hotspot-compiler-dev Digest, Vol 145, Issue 95

[John Rose]

On Jul 10, 2019, at 7:25 AM, Roland Westrelin <rwestrel at redhat.com> wrote:
> 
> Some optimizations
> also have to be tuned down because the type system is not trusted. For
> instance, it happens that c2 IR contains cast node to Object eventhough
> there are obviously not needed. The primary motivation for the change in
> the way interfaces are handled is to remove work arounds, make sure we
> need no new ones, and allow C2 to trust its own type system. So it's a
> lot more about stability, maintainability than performance.

A principled fix to C2 would be to ignore all interface information, detuning
(known, loaded) interfaces to java.lang.Object.

(Detuning unknown types to Object would also be a good move.)

All the workarounds you speak of are really attempts to make interfaces
work-alikes for Object.  But that has always been a game of whack-a-mole.
I’m sad (but not surprised) that there are still moles to whack there.

When we wrote C2 in the early days of HotSpot the meaning of interfaces
in the JVM verifier was widely misunderstood.  Folks thought “Oh, there’s
a named type here so it must mean something; I’ll just patch around my
local problem since it’s probably just me.”  But it’s not me, it’s them.

— John