Reduced MaxVectorSize and vector type initialization

[Nils Eliasson]

Deans suggestion with making the TypeVect initialization unconditional 
also removes all platform dependencies on type.cpp:

http://cr.openjdk.java.net/~neliasso/maxvectorsize/webrev.02/

Regards,
Nils

On 2017-10-25 00:02, Vladimir Kozlov 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 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/
>>>
>>