Reduced MaxVectorSize and vector type initialization

[Vladimir Kozlov]

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 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/
>>
>> Feedback appreciated,
>>
>> Regards,
>> Nils Eliasson
>>
>>
>>
>>
>>
>> http://cr.openjdk.java.net/~neliasso/maxvectorsize/webrev/
>>
>