RFR: 8331764: C2 SuperWord: refactor _align_to_ref/_mem_ref_for_main_loop_alignment

[Vladimir Kozlov]

On Tue, 7 May 2024 09:26:11 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> This PR accomplishes these things:
> - Rename `_align_to_ref` -> `_mem_ref_for_main_loop_alignment`.
> - Move the `mem_ref` finding for alignment out of `SuperWord::find_adjacent_refs`. This is too early, and we don't even know if the relevant `mem_ref` is going to be vectorized. It makes more sense to pick a `mem_ref` directly in `SuperWord::adjust_pre_loop_limit_to_align_main_loop_vectors`, where we already know what packs are going to be vectorized.
> - For the alignment width (aw), we can use the `vector_width` of the pack to which the `mem_ref` belongs, rather than the potentially much larger `vector_width_in_bytes`. I track this with `_aw_for_main_loop_alignment` now.
> 
> I need this for https://github.com/openjdk/jdk/pull/18822, and decided to split it out into an independent change.

src/hotspot/share/opto/superword.cpp line 3407:

> 3405:     if (first == nullptr) { continue; }
> 3406: 
> 3407:     int vw = first->memory_size() * pack->size();

I assume `first` is verified already and `first->memory_size()` is reasonable (size of primitive type).

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PR Review Comment: https://git.openjdk.org/jdk/pull/19115#discussion_r1592872102