[vector api] Operations on Mask-s

[Ningsheng Jian (Arm Technology China)]

Hi,

> -----Original Message-----
> From: panama-dev <panama-dev-bounces at openjdk.java.net> On Behalf Of
> Vladimir Ivanov
> Sent: Friday, January 4, 2019 9:02 AM
> To: Tomasz Kowalczewski <tomasz.kowalczewski at gmail.com>; panama-
> dev at openjdk.java.net
> Subject: Re: [vector api] Operations on Mask-s
>
> Tomasz,
>
> Yes, I had similar experience with complex use cases involving masks.
> The API works well for lane-wise cases, but falls apart when cross-lane
> operations are needed, requiring user to go through intermediate
> representations which aren't optimized well.
>
> There are different ways to fix that gap on API level, e.g.:
>    * declare more operations on Mask
>    * operate on long: Mask <=> long
>    * operate on Vector: Mask <=> Vector or Mask <: BoolVector <: Vector
>
> But IMO the main limiting factor to proceed is implementation considerations.
>
> On x86 there are 2 ways to represent masks: (1) as high bits in vector registers
> (pre-AVX512) and (2) as opmask registers (k0-k7). (I believe it's similar on ARM
> with NEON and SVE.)
>

Yes, and since SVE has fully predicate (mask) support, we would like to implement it in the mask register way, but need to keep compatible with non-mask Arches.

> C2 doesn't have full AVX512 support yet (e.g., no opmask register support in
> register allocator) and it was decided to focus on pre-AVX512 model first. So,
> current implementation represents masks as vectors uniformly across pre-
> AVX512 and AVX512-capable CPUs. And that's probably the main reason why
> Mask hasn't got enough attention yet.
>

Do you (or anyone else) have any plan to work on opmask register support in register allocator?

Thanks,
Ningsheng
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