[10] RFR (M): 6986483: CHA: optimize calls through interfaces

[Paul Sandoz]

Hi,

This is a nice surprise :-)

I happened to be separately looking at this area, analysing the performance of interfaces/abstract classes to apply a certain implementation technique to avoid going megamorphic.

I concluded that the implementation technique was not viable, but with your patch i think i can revive it and perform more detailed analysis.

Based on my benchmark the performance improvement looks good.

Thanks!
Paul.

> On 14 Feb 2017, at 10:51, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> http://cr.openjdk.java.net/~vlivanov/6986483/webrev.00/
> https://bugs.openjdk.java.net/browse/JDK-6986483
> 
> Proposed change adds CHA support in C2 for interface calls.
> 
> Consider the following hierarchy:
> 
>  interface Intf { m(); }
>  class C implements Intf { public m() { ... } }
>  class C1 extends C { /* doesn't override m() */ }
>  ...
>  class Cn extends C { /* doesn't override m() */ }
> 
> Call site: invokeinterface Intf.m() ...
> 
> If Intf were an abstract class, CHA could deduce that Intf::m() can be replaced with C::m(), but it doesn't work for interfaces. Verifier doesn't check interface types in bytecode, so CHA can't assume the receiver implements Intf.
> 
> CHA in C1 handles such call sites for interfaces with a single implementor. It replaces invokeinterface Intf.m() with invokevirtual C.m() guarded by a subtype check (instanceof C). C2 doesn't do that and this request is about adding that. Type profiling doesn't help here (the call site is usually megamorphic), so C2 can't inline it.
> 
> The proposed implementation is similar to C1, except that the code deoptimizes when subtype check fails and ICCE is thrown from the interpreter.
> 
> While working on it, I spotted and fixed a couple of inefficiencies in C1 implementation:
> 
>  (1) dependency context being used was broader than necessary - resolved instead of declared interface (hence, possibility of unnecessary invalidations);
> 
>  (2) didn't work for interfaces w/ any default methods: CHA doesn't support default methods at the moment, so what matters is whether Intf::m() is default or not and not whether Intf has *any* concrete methods.
> 
> Testing: unit tests on CHA, JPRT, RBT (hs-comp-tier0, in progress), LogCompilation tool.
> 
> Thanks!
> 
> Best regards,
> Vladimir Ivanov

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