Using the Vector API to access SIMD instructions

[Raffaello Giulietti]

On 2021-10-08 18:50, Remi Forax wrote:
> ----- Original Message -----
>> From: "raffaello giulietti" <raffaello.giulietti at gmail.com>
>> To: "panama-dev at openjdk.java.net'" <panama-dev at openjdk.java.net>
>> Sent: Vendredi 8 Octobre 2021 17:16:41
>> Subject: Using the Vector API to access SIMD instructions
> 
>> Hello,
>>
>> I'm implementing two decimal floating-point formats, as defined by the
>> IEEE 754 spec. I'm anticipating that they will become primitive classes
>> (JEP 401) if I can make them perform reasonably well.
>>
>> In Decimal128 I find myself coding something like
>>
>>      int c0, c1, c2, c3;
>>      long m;
>>      int f, ph;
>>
>>      ...
>>
>>      int d0 = (int) ((m * c0) >>> f);
>>      int d1 = (int) ((m * c1) >>> f);
>>      int d2 = (int) ((m * c2) >>> f);
>>      int d3 = (int) ((m * c3) >>> f);
>>
>>      int e0 = c0 - ph * d0;
>>      int e1 = c1 - ph * d1;
>>      int e2 = c2 - ph * d2;
>>      int e3 = c3 - ph * d3;
>>
>> I would like to code these repetitive 4 + 4 lines as SIMD operations
>> using the Vector API.
>>
>> However, it seems to me that I would have to re-code them 3 times,
>> depending on the preferred size of the underlying SIMD registers and
>> hoping that the preferred size can be constant folded and dead code be
>> eliminated by C2.
> 
> You can use IntVector.SPECIES_128
>    https://docs.oracle.com/en/java/javase/17/docs/api/jdk.incubator.vector/jdk/incubator/vector/IntVector.html#SPECIES_128
> and write the code once and run it everywhere :)
> 

This is less than optimal if the platform supports larger registers, I 
guess. For example, even my old laptop supports 256 bit YMM registers.

And there seems to be no way to "load/unload" a Vector without passing 
through an array or a ByteBuffer.

In the end I think that the Vector API is design more in supporting 
vector operation on large arrays than on small fixed size data as here. 
My attempts to vectorize the above scalar code end up being rather ugly 
and probably not worth the possible performance enhancements.


Greetings
Raffaello