RFR(M): 8223029: [lworld] C2 support for widening/narrowing conversion "[QFoo;" <: "[LFoo;”

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8223029
http://cr.openjdk.java.net/~thartmann/8223029/webrev.00/

This is based on Mr. Simms' patch for 8223017 [1].

I've changed the type system (ciTypeFlow.cpp and type.cpp) to not fall back to [Object when meeting
[V and [V? because [V <: [V? allows conversion between the two. Whenever we access a [V?, we now
need to check if the actual type is a (potentially flattened) [V. I've changed the array_load and
array_store implementation in parse2.cpp accordingly. In addition, I disabled folding of klass loads
for [V? in memnode.cpp because the actual type might be [V.

I've noticed that GraphKit::gen_value_type_array_guard() currently always deoptimizes when storing
null to a [V? because we only check if the array element is a value type but not if the array is
really null-free. I've fixed that and added a corresponding test (TestArrayAccessDeopt.java).

I've also noticed that sometimes we were only checking ValueArrayFlatten where we should really also
check vk->flatten_array() if the klass is known. I've fixed that as well and added a test
(TestFlatArrayThreshold.java)

I've added lots of tests to TestNullableArrays to verify correctness of widening and narrowing
conversions.

Changes to other files are refactoring.

Thanks,
Tobias

[1] http://cr.openjdk.java.net/~dsimms/valhalla/8223017/