RFR(M): 8141551: C2 can not handle returns with incompatible interface arrays

[Volker Simonis]

Hi John, Andrew,

thanks for the nice references and history behind the implementation
(altough two of the bugs are not visible :(

But the link to 8028165 was especially helpful. I think it describes a
similar problem. I've just managed to reproduce it and I'm now testing
if my fix also helps there. While doing the tests I've also discovered
a problem with my fix which I've addressed in this new version of the
webrev:

http://cr.openjdk.java.net/~simonis/webrevs/2015/8141551.v2/

The problem was that I casted the class-array to an interface instead
of casting it to an array of interface in Parse::return_current()

-        value = _gvn.transform(new CheckCastPPNode(0,value,tr));
+          value = _gvn.transform(new CheckCastPPNode(0, value,
phi->bottom_type()));

And I've also put the whole new part which handles oop-arrays vs.
arrays-of-interface into an else branch because it is mutually
exclusive with the existing part which handles returning oops to an
interface-return.

Regards,
Volker


On Thu, Nov 12, 2015 at 11:27 AM, Andrew Dinn <adinn at redhat.com> wrote:
> On 12/11/15 01:33, John Rose wrote:
>> Ughh, that again.  This is 15-year-old technical debt in the C2 type system.
>> It has almost earned VP-level recognition as an organizational cost center.
>>
>> Here is some of the trail of tears:
>> https://bugs.openjdk.java.net/browse/JDK-6312651
>> https://bugs.openjdk.java.net/browse/JDK-4641534
>> https://bugs.openjdk.java.net/browse/JDK-6837094
>> https://bugs.openjdk.java.net/browse/JDK-8028165?focusedCommentId=13614504&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13614504
>
> A shame that the first two links seem not to be traversable (from
> outside of Oracle at least). The final link is well worth a visit, not
> just to the referenced comment but also to the opener -- which is not
> afraid to name names.
>
> I think I resolved the references correctly as follows:
>
> Davey/Priestley to
>
>
> http://www.cambridge.org/gb/academic/subjects/mathematics/algebra/introduction-lattices-and-order-2nd-edition
>
> and Nielson/Nielson/Hankin to
>
>   http://www.springer.com/us/book/9783540654100
>
> Hecht to
>
>   http://dl.acm.org/citation.cfm?id=540175
>
> The first one looks like it is just basic lattice theory whereas the
> second one looks to apply that theory to the compilation task and seems
> to be highly relevant to what C2 does (1999 pub date :-). The third text
> also seems to be about applications of lattices to compiler analysis but
> dates back to the 70s. I assume it is one of those 'seminal works'. A
> reference to the 2nd text in type.hpp would probably have helped me a
> lot about 2 years ago.
>
> A Happy Christmas (or alter/non-denominational festive vacation) to all
> our readers.
>
> regards,
>
>
> Andrew Dinn
> -----------
>