Casting reference array to any-T array.

Brian Goetz brian.goetz at oracle.com
Thu Jan 8 02:12:02 UTC 2015


What makes you say that?  Right now, specialization is class-load time. 
  With some possible changes to the VM (to be revealed when more baked) 
we can push it later and get more sharing.

On 1/7/2015 7:35 PM, Vitaly Davidovich wrote:
> Problem is specialization is javac-time, VM not involved.
>
> Sent from my phone
>
> On Jan 7, 2015 7:33 PM, "Michael Barker" <mikeb01 at gmail.com
> <mailto:mikeb01 at gmail.com>> 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 gmail.com
>     <mailto:vitalyd at gmail.com>> 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):
>         http://joeduffyblog.com/2011/10/23/on-generics-and-some-of-the-associated-overheads/
>
>         On Wed, Jan 7, 2015 at 5:23 PM, Michael Barker
>         <mikeb01 at gmail.com <mailto:mikeb01 at gmail.com>> 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 gmail.com <mailto:vitalyd at gmail.com>> 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 gmail.com <mailto:mikeb01 at gmail.com>> 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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