On Tue, 2 Dec 2025 10:36:04 GMT, Damon Fenacci <[email protected]> wrote:
> # 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... This pull request has now been integrated. Changeset: 4ffaf147 Author: Damon Fenacci <[email protected]> URL: https://git.openjdk.org/valhalla/commit/4ffaf1477da5e8236cf13a45bff0a760bcf0cea9 Stats: 73 lines in 6 files changed: 27 ins; 11 del; 35 mod 8369045: [lworld] valhalla/valuetypes/WeakReferenceTest.java has an unschedulable graph Reviewed-by: thartmann ------------- PR: https://git.openjdk.org/valhalla/pull/1768
