RFR: 8326139: C2 SuperWord: split packs (match use/def packs, implemented, mutual independence) [v2]

[Emanuel Peter]

On Mon, 26 Feb 2024 18:10:57 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> src/hotspot/share/opto/superword.cpp line 969:
>> 
>>> 967:   }
>>> 968: 
>>> 969:   if (isomorphic(s1, s2) && !is_populate_index(s1, s2)) {
>> 
>> Note: As explained in `test7a`:
>> If we pack the `iv + 1`, `iv + 2`, `iv + 3` ... `AddI` nodes, then we get a pack that is non-power-of-2 in most cases, since `iv + 0` has collapsed to `iv`, and is not an `AddI` but a `Phi` node. That then causes `split_packs_at_use_def_boundaries` to cut the use-packs, and things unravel from there.
>> 
>> So I had 2 options:
>> 1. Create `Phi, AddI, AddI, AddI, ...` packs, and deal with the consequences of having nodes of different types in a pack. Plus, we would have the `iv` in a pack, which also has consequences all over the place... Not great.
>> 2. Forbid anything that looks like a `PopulateIndex` to be packed. It does not need to be packed, and will still be vectorized, see `SuperWord::vector_opd`. I went with this option.
>
> Agree with option 2.

Great :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/17848#discussion_r1503806812