From 16746d924599f0d18eaa04124406c495c817d4d2 Mon Sep 17 00:00:00 2001
From: Mark Shannon <mark@hotpy.org>
Date: Tue, 26 Jul 2022 15:10:34 +0100
Subject: [PATCH 1/2] Move weakreflist into the pre-header.

---
 Include/internal/pycore_object.h |  2 +-
 Objects/typeobject.c             | 34 +++++++++++++++++++++-----------
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index 173d36784cf138..ab7657a672ba4e 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -263,7 +263,7 @@ extern int _Py_CheckSlotResult(
 
 // Test if a type supports weak references
 static inline int _PyType_SUPPORTS_WEAKREFS(PyTypeObject *type) {
-    return (type->tp_weaklistoffset > 0);
+    return (type->tp_weaklistoffset != 0);
 }
 
 extern PyObject* _PyType_AllocNoTrack(PyTypeObject *type, Py_ssize_t nitems);
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 0801d9f3d5f1c6..59d4973742acdf 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2483,8 +2483,9 @@ subtype_getweakref(PyObject *obj, void *context)
         return NULL;
     }
     _PyObject_ASSERT((PyObject *)type,
-                     type->tp_weaklistoffset > 0);
+                     type->tp_weaklistoffset != 0);
     _PyObject_ASSERT((PyObject *)type,
+                     (type->tp_weaklistoffset == -4 * ((int)sizeof(PyObject *))) ||
                      ((type->tp_weaklistoffset + sizeof(PyObject *))
                       <= (size_t)(type->tp_basicsize)));
     weaklistptr = (PyObject **)((char *)obj + type->tp_weaklistoffset);
@@ -3078,19 +3079,28 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type)
         }
     }
 
-    if (ctx->add_dict && ctx->base->tp_itemsize) {
-        type->tp_dictoffset = -(long)sizeof(PyObject *);
-        slotoffset += sizeof(PyObject *);
+    if (ctx->add_dict) {
+        if (ctx->base->tp_itemsize) {
+            type->tp_dictoffset = -(long)sizeof(PyObject *);
+            slotoffset += sizeof(PyObject *);
+        }
+        else {
+            assert((type->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
+            type->tp_flags |= Py_TPFLAGS_MANAGED_DICT;
+        }
     }
 
     if (ctx->add_weak) {
         assert(!ctx->base->tp_itemsize);
-        type->tp_weaklistoffset = slotoffset;
-        slotoffset += sizeof(PyObject *);
+        if (type->tp_flags & Py_TPFLAGS_MANAGED_DICT) {
+            type->tp_weaklistoffset = -4 * sizeof(PyObject *);
+        }
+        else {
+            type->tp_weaklistoffset = slotoffset;
+            slotoffset += sizeof(PyObject *);
+        }
     }
-    if (ctx->add_dict && ctx->base->tp_itemsize == 0) {
-        assert((type->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
-        type->tp_flags |= Py_TPFLAGS_MANAGED_DICT;
+    if (type->tp_flags & Py_TPFLAGS_MANAGED_DICT) {
         type->tp_dictoffset = -slotoffset - sizeof(PyObject *)*3;
     }
 
@@ -5070,11 +5080,13 @@ compatible_for_assignment(PyTypeObject* oldto, PyTypeObject* newto, const char*
          !same_slots_added(newbase, oldbase))) {
         goto differs;
     }
-    /* The above does not check for managed __dicts__ */
+    /* The above does not check for managed __dict__ or __weakref__ */
     if ((oldto->tp_flags & Py_TPFLAGS_MANAGED_DICT) ==
         ((newto->tp_flags & Py_TPFLAGS_MANAGED_DICT)))
     {
-        return 1;
+        if (oldto->tp_weaklistoffset == newto->tp_weaklistoffset) {
+            return 1;
+        }
     }
 differs:
     PyErr_Format(PyExc_TypeError,

From bcf23ceeda8f82152103cf7f925d4b17bdcf959e Mon Sep 17 00:00:00 2001
From: Mark Shannon <mark@hotpy.org>
Date: Thu, 4 Aug 2022 16:04:30 +0100
Subject: [PATCH 2/2] Add news

---
 .../2022-08-04-16-04-18.gh-issue-95245.N4gOUV.rst              | 3 +++
 1 file changed, 3 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-08-04-16-04-18.gh-issue-95245.N4gOUV.rst

diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-08-04-16-04-18.gh-issue-95245.N4gOUV.rst b/Misc/NEWS.d/next/Core and Builtins/2022-08-04-16-04-18.gh-issue-95245.N4gOUV.rst
new file mode 100644
index 00000000000000..4449ddd8ded8a0
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-08-04-16-04-18.gh-issue-95245.N4gOUV.rst	
@@ -0,0 +1,3 @@
+Reduces the size of a "simple" Python object from 8 to 6 words by moving the
+weakreflist pointer into the pre-header directly before the object's
+dict/values pointer.