[lworld] RFR: 8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph [v2]

[Damon Fenacci]

> # Issue
> A few test failed intermittently with -Xcomp on Mac due to an unschedulable graph in C2:
> - valhalla/valuetypes/WeakReferenceTest.java
> - valhalla/valuetypes/ProxyTest.java
> - valhalla/valuetypes/ObjectNewInstance.java
> - valhalla/valuetypes/ObjectMethodsViaCondy.java
> 
> # Causes
> 
> The origin of the issue seems to be coming from strength reduction (`process_late_inline_calls_no_inline`) where we replace virtual and MH calls with direct calls. 
> https://github.com/dafedafe/valhalla/blob/75e2dd95df5d847d7d6e35a23016d22705681cf4/src/hotspot/share/opto/compile.cpp#L3072
> If the return type of the methods are not loaded, we add a call to runtime's `store_inline_type_fields_to_buf` right after the actual method call, to save the scalarized return into a oop. This happens first for the caller at parse time and then for the callee when strength-reducing the virtual call to a direct one. The return projections of the inline fields of the call are added to `store_inline_type_fields_to_buf` and its oop projection is then added as input to the other `store_inline_type_fields_to_buf` which fundamentally leaves the graph around it in an awkward state.
> 
> If this happens in combination with loop unswitching it can lead to a graph not being schedulable, which is what happens in the failure of this issue, where we have:
> * a virtual call with a following `store_inline_type_fields_to_buf` (1) in a loop.
> * the loop undergoes unswitching, which creates 2 copies of the body (including a copy of the virtual call). All outputs of the new virtual call are phi-d with the one of the other path as input of the `store_inline_type_fields_to_buf`.
> * the new copy of the virtual call is later replaced with a direct call: the creation of the new direct call adds a `store_inline_type_fields_to_buf` (2) right after the direct call. All the inline type return values of the call being inlined are removed, so the phis now only have one input and are removed by a later GVN pass.
> <img width="600" alt="Screenshot 2025-11-26 at 10 33 51" src="https://github.com/user-attachments/assets/35e947c2-350e-414f-9d44-72559d480a88" />
> 
> * this creates an issue later during GCM since the `store_inline_type_fields_to_buf` (1) call is logically not dominated by any of the 2 sides of the unswitched loop and lands in a separate dominance path of the arguments whose phis have been removed (which are still dominated by the original virtual call).
> 
> # Solution
> 
> The issue happens only when strength-reducing to a direct ca...

Damon Fenacci has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8369045: remove strength reduction flag

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1768/files
  - new: https://git.openjdk.org/valhalla/pull/1768/files/76f2e96f..ccf58125

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1768&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1768&range=00-01

  Stats: 9 lines in 3 files changed: 0 ins; 6 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/1768.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1768/head:pull/1768

PR: https://git.openjdk.org/valhalla/pull/1768