RFR: 8338023: Support two vector selectFrom API [v13]

[Sandhya Viswanathan]

On Mon, 30 Sep 2024 21:28:22 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ByteVector.java line 551:
>> 
>>> 549:         return ((ByteVector)src1).vectorFactory(res);
>>> 550:     }
>>> 551: 
>> 
>> This could instead be:
>>    src1.rearrange(this.lanewise(VectorOperators.AND, 2 * VLENGTH - 1).toShuffle(), src2);
>> Or even simplified to:
>>    src1.rearrange(this.toShuffle(), src2);
>
> I think you have to do the masking before conversion - `vec.lanewise(VectorOperators.AND, 2 * VLENGTH - 1).toShuffle()` is not the same as `vec.toShuffle()` for all inputs.
> 
> 
> jshell> IntVector indexes = IntVector.fromArray(IntVector.SPECIES_256, new int[] {0, 1, 8, 9, 16, 17, 24, 25}, 0);
> indexes ==> [0, 1, 8, 9, 16, 17, 24, 25]
> 
> jshell> indexes.lanewise(VectorOperators.AND, indexes.length() * 2 - 1)
> $19 ==> [0, 1, 8, 9, 0, 1, 8, 9]
> 
> jshell> indexes.lanewise(VectorOperators.AND, indexes.length() * 2 - 1).toShuffle()
> $20 ==> Shuffle[0, 1, -8, -7, 0, 1, -8, -7]
> 
> jshell> indexes.toShuffle()
> $21 ==> Shuffle[0, 1, -8, -7, -8, -7, -8, -7]

Thanks for the example. Yes, you have a point there.  So we would need to do:
   src1.rearrange(this.lanewise(VectorOperators.AND, 2 * VLENGTH - 1).toShuffle(), src2);

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

PR Review Comment: https://git.openjdk.org/jdk/pull/20508#discussion_r1781859166