[lworld] [Rev 01] 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

Tobias Hartmann has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev
excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since
the last revision:

 - Merge branch 'lworld' into JDK-8246603
 - 8246603: [lworld] C2 does not scalarize inline types wrapped into non-escaping box objects

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

Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/71/files
  - new: https://git.openjdk.java.net/valhalla/pull/71/files/d6fcaa5a..f22cd1d4

Webrevs:
 - full: https://webrevs.openjdk.java.net/valhalla/71/webrev.01
 - incr: https://webrevs.openjdk.java.net/valhalla/71/webrev.00-01

  Stats: 396438 lines in 5079 files changed: 137186 ins; 239638 del; 19614 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