserting atomic post loop after vector main loop.
>> 
>> I would really appreciate some kind of ascii art here. It should show the pre, main, vectorized-post and post loop.
>> And the relevant zero-trip guards, etc.
>
> And maybe you can also draw how the rewiring happens, i.e. where you cut/glue the graph back together.

I think that will make reviewing much easier :)

Like maybe trace the trip phi, and some other imaginary/memory data phi.
The rewiring here looks tricky, and I'll have to spend some time looking at it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22629#discussion_r1878197648