gh-99845: Clean up _PyObject_VAR_SIZE() usage (#99847)

* code_sizeof() now uses an unsigned type (size_t) to compute the result.
* Fix _PyObject_ComputedDictPointer(): cast _PyObject_VAR_SIZE() to
  Py_ssize_t, rather than long: it's a different type on 64-bit Windows.
* Clarify that _PyObject_VAR_SIZE() uses an unsigned type (size_t).

Modules/gcmodule.c

@@ -2329,15 +2329,14 @@ _PyObject_GC_New(PyTypeObject *tp)
 PyVarObject *
 _PyObject_GC_NewVar(PyTypeObject *tp, Py_ssize_t nitems)
 {
-    size_t size;
     PyVarObject *op;
 
     if (nitems < 0) {
         PyErr_BadInternalCall();
         return NULL;
     }
     size_t presize = _PyType_PreHeaderSize(tp);
-    size = _PyObject_VAR_SIZE(tp, nitems);
+    size_t size = _PyObject_VAR_SIZE(tp, nitems);
     op = (PyVarObject *)gc_alloc(size, presize);
     if (op == NULL) {
         return NULL;
@@ -2351,7 +2350,7 @@ _PyObject_GC_Resize(PyVarObject *op, Py_ssize_t nitems)
 {
     const size_t basicsize = _PyObject_VAR_SIZE(Py_TYPE(op), nitems);
     _PyObject_ASSERT((PyObject *)op, !_PyObject_GC_IS_TRACKED(op));
-    if (basicsize > PY_SSIZE_T_MAX - sizeof(PyGC_Head)) {
+    if (basicsize > (size_t)PY_SSIZE_T_MAX - sizeof(PyGC_Head)) {
         return (PyVarObject *)PyErr_NoMemory();
     }
 

Objects/codeobject.c

@@ -1867,15 +1867,13 @@ static PyGetSetDef code_getsetlist[] = {
 static PyObject *
 code_sizeof(PyCodeObject *co, PyObject *Py_UNUSED(args))
 {
-    Py_ssize_t res = _PyObject_VAR_SIZE(Py_TYPE(co), Py_SIZE(co));
-
+    size_t res = _PyObject_VAR_SIZE(Py_TYPE(co), Py_SIZE(co));
     _PyCodeObjectExtra *co_extra = (_PyCodeObjectExtra*) co->co_extra;
     if (co_extra != NULL) {
-        res += sizeof(_PyCodeObjectExtra) +
-               (co_extra->ce_size-1) * sizeof(co_extra->ce_extras[0]);
+        res += sizeof(_PyCodeObjectExtra);
+        res += ((size_t)co_extra->ce_size - 1) * sizeof(co_extra->ce_extras[0]);
     }
-
-    return PyLong_FromSsize_t(res);
+    return PyLong_FromSize_t(res);
 }
 
 static PyObject *

Objects/object.c

@@ -1043,22 +1043,25 @@ PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
 PyObject **
 _PyObject_ComputedDictPointer(PyObject *obj)
 {
-    Py_ssize_t dictoffset;
     PyTypeObject *tp = Py_TYPE(obj);
-
     assert((tp->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
-    dictoffset = tp->tp_dictoffset;
-    if (dictoffset == 0)
+
+    Py_ssize_t dictoffset = tp->tp_dictoffset;
+    if (dictoffset == 0) {
         return NULL;
+    }
+
     if (dictoffset < 0) {
         assert(dictoffset != -1);
+
         Py_ssize_t tsize = Py_SIZE(obj);
         if (tsize < 0) {
             tsize = -tsize;
         }
         size_t size = _PyObject_VAR_SIZE(tp, tsize);
+        assert(size <= (size_t)PY_SSIZE_T_MAX);
+        dictoffset += (Py_ssize_t)size;
 
-        dictoffset += (long)size;
         _PyObject_ASSERT(obj, dictoffset > 0);
         _PyObject_ASSERT(obj, dictoffset % SIZEOF_VOID_P == 0);
     }