[vectorapi] Shape of the preferred species with UseAVX=0

[Quân Anh Mai]

Hi,

In x86 there are 2 vector extension generations, SSE and AVX, and the flags
controlling these are UseSSE and UseAVX, respectively. By setting UseAVX to
0, you are emulating an SSE4 machine, which has 16-byte vector registers.
64-bit VM requires SSE2, so you cannot set a value lower than 2 for UseSSE,
and for all such values there are always 16-byte vector registers
available. As a result, the minimum preferred vector size on x86_64 is 16
bytes.

Regards,
Quan Anh

On Wed, 24 May 2023 at 19:39, Chris Hegarty <chegar999 at gmail.com> wrote:

> Hi,
>
> Over in Lucene-land we're experimenting with the Vector API, and ran
> into an issue when testing against different environments. We're
> effectively simulating different environments with Hotspot's command
> line flags, and see an issue with `-XX:UseAVX=0`. With this option we
> expect the shape of the preferred species to fallback to 64 bits, but it
> is actually 128.
>
> Trivial reproducer that just prints the preferred int species:
>
> $ java -version
> openjdk version "21-ea" 2023-09-19
> OpenJDK Runtime Environment (build 21-ea+23-1988)
> OpenJDK 64-Bit Server VM (build 21-ea+23-1988, mixed mode, sharing)
>
> $ cat PrintPreferredVectorSize.java
> import jdk.incubator.vector.*;
>
> public class PrintPreferredVectorSize {
>
>      public static void main(String... args) {
>          System.out.println(IntVector.SPECIES_PREFERRED);
>      }
> }
>
> // Start with the default - no flags, then downsize with MaxVectorSize.
> // All looks good.
> $ java --add-modules jdk.incubator.vector PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 16, S_512_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:MaxVectorSize=64
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 16, S_512_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:MaxVectorSize=32
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 8, S_256_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:MaxVectorSize=16
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 4, S_128_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:MaxVectorSize=8
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 2, S_64_BIT]
>
> ^^^ this all is as expected ^^^
>
> Now do similar(ish) with UseAVX
>
> $ java --add-modules jdk.incubator.vector -XX:UseAVX=3
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 16, S_512_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:UseAVX=2
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 8, S_256_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:UseAVX=1
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 4, S_128_BIT]
>
> $ java --add-modules jdk.incubator.vector -XX:UseAVX=0
> PrintPreferredVectorSize
> WARNING: Using incubator modules: jdk.incubator.vector
> Species[int, 4, S_128_BIT]
>
> It is this last one that is surprising to us. We expect similar
> -XX:MaxVectorSize=8, which is Species[int, 2, S_64_BIT]
>
> Firstly, is this a bug? Is our expectation correct? If not, then I'm
> failing to understand something.
>
> If I'm not mistaken, then I can see that the
> Matcher::vector_width_in_bytes assumes a minimum of 16 regardless of
> whether UseAVX is < 1. Trivially, and as a hack, I just retrofitted
> vector_width_in_bytes to return 0 if UseAVX < 1. And we get
> Species[int, 2, S_64_BIT].
>
> The layer of these flags is not straightforward, so I won't pretend that
> my hack is the right way to fix this, but I just wanted to ensure that I
> was looking in the correct general area.
>
> Thanks,
> -Chris.
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/panama-dev/attachments/20230524/b4438e7e/attachment.htm>