Use Sam's suggestion to avoid branching

Author: markshannon

Include/internal/pycore_stackref.h

@@ -375,7 +375,7 @@ PyStackRef_AsStrongReference(_PyStackRef stackref)
 // With GIL
 
 #define Py_TAG_BITS 3
-#define Py_TAG_IMMORTAL 3
+#define Py_TAG_IMMORTAL _Py_IMMORTAL_FLAGS
 #define Py_TAG_REFCNT 1
 #define BITS_TO_PTR(REF) ((PyObject *)((REF).bits))
 #define BITS_TO_PTR_MASKED(REF) ((PyObject *)(((REF).bits) & (~Py_TAG_BITS)))
@@ -460,7 +460,12 @@ static inline _PyStackRef
 PyStackRef_FromPyObjectSteal(PyObject *obj)
 {
     assert(obj != NULL);
+    unsigned int tag;
+#if SIZEOF_VOID_P > 4
+    tag = obj->ob_flags & Py_TAG_BITS;
+#else
     unsigned int tag = _Py_IsImmortal(obj) ? Py_TAG_IMMORTAL : 0;
+#endif
     _PyStackRef ref = ((_PyStackRef){.bits = ((uintptr_t)(obj)) | tag});
     PyStackRef_CheckValid(ref);
     return ref;

Include/object.h

@@ -124,11 +124,13 @@ struct _object {
         PY_INT64_T ob_refcnt_full; /* This field is needed for efficient initialization with Clang on ARM */
         struct {
 #  if PY_BIG_ENDIAN
-            PY_UINT32_T ob_flags;
-            PY_UINT32_T ob_refcnt;
+            uint16_t ob_flags;
+            uint16_t ob_overflow;
+            uint32_t ob_refcnt;
 #  else
-            PY_UINT32_T ob_refcnt;
-            PY_UINT32_T ob_flags;
+            uint32_t ob_refcnt;
+            uint16_t ob_overflow;
+            uint16_t ob_flags;
 #  endif
         };
 #else

Include/refcount.h

@@ -20,7 +20,8 @@ cleanup during runtime finalization.
 */
 
 /* Leave the low bits for refcount overflow for old stable ABI code */
-#define _Py_STATICALLY_ALLOCATED_FLAG (1 << 7)
+#define _Py_STATICALLY_ALLOCATED_FLAG 4
+#define _Py_IMMORTAL_FLAGS 3
 
 #if SIZEOF_VOID_P > 4
 /*
@@ -43,7 +44,8 @@ be done by checking the bit sign flag in the lower 32 bits.
 
 */
 #define _Py_IMMORTAL_INITIAL_REFCNT (3UL << 30)
-#define _Py_STATIC_IMMORTAL_INITIAL_REFCNT ((Py_ssize_t)(_Py_IMMORTAL_INITIAL_REFCNT | (((Py_ssize_t)_Py_STATICALLY_ALLOCATED_FLAG) << 32)))
+#define _Py_STATIC_FLAG_BITS ((Py_ssize_t)(_Py_STATICALLY_ALLOCATED_FLAG | _Py_IMMORTAL_FLAGS))
+#define _Py_STATIC_IMMORTAL_INITIAL_REFCNT (((Py_ssize_t)_Py_IMMORTAL_INITIAL_REFCNT) | (_Py_STATIC_FLAG_BITS << 48))
 
 #else
 /*
@@ -114,7 +116,6 @@ PyAPI_FUNC(Py_ssize_t) Py_REFCNT(PyObject *ob);
     #endif
 #endif
 
-
 static inline Py_ALWAYS_INLINE int _Py_IsImmortal(PyObject *op)
 {
 #if defined(Py_GIL_DISABLED)

Objects/object.c

@@ -2524,6 +2524,9 @@ _Py_SetImmortalUntracked(PyObject *op)
     op->ob_ref_local = _Py_IMMORTAL_REFCNT_LOCAL;
     op->ob_ref_shared = 0;
     _Py_atomic_or_uint8(&op->ob_gc_bits, _PyGC_BITS_DEFERRED);
+#elif SIZEOF_VOID_P > 4
+    op->ob_flags = _Py_IMMORTAL_FLAGS;
+    op->ob_refcnt = _Py_IMMORTAL_INITIAL_REFCNT;
 #else
     op->ob_refcnt = _Py_IMMORTAL_INITIAL_REFCNT;
 #endif