RFR: 8338023: Support two vector selectFrom API [v10]

[Paul Sandoz]

On Tue, 17 Sep 2024 07:02:15 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/X-Vector.java.template line 561:
>> 
>>> 559:         for (int i = 0; i < vlen; i++) {
>>> 560:             int index = ((int)vecPayload1[i]);
>>> 561:             res[i] = index >= vlen ? vecPayload3[index & (vlen - 1)] : vecPayload2[index];
>> 
>> This is incorrect as the index could be negative. You need to wrap in the range `[0, 2 * vlen - 1]` before the comparison and selection.
>> 
>> int index = ((int)vecPayload1[i]) & ((vlen << 1) - 1));
>> res[i] = index < vlen ? vecPayload2[index] : vecPayload3[index - vlen];
>
> Hi @PaulSandoz , we already pass wrapped indexes to this helper routine called from fallback implementation.

Opps yes, the masking was throwing me off.

Can you please add a comment and/or rename the parameters e.g., so `v1` is renamed to `wrappedIndex`?
Also i would recommend not doing the masking, it is very misleading and instead do the subtraction.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20508#discussion_r1763582764