gh-123358: Use `_PyStackRef` in `LOAD_DEREF` (gh-130064)

Concurrent accesses from multiple threads to the same `cell` object did not
scale well in the free-threaded build. Use `_PyStackRef` and optimistically
avoid locking to improve scaling.

With the locks around cell reads gone, some of the free threading tests were
prone to starvation: the readers were able to run in a tight loop and the
writer threads weren't scheduled frequently enough to make timely progress.
Adjust the tests to avoid this.

Co-authored-by: Donghee Na <donghee.na@python.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h
index 493d99d..f6a538e 100644
--- a/Python/generated_cases.c.h
+++ b/Python/generated_cases.c.h
@@ -8822,14 +8822,18 @@
             INSTRUCTION_STATS(LOAD_DEREF);
             _PyStackRef value;
             PyCellObject *cell = (PyCellObject *)PyStackRef_AsPyObjectBorrow(GETLOCAL(oparg));
-            PyObject *value_o = PyCell_GetRef(cell);
-            if (value_o == NULL) {
+            _PyFrame_SetStackPointer(frame, stack_pointer);
+            value = _PyCell_GetStackRef(cell);
+            stack_pointer = _PyFrame_GetStackPointer(frame);
+            if (PyStackRef_IsNull(value)) {
+                stack_pointer[0] = value;
+                stack_pointer += 1;
+                assert(WITHIN_STACK_BOUNDS());
                 _PyFrame_SetStackPointer(frame, stack_pointer);
                 _PyEval_FormatExcUnbound(tstate, _PyFrame_GetCode(frame), oparg);
                 stack_pointer = _PyFrame_GetStackPointer(frame);
                 JUMP_TO_LABEL(error);
             }
-            value = PyStackRef_FromPyObjectSteal(value_o);
             stack_pointer[0] = value;
             stack_pointer += 1;
             assert(WITHIN_STACK_BOUNDS());