VectorMask.firstTrue()/lastTrue()

[Vladimir Ivanov]

> I would like to add intrinsic operations for counting leading and trailing zeros (in addition to popcount), then use those operations for implementing first/lastTrue.
> 
> firstTrue == count-leading-zeros
> 
> lastTrue == length - count-trailing-zeros - 1
> 
> The above should help answer your question about the return value. But it does make me wonder if the result for "not found" is non-obvious and we should at least clarify in the Javadoc. The method names likely do not help in this regard.

In order to rely on leading/trailing zeroes, you have to convert the 
mask in canonical form (bit per element) first.

There's already VectorMask.toLong() present which has to be intrinsified 
first. Then you can rely on existing intrinsics 
Long.numberOfTrailingZeros()/numberOfTrailingZeros().

(Also, VectorMask.toLong() is not SVE-friendly where a predicate 
register can hold a value up to 256-bit in size.)

Best regards,
Vladimir Ivanov

>> On Apr 14, 2021, at 5:35 AM, Remi Forax <forax at univ-mlv.fr> wrote:
>>
>> There is a way to implement firstTrue and lastTrue using only vector operations.
>>
>> The idea is to use to mask iota with the mask and then do a reduceLanes with MIN or MAX.
>> By example to for firstTrue() that returns -1 if the mask is empty,
>> first create iotaPlusOne, blend with zero, then reduceLanes with MIN, then substract 1
>>
>>   VectorMask mask = ...
>>   var iotaPlusOne = ByteVector.broadcast(SPECIES, 1).addIndex(1);
>>   ByteVector.zero(SPECIES).blend(iotaPlusOne, mask).reduceLanes(VectorOperators.MIN) - 1;
>>
>> BTW, i wonder why firstTrue return the mask length instead of -1.
>>
>> regards,
>> Rémi
>