[lworld] RFR: 8246603: [lworld] C2 does not scalarize inline types wrapped into non-escaping box objects

[Tobias Hartmann]

C2 fails to scalarize inline types wrapped into non-inline, non-escaping (box) objects.

For example, in TestLWorld::test109, C2 successfully scalar replaces the InterfaceBox object but not the LongWrapper
object it contains because of the complex control flow in LongWrapper::wrap. However, since LongWrapper is an inline
type, we don't need to rely on Escape Analysis to be able to scalar replace. Now the problem is that LongWrapper is
stored as oop in a field of type WrapperInterface and we don't keep track of the ValueType(Ptr)Node long enough (i.e.
until after EA) for the load to be removed and the buffer allocation to go away.

The fix contains the following changes:
- Use ValueTypePtrNode instead of the oop whenever possible to keep track of field values. PhiNode::Ideal will then push
  such ValueTypePtrNode down and LoadNode::Identity will fold the loads.
- Keep ValueTypePtrNodes such that we can still fold loads after EA removed potential non-inline, wrapper objects that
  prevented scalarization during parsing. Only remove them after EA is done.
- Piggy-backing on PhaseMacroExpand::eliminate_allocate_node to eliminate unused inline type allocations and removed
  Allocate::Ideal which is not needed anymore (it also did not remove allocations that still had initializing stores).
- Added code to remove the membar added after inline type allocation (it will otherwise block loop opts).
- Make sure phis are always split if all inputs are mergemems to remove useless memory merges that block optimizations
  (see JDK-8247216)
- Added regression tests and a benchmark (provided by Maurizio)

Performance without fix:

Benchmark                    Mode  Cnt  Score   Error  Units
TestBoxing.pojo_loop         avgt   30  4.699 ± 0.045  ms/op
TestBoxing.box_generic_loop  avgt   30  4.540 ± 0.058  ms/op
TestBoxing.box_inline_loop   avgt   30  0.527 ± 0.009  ms/op
TestBoxing.box_intf_loop     avgt   30  4.512 ± 0.050  ms/op
TestBoxing.box_ref_loop      avgt   30  4.551 ± 0.037  ms/op
TestBoxing.inline_loop       avgt   30  0.524 ± 0.013  ms/op

Performance with fix:

Benchmark                    Mode  Cnt  Score   Error  Units
TestBoxing.pojo_loop         avgt   30  4.818 ± 0.166  ms/op
TestBoxing.box_generic_loop  avgt   30  0.517 ± 0.007  ms/op
TestBoxing.box_inline_loop   avgt   30  0.513 ± 0.007  ms/op
TestBoxing.box_intf_loop     avgt   30  0.523 ± 0.024  ms/op
TestBoxing.box_ref_loop      avgt   30  0.511 ± 0.010  ms/op
TestBoxing.inline_loop       avgt   30  0.514 ± 0.012  ms/op

-------------

Commit messages:
 - 8246603: [lworld] C2 does not scalarize inline types wrapped into non-escaping box objects

Changes: https://git.openjdk.java.net/valhalla/pull/71/files
 Webrev: https://webrevs.openjdk.java.net/valhalla/71/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246603
  Stats: 587 lines in 19 files changed: 432 ins; 91 del; 64 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/71/head:pull/71

PR: https://git.openjdk.java.net/valhalla/pull/71