RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v15]

[Andrew Haley]

On Wed, 16 Jul 2025 08:35:38 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

>> But why would the Neon implementation fail if UseSVE ==1? Surely it would still work, and if it still works this comment is wrong.
>
> @theRealAph Thanks for your comments. The Neon implementation would not fail if UseSVE == 1 (Does the comment imply something like this?). Only that we are making a choice of generating Neon instructions for UseSVE = 1 and vec_len = 16. 
> 
> The conditions that can reach this method are - UseSVE = 0, 1 with vec_len = 8 or 16 and UseSVE = 2 with any vec_len (based on the conditions in `Matcher::match_rule_supported_vector()`). We have already filtered out `UseSVE = 1` with `vec_len = 8` and `UseSVE = 2` at line #2904. So if the control reaches #2915 then it's either `UseSVE = 0` with any vec_len and `UseSVE = 1` with `vec_len = 16` and that's what the comment mentions.

Then that's what your comment should say. It does not: it says "conditions must satisfy" which implies that this is something the following code needs for correct operation.

The language in your reply here is fine. Say that instead of "one of these conditions must satisfy".

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2210089819