Casting reference array to any-T array.

Vitaly Davidovich vitalyd at
Thu Jan 8 00:35:25 UTC 2015

Problem is specialization is javac-time, VM not involved.

Sent from my phone
On Jan 7, 2015 7:33 PM, "Michael Barker" <mikeb01 at> wrote:

> Yes, but with a slight step further.  As you point out specialising
> everything will lead to bloating the number of classes.  I was thinking
> about Hotspot specialising some combinations of generic classes and
> specific reference types based on some heuristic/profiling information.
> On 8 January 2015 at 11:32, Vitaly Davidovich <vitalyd at> wrote:
>> Ah, you're talking about specialized classes as a whole (I was referring
>> to just the arrays aspect).  Yes, if it were to specialize every single
>> type, then you'd get better type information.  Downside is you now explode
>> the number of method definitions in the runtime.  In .NET, for example,
>> generic methods are not specialized for reference types, in part for this
>> reason I believe.  Generally speaking, the downside to creating distinct
>> structures per type is the explosion in the number of types at runtime.  I
>> encourage you to read this oldish blog post by Joe Duffy (MSFT engineer):
>> On Wed, Jan 7, 2015 at 5:23 PM, Michael Barker <mikeb01 at> wrote:
>>> My understand is that it does type profiling at the callsite and
>>> something like HashMap.hash() will encounter such wide variety of classes
>>> that it will rarely be anything other than fully mega-morphic.  My guess
>>> was that if there was specialised class for a specific reference type then
>>> this could become mono-morphic.
>>> On 8 January 2015 at 11:12, Vitaly Davidovich <vitalyd at> wrote:
>>>> Right, but the reason I'm doubtful that this will have any impact is
>>>> because the JIT already does type profiling, and the runtime types it sees
>>>> (and the statistics around that) won't change due to erasure.  My "make its
>>>> life easier" comment was a guess that perhaps some code paths in the
>>>> optimizer don't need to be taken (e.g. don't look at profiling info if it
>>>> now knows statically that an array is composed of final classes).
>>>> On Wed, Jan 7, 2015 at 5:09 PM, Michael Barker <mikeb01 at>
>>>> wrote:
>>>>> The current functionality continues with the erasure plan.  However, I
>>>>>>> wouldn't mind doing better!
>>>>>> Yeah, I can't immediately think of a critical reason why it can't
>>>>>> stay erased.  For JIT optimizer, having a narrower upper bound on the type
>>>>>> may make its life easier, although I don't know if it'll have any material
>>>>>> difference.  The one question is what reflection will do (and any code
>>>>>> based on reflection, such as custom serialization, code generation, etc):
>>>>> (Caveat, I'm not a compiler expert so this is a bit of a guess.)
>>>>> One possible place where this could be used with within the
>>>>> optimiser.  E.g. if Hotspot could see a specialised HashMap<String, String>
>>>>> instead of an erased one, then it could determine that calls to hashCode
>>>>> and equals would be mono-morphic and apply more aggressive in-lining.  This
>>>>> could lead to jump in performance across a broad ranges of apps (hands up
>>>>> who uses Strings and HashMaps :-).  My understand is that the mega-morhpic
>>>>> dispatch (of hashCode/equals) is one of the more significant costs within
>>>>> HashMap.
>>>>> If that was possible then it would be pretty cool!
>>>>> Mike.

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