Reduced MaxVectorSize and vector type initialization

[Vladimir Kozlov]

Thank you, Rohit

Do you plan to propose changes to increase vector size to 32 for 15h and 16h? Or AMD is fine with current settings?

Thanks,
Vladimir

On 10/25/17 9:48 PM, Rohit Arul Raj wrote:
> Hello Vladimir,
> 
> 
> Please find the requested details:
> 
> 
> AVX/AVX2 support availability on AMD Processors:
> 
> Family 14h and earlier – No AVX support
> 
> Family 15h -  (1^st -gen), (2nd-gen), (3rd-gen) AVX support available, max vector width is 32 bytes (we limit the vector 
> size to 16 bytes in openJDK).
> 
> Family 16h – AVX support available, max vector width is 32 bytes (we limit the vector size to 16 bytes in openJDK).
> 
> Family 15h -  (4^th -gen) AVX, AVX2 support available, max vector width is 32 bytes (we limit the vector size to 16 
> bytes in openJDK).
> 
> Family 17h – AVX, AVX2 support available, max vector width is 32 bytes (our proposed changes have vector size set to 32 
> bytes in openJDK).
> 
> AVX3 support is not available on AMD processors yet.
> 
> 
>  From the comments below, Dean's suggestions seems reasonable.
> 
> Regards,
> Rohit
> 
> 
> On Wed, Oct 25, 2017 at 3:32 AM, Vladimir Kozlov <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>> wrote:
> 
>     We can't use platform specific UseAVX flag in shared code in type.cpp.
> 
>     I would say we should not support AVX (set UseAVX to 0) on AMD < 17h.
>     And we need to ask AMD which AMD cpus are supporting AVX, AVX2, AVX3 and corresponding vectors 32 and 64 bytes.
>     If AMD's Instructions Set before 17h does not support whole 32 bytes vectors we can't call it AVX.
> 
>     Thanks,
>     Vladimir
> 
>     On 10/18/17 10:01 AM, dean.long at oracle.com <mailto:dean.long at oracle.com> wrote:
> 
>         How about initializing TypeVect::VECTY and friends unconditionally?  I am nervous about exchanging one guarding
>         condition for another.
> 
>         dl
> 
> 
>         On 10/18/17 1:03 AM, Nils Eliasson wrote:
> 
> 
>             HI,
> 
>             I ran into a problem with the interaction between MaxVectorSize and the UseAVX. For some AMD CPUs we limit
>             the vector size to 16 because it gives the best performance.
> 
>                 +    if (cpu_family() < 0x17 && MaxVectorSize > 16) {
>                 +      // Limit vectors size to 16 bytes on AMD cpus < 17h.
>                         FLAG_SET_DEFAULT(MaxVectorSize, 16);
>                       }
> 
> 
>             Whenf MaxVecorSize is set to 16 it has the sideeffect that the TypeVect::VECTY and mreg2type[Op_VecY] won't
>             be initalized even though the platform has the capability.
> 
>             Type.cpp:~660
> 
>             [...]
>              >   if (Matcher::vector_size_supported(T_FLOAT,8)) {
>              >     TypeVect::VECTY = TypeVect::make(T_FLOAT,8);
>              >   }
>             [...]
>              >   mreg2type[Op_VecY] = TypeVect::VECTY;
> 
> 
>             In the ad-files feature flags (UseAVX etc.) are used to control what rules should be matched if it has
>             effects on specific vector registers. Here we have a mismatch.
> 
>             On a platform that supports AVX2 but have MaxVectorSize limited to 16, the VM will fail when the
>             TypeVect::VECTY/mreg2type[Op_VecY] is uninitialized. We will also hit asserts in a few places like:
>             assert(Matcher::vector_size_supported(T_FLOAT,RegMask::SlotsPerVecY), "sanity");
> 
>             Shouldn't the type initialization in type.cpp be dependent on feature flag (UseAVX etc.) instead of
>             MaxVectorSize? (The type for the vector registers are initialized if the platform supports them, but they
>             might not be used if MaxVectorSize is limited.)
> 
>             This is a patch that solves the problem, but I have not convinced myself that it is the right way:
>             http://cr.openjdk.java.net/~neliasso/maxvectorsize/webrev/
>             <http://cr.openjdk.java.net/~neliasso/maxvectorsize/webrev/>
> 
>             Feedback appreciated,
> 
>             Regards,
>             Nils Eliasson
> 
> 
> 
> 
> 
>             http://cr.openjdk.java.net/~neliasso/maxvectorsize/webrev/
>             <http://cr.openjdk.java.net/~neliasso/maxvectorsize/webrev/>
> 
> 
>