Use int immortality bit for small int check

python · eendebakpt · Dec 4, 2024 · Dec 4, 2024 · Dec 5, 2024 · Dec 5, 2024
commit 46396428b45085d9643b99e33f51d1491163cb3c
diff --git a/Include/internal/pycore_long.h b/Include/internal/pycore_long.h
@@ -157,13 +157,14 @@ PyAPI_FUNC(int) _PyLong_Size_t_Converter(PyObject *, void *);
 
 /* Long value tag bits:
  * 0-1: Sign bits value = (1-sign), ie. negative=2, positive=0, zero=1.
- * 2: Reserved for immortality bit
+ * 2: Reserved for immortality bit. Set to 1 for the small ints
  * 3+ Unsigned digit count
  */
 #define SIGN_MASK 3
 #define SIGN_ZERO 1
 #define SIGN_NEGATIVE 2
 #define NON_SIZE_BITS 3
+#define IMMORTALITY_BIT_MASK (1 << 2)
 
 /* The functions _PyLong_IsCompact and _PyLong_CompactValue are defined
  * in Include/cpython/longobject.h, since they need to be inline.
@@ -262,6 +263,8 @@ _PyLong_SameSign(const PyLongObject *a, const PyLongObject *b)
     return (a->long_value.lv_tag & SIGN_MASK) == (b->long_value.lv_tag & SIGN_MASK);
 }
 
+#define IMMORTAL_BIT(val) ((_PY_NSMALLNEGINTS <= val < _PY_NSMALLPOSINTS) * IMMORTALITY_BIT_MASK)
+
 #define TAG_FROM_SIGN_AND_SIZE(sign, size) \
     ((uintptr_t)(1 - (sign)) | ((uintptr_t)(size) << NON_SIZE_BITS))
 
@@ -296,7 +299,7 @@ _PyLong_FlipSign(PyLongObject *op) {
         .long_value  = { \
             .lv_tag = TAG_FROM_SIGN_AND_SIZE( \
                 (val) == 0 ? 0 : ((val) < 0 ? -1 : 1), \
-                (val) == 0 ? 0 : 1), \
+                (val) == 0 ? 0 : 1) | IMMORTAL_BIT(val), \
             { ((val) >= 0 ? (val) : -(val)) }, \
         } \
     }

diff --git a/Modules/_testcapi/immortal.c b/Modules/_testcapi/immortal.c
@@ -26,7 +26,15 @@ static PyObject *
 test_immortal_small_ints(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
     for (int i = -5; i <= 256; i++) {
-        assert(verify_immortality(PyLong_FromLong(i)));
+        PyObject *obj = PyLong_FromLong(i);
+        assert(verify_immortality(obj));
+        int has_int_immortal_bit = ((PyLongObject *)obj)->long_value.lv_tag & (1 << 2);
+        assert(has_int_immortal_bit);
+    }
+    for (int i = 257; i <= 260; i++) {
+        PyObject *obj = PyLong_FromLong(i);
+        int has_int_immortal_bit = ((PyLongObject *)obj)->long_value.lv_tag & (1 << 2);
+        assert(!has_int_immortal_bit);
     }
     Py_RETURN_NONE;
 }

diff --git a/Objects/longobject.c b/Objects/longobject.c
@@ -3615,9 +3615,11 @@ static inline int
 /// Return 1 if the object is one of the immortal small ints
 _long_is_small_int(PyObject *op)
 {
-    return (((PyObject *)(&_PyLong_SMALL_INTS[0]) <= op)
-             && (op <= (PyObject *)&_PyLong_SMALL_INTS[_PY_NSMALLNEGINTS +_PY_NSMALLPOSINTS]));
+    PyLongObject *long_object = (PyLongObject *)op;
+
+    return long_object->long_value.lv_tag | IMMORTALITY_BIT_MASK;
 }
+
 static void
 long_dealloc(PyObject *self)
 {