Intel AMX and feature detection

[Viswanathan, Sandhya]

It is possible do the “down bit’ing” today by setting -XX:MaxVectorSize=32 on JVM command line. This sets the preferred species to 256-bit vector size with AVX-512 ISA.  Would that work by any chance? Or I guess that is not what we want ...

Best Regards,
Sandhya

-----Original Message-----
From: panama-dev <panama-dev-retn at openjdk.org> On Behalf Of Paul Sandoz
Sent: Thursday, June 27, 2024 7:59 AM
To: John Rose <john.r.rose at oracle.com>
Cc: Uwe Schindler <uschindler at apache.org>; panama-dev at openjdk.org
Subject: Re: Intel AMX and feature detection



> On Jun 26, 2024, at 3:55 PM, John Rose <john.r.rose at oracle.com> wrote:
> 
> Actually we have VectorShape.S_64_BIT which, if it is the preferred 
> shape, is really telling you the “vector” processing is inside the CPU 
> not the VPU.
> That’s a good-enough hint to avoid the Vector API, right?

Yes, I think that is a good proxy for lack of any compiler support.

I would like to hear opinions from the Intel’s folks on “down bit’ing” from 512 to 256 on AVX-512 without VBMI2. It seems pragmatic. What about the auto-vectorizer? We also have HotSpot flags to say uses AVX2 on an AVX-512 machine as a workaround. If we do this perhaps we need a flag to override the “down bit’ing”.

I will log issues for both.

Paul.