gh-127119: Faster check for small ints in long_dealloc

Include/cpython/longintrepr.h

@@ -76,8 +76,8 @@ typedef long stwodigits; /* signed variant of twodigits */
  - 1: Zero
  - 2: Negative
 
- The third lowest bit of lv_tag is reserved for an immortality flag, but is
-not currently used.
+ The third lowest bit of lv_tag is
+set to 1 for the small ints.
 
  In a normalized number, ob_digit[ndigits-1] (the most significant
  digit) is never zero. Also, in all cases, for all valid i,

Include/internal/pycore_long.h

@@ -159,13 +159,14 @@ 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: 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.
@@ -196,7 +197,7 @@ PyAPI_FUNC(int) _PyLong_Size_t_Converter(PyObject *, void *);
 static inline int
 _PyLong_IsNonNegativeCompact(const PyLongObject* op) {
  assert(PyLong_Check(op));
- return op->long_value.lv_tag <= (1 << NON_SIZE_BITS);
+ return ((op->long_value.lv_tag & ~IMMORTALITY_BIT_MASK) <= (1 << NON_SIZE_BITS));
 }
 
 
@@ -298,7 +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) | IMMORTALITY_BIT_MASK, \
  { ((val) >= 0 ? (val) : -(val)) }, \
  } \
  }

Misc/NEWS.d/next/Core_and_Builtins/2024-12-04-22-14-40.gh-issue-127119._hpyFE.rst

@@ -0,0 +1 @@
+Slightly optimize the :class:`int` deallocator.

Modules/_testcapi/immortal.c

@@ -1,5 +1,8 @@
 #include "parts.h"
 
+#define Py_BUILD_CORE
+#include "internal/pycore_long.h" // IMMORTALITY_BIT_MASK
+
 int verify_immortality(PyObject *object)
 {
  assert(_Py_IsImmortal(object));
@@ -26,7 +29,17 @@ 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 & IMMORTALITY_BIT_MASK;
+ assert(has_int_immortal_bit);
+ }
+ for (int i = 257; i <= 260; i++) {
+ PyObject *obj = PyLong_FromLong(i);
+ assert(obj);
+ int has_int_immortal_bit = ((PyLongObject *)obj)->long_value.lv_tag & IMMORTALITY_BIT_MASK;
+ assert(!has_int_immortal_bit);
+ Py_DECREF(obj);
  }
  Py_RETURN_NONE;
 }

Objects/longobject.c

@@ -3616,32 +3616,25 @@ long_richcompare(PyObject *self, PyObject *other, int op)
 }
 
 static inline int
-compact_int_is_small(PyObject *self)
+/// Return 1 if the object is one of the immortal small ints
+_long_is_small_int(PyObject *op)
 {
- PyLongObject *pylong = (PyLongObject *)self;
- assert(_PyLong_IsCompact(pylong));
- stwodigits ival = medium_value(pylong);
- if (IS_SMALL_INT(ival)) {
- PyLongObject *small_pylong = (PyLongObject *)get_small_int((sdigit)ival);
- if (pylong == small_pylong) {
- return 1;
- }
- }
- return 0;
+ PyLongObject *long_object = (PyLongObject *)op;
+ int is_small_int = (long_object->long_value.lv_tag & IMMORTALITY_BIT_MASK) != 0;
+ assert((!is_small_int) || PyLong_CheckExact(op));
+ return is_small_int;
 }
 
 void
 _PyLong_ExactDealloc(PyObject *self)
 {
  assert(PyLong_CheckExact(self));
+ if (_long_is_small_int(self)) {
+ // See PEP 683, section Accidental De-Immortalizing for details
+ _Py_SetImmortal(self);
+ return;
+ }
  if (_PyLong_IsCompact((PyLongObject *)self)) {
- #ifndef Py_GIL_DISABLED
- if (compact_int_is_small(self)) {
- // See PEP 683, section Accidental De-Immortalizing for details
- _Py_SetImmortal(self);
- return;
- }
- #endif
  _Py_FREELIST_FREE(ints, self, PyObject_Free);
  return;
  }
@@ -3651,24 +3644,20 @@ _PyLong_ExactDealloc(PyObject *self)
 static void
 long_dealloc(PyObject *self)
 {
- assert(self);
- if (_PyLong_IsCompact((PyLongObject *)self)) {
- if (compact_int_is_small(self)) {
- /* This should never get called, but we also don't want to SEGV if
- * we accidentally decref small Ints out of existence. Instead,
- * since small Ints are immortal, re-set the reference count.
- *
- * See PEP 683, section Accidental De-Immortalizing for details
- */
- _Py_SetImmortal(self);
- return;
- }
- if (PyLong_CheckExact(self)) {
- _Py_FREELIST_FREE(ints, self, PyObject_Free);
- return;
- }
+ if (_long_is_small_int(self)) {
+ /* This should never get called, but we also don't want to SEGV if
+ * we accidentally decref small Ints out of existence. Instead,
+ * since small Ints are immortal, re-set the reference count.
+ *
+ * See PEP 683, section Accidental De-Immortalizing for details
+ */
+ _Py_SetImmortal(self);
+ return;
+ }
+ if (PyLong_CheckExact(self) && _PyLong_IsCompact((PyLongObject *)self)) {
+ _Py_FREELIST_FREE(ints, self, PyObject_Free);
+ return;
  }
-
  Py_TYPE(self)->tp_free(self);
 }
 
@@ -6030,7 +6019,7 @@ long_subtype_new(PyTypeObject *type, PyObject *x, PyObject *obase)
  return NULL;
  }
  assert(PyLong_Check(newobj));
- newobj->long_value.lv_tag = tmp->long_value.lv_tag;
+ newobj->long_value.lv_tag = tmp->long_value.lv_tag & ~IMMORTALITY_BIT_MASK;
  for (i = 0; i < n; i++) {
  newobj->long_value.ob_digit[i] = tmp->long_value.ob_digit[i];
  }

Tools/gdb/libpython.py

@@ -890,7 +890,7 @@ class PyLongObjectPtr(PyObjectPtr):
 
  def proxyval(self, visited):
  '''
- Python's Include/longinterpr.h has this declaration:
+ Python's Include/cpython/longinterpr.h has this declaration:
 
  typedef struct _PyLongValue {
  uintptr_t lv_tag; /* Number of digits, sign and flags */
@@ -909,8 +909,7 @@ def proxyval(self, visited):
  - 0: Positive
  - 1: Zero
  - 2: Negative
- The third lowest bit of lv_tag is reserved for an immortality flag, but is
- not currently used.
+ The third lowest bit of lv_tag is set to 1 for the small ints and 0 otherwise.
 
  where SHIFT can be either:
  #define PyLong_SHIFT 30