Intel AMX and feature detection

[Vladimir Ivanov]

> 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 ...

HotSpot JVM already does that: by default, AVX512 is not used on Skylake 
CPUs [1] even though they support AVX512F et all  (unless user 
explicitly specifies -XX:UseAVX=3). Maybe the JVM should be more 
aggressive, hard to say. (It'll affect Cascade Lake.) The choice is not 
specific to Vector API, but affects the whole JVM (in particular, VM 
intrinsics and C2 auto-vectorizer).

Best regards,
Vladimir Ivanov

[1] 
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/vm_version_x86.cpp#L1005

> -----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.