bpo-40521: Make slice cache per-interpreter (GH-20637)

Each interpreter now has its own slice cache:

* Move slice cache into PyInterpreterState.
* Add tstate parameter to _PySlice_Fini().

Include/internal/pycore_interp.h

@@ -187,6 +187,10 @@ struct _is {
 #endif
     struct _Py_tuple_state tuple;
     struct _Py_float_state float_state;
+
+    /* Using a cache is very effective since typically only a single slice is
+       created and then deleted again. */
+    PySliceObject *slice_cache;
 };
 
 /* Used by _PyImport_Cleanup() */

Include/internal/pycore_pylifecycle.h

@@ -65,7 +65,7 @@ extern void _PyList_Fini(void);
 extern void _PySet_Fini(void);
 extern void _PyBytes_Fini(void);
 extern void _PyFloat_Fini(PyThreadState *tstate);
-extern void _PySlice_Fini(void);
+extern void _PySlice_Fini(PyThreadState *tstate);
 extern void _PyAsyncGen_Fini(void);
 
 extern void PyOS_FiniInterrupts(void);

Misc/NEWS.d/next/Core and Builtins/2020-05-20-01-17-34.bpo-40521.wvAehI.rst

@@ -1,2 +1,3 @@
-Tuple free lists, empty tuple singleton, and float free list are no longer
-shared by all interpreters: each interpreter now its own free lists.
+The tuple free lists, the empty tuple singleton, the float free list, and the
+slice cache are no longer shared by all interpreters: each interpreter now has
+its own free lists and caches.

Objects/sliceobject.c

@@ -15,7 +15,7 @@ this type and there is exactly one in existence.
 
 #include "Python.h"
 #include "pycore_abstract.h"      // _PyIndex_Check()
-#include "pycore_object.h"
+#include "pycore_object.h"        // _PyObject_GC_TRACK()
 #include "structmember.h"         // PyMemberDef
 
 static PyObject *
@@ -95,16 +95,13 @@ PyObject _Py_EllipsisObject = {
 
 /* Slice object implementation */
 
-/* Using a cache is very effective since typically only a single slice is
- * created and then deleted again
- */
-static PySliceObject *slice_cache = NULL;
 
-void _PySlice_Fini(void)
+void _PySlice_Fini(PyThreadState *tstate)
 {
-    PySliceObject *obj = slice_cache;
+    PyInterpreterState *interp = tstate->interp;
+    PySliceObject *obj = interp->slice_cache;
     if (obj != NULL) {
-        slice_cache = NULL;
+        interp->slice_cache = NULL;
         PyObject_GC_Del(obj);
     }
 }
@@ -116,10 +113,11 @@ void _PySlice_Fini(void)
 PyObject *
 PySlice_New(PyObject *start, PyObject *stop, PyObject *step)
 {
+    PyInterpreterState *interp = _PyInterpreterState_GET();
     PySliceObject *obj;
-    if (slice_cache != NULL) {
-        obj = slice_cache;
-        slice_cache = NULL;
+    if (interp->slice_cache != NULL) {
+        obj = interp->slice_cache;
+        interp->slice_cache = NULL;
         _Py_NewReference((PyObject *)obj);
     } else {
         obj = PyObject_GC_New(PySliceObject, &PySlice_Type);
@@ -324,14 +322,17 @@ Create a slice object.  This is used for extended slicing (e.g. a[0:10:2]).");
 static void
 slice_dealloc(PySliceObject *r)
 {
+    PyInterpreterState *interp = _PyInterpreterState_GET();
     _PyObject_GC_UNTRACK(r);
     Py_DECREF(r->step);
     Py_DECREF(r->start);
     Py_DECREF(r->stop);
-    if (slice_cache == NULL)
-        slice_cache = r;
-    else
+    if (interp->slice_cache == NULL) {
+        interp->slice_cache = r;
+    }
+    else {
         PyObject_GC_Del(r);
+    }
 }
 
 static PyObject *

Python/pylifecycle.c

@@ -1265,9 +1265,9 @@ finalize_interp_types(PyThreadState *tstate, int is_main_interp)
 
     if (is_main_interp) {
         _PyDict_Fini();
-        _PySlice_Fini();
     }
 
+    _PySlice_Fini(tstate);
     _PyWarnings_Fini(tstate->interp);
 
     if (is_main_interp) {