performance surprise with Object.hashCode()

[Andy Nuss]

Can you guys tell me, does a pure Object instance even have a vtable?  Is there any reason why hotspot is special-casing the object hashCode() function thru "instrinsics", when instead it could make hashCode() non-native, and forward by default to the equivalent of System.identityHashCode()?



________________________________
 From: Aleksey Shipilev <aleksey.shipilev at oracle.com>
To: Vitaly Davidovich <vitalyd at gmail.com> 
Cc: hotspot compiler <hotspot-compiler-dev at openjdk.java.net>; Andy Nuss <andrew_nuss at yahoo.com> 
Sent: Monday, May 13, 2013 1:04 PM
Subject: Re: performance surprise with Object.hashCode()
 

On 05/14/2013 12:00 AM, Vitaly Davidovich wrote:
> I'm comparing i_i vs o_i difference when intrinsic is enabled/disabled. 
> Based on your results, when intrinsic is disabled the o_i case is
> marginally slower than i_i, whereas it's noticeably slower otherwise.

Ah yes. That's because i_i is perfectly devirtualized without any
typechecks (re-reading my original note, there is mention of typecheck,
disregard that, see the hot loop in assembly, it is perfect). o_i
without intrinsic has the typecheck nevertheless, and we are wasting
some of the time there. o_i with intrinsic seems to lose the
devirtualization completely.

-Aleksey.
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