vcall wish for hotspot

Remi Forax forax at
Sun May 19 03:57:37 PDT 2013

On 05/19/2013 11:38 AM, Andrew Haley wrote:
> On 05/19/2013 05:37 AM, Andy Nuss wrote:
>> Well, I guess I don't know what I'm doing after all.
>> I ported my benchmark to C++, and found that C++ overhead for the
>> virtual call was 6 nanos, way slower than the 0.5 nanos I was
>> getting for hotspot.
>> Then I guessed, maybe Clemens is right, hotspot is seeing that Link1
>> and Link2 are very similar in structure, and even though the
>> iteration thru the list of base types would seem to force a virtual
>> call, hotspot is just turning it into an if-else, and inlining
>> everything else, and the extra 0.5 nanos is simply due to branch
>> mispredict.
>> So I create 10 variants of my Link class, Link1 thru Link10, and
>> hotspot was about the same as C++, 6 nanos.
>> But that could mean that hotspot was now using a vtable, or it could
>> mean hotspot was paying for 10 branch mispredictions.
>> So I tried 20 variants of my Link class, Link1 thru Link20, and
>> hotspot was now about 11 nanos.  So it would seem that hotspot, for
>> simple variants of a base class doesn't use a vtable at all, but
>> just creates one big if-else inline.  Whereas C++, even with just 2
>> variants of the class, always uses a vtable.
>> If this is true, that itself is very interesting.  Any ideas how to
>> write a test that really forces hotspot to use the vtable?
> If I were you I'd stop guessing, build a debug hotspot, and have a look
> at the generated code.  The is no substitute.

I agree :)
and a debug VM is not a requirement anymore

> Andrew.


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