bpo-42161: Add _PyLong_GetZero() and _PyLong_GetOne() (GH-22993)

Add _PyLong_GetZero() and _PyLong_GetOne() functions and a new
internal pycore_long.h header file.

Python cannot be built without small integer singletons anymore.

Include/internal/pycore_interp.h

@@ -163,6 +163,11 @@ struct _Py_exc_state {
 #define _PY_NSMALLPOSINTS           257
 #define _PY_NSMALLNEGINTS           5
 
+// _PyLong_GetZero() and _PyLong_GetOne() must always be available
+#if _PY_NSMALLPOSINTS < 2
+#  error "_PY_NSMALLPOSINTS must be greater than 1"
+#endif
+
 // The PyInterpreterState typedef is in Include/pystate.h.
 struct _is {
 
@@ -233,14 +238,12 @@ struct _is {
 
     PyObject *audit_hooks;
 
-#if _PY_NSMALLNEGINTS + _PY_NSMALLPOSINTS > 0
     /* Small integers are preallocated in this array so that they
        can be shared.
        The integers that are preallocated are those in the range
        -_PY_NSMALLNEGINTS (inclusive) to _PY_NSMALLPOSINTS (not inclusive).
     */
     PyLongObject* small_ints[_PY_NSMALLNEGINTS + _PY_NSMALLPOSINTS];
-#endif
     struct _Py_bytes_state bytes;
     struct _Py_unicode_state unicode;
     struct _Py_float_state float_state;

Include/internal/pycore_long.h

@@ -0,0 +1,43 @@
+#ifndef Py_INTERNAL_LONG_H
+#define Py_INTERNAL_LONG_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+#include "pycore_interp.h"        // PyInterpreterState.small_ints
+#include "pycore_pystate.h"       // _PyThreadState_GET()
+
+// Don't call this function but _PyLong_GetZero() and _PyLong_GetOne()
+static inline PyObject* __PyLong_GetSmallInt_internal(int value)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+#ifdef Py_DEBUG
+    _Py_EnsureTstateNotNULL(tstate);
+#endif
+    assert(-_PY_NSMALLNEGINTS <= value && value < _PY_NSMALLPOSINTS);
+    size_t index = _PY_NSMALLNEGINTS + value;
+    PyObject *obj = (PyObject*)tstate->interp->small_ints[index];
+    // _PyLong_GetZero() and _PyLong_GetOne() must not be called
+    // before _PyLong_Init() nor after _PyLong_Fini()
+    assert(obj != NULL);
+    return obj;
+}
+
+// Return a borrowed reference to the zero singleton.
+// The function cannot return NULL.
+static inline PyObject* _PyLong_GetZero(void)
+{ return __PyLong_GetSmallInt_internal(0); }
+
+// Return a borrowed reference to the one singleton.
+// The function cannot return NULL.
+static inline PyObject* _PyLong_GetOne(void)
+{ return __PyLong_GetSmallInt_internal(1); }
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_LONG_H */

Makefile.pre.in

@@ -1117,6 +1117,7 @@ PYTHON_HEADERS= \
 		$(srcdir)/Include/internal/pycore_initconfig.h \
 		$(srcdir)/Include/internal/pycore_interp.h \
 		$(srcdir)/Include/internal/pycore_list.h \
+		$(srcdir)/Include/internal/pycore_long.h \
 		$(srcdir)/Include/internal/pycore_object.h \
 		$(srcdir)/Include/internal/pycore_pathconfig.h \
 		$(srcdir)/Include/internal/pycore_pyerrors.h \

Objects/longobject.c

@@ -5,6 +5,7 @@
 #include "Python.h"
 #include "pycore_bitutils.h"      // _Py_popcount32()
 #include "pycore_interp.h"        // _PY_NSMALLPOSINTS
+#include "pycore_long.h"          // __PyLong_GetSmallInt_internal()
 #include "pycore_object.h"        // _PyObject_InitVar()
 #include "pycore_pystate.h"       // _Py_IsMainInterpreter()
 #include "longintrepr.h"
@@ -19,8 +20,8 @@ class int "PyObject *" "&PyLong_Type"
 [clinic start generated code]*/
 /*[clinic end generated code: output=da39a3ee5e6b4b0d input=ec0275e3422a36e3]*/
 
-#define NSMALLPOSINTS           _PY_NSMALLPOSINTS
 #define NSMALLNEGINTS           _PY_NSMALLNEGINTS
+#define NSMALLPOSINTS           _PY_NSMALLPOSINTS
 
 _Py_IDENTIFIER(little);
 _Py_IDENTIFIER(big);
@@ -34,7 +35,6 @@ _Py_IDENTIFIER(big);
 PyObject *_PyLong_Zero = NULL;
 PyObject *_PyLong_One = NULL;
 
-#if NSMALLNEGINTS + NSMALLPOSINTS > 0
 #define IS_SMALL_INT(ival) (-NSMALLNEGINTS <= (ival) && (ival) < NSMALLPOSINTS)
 #define IS_SMALL_UINT(ival) ((ival) < NSMALLPOSINTS)
 
@@ -42,8 +42,7 @@ static PyObject *
 get_small_int(sdigit ival)
 {
     assert(IS_SMALL_INT(ival));
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    PyObject *v = (PyObject*)interp->small_ints[ival + NSMALLNEGINTS];
+    PyObject *v = __PyLong_GetSmallInt_internal(ival);
     Py_INCREF(v);
     return v;
 }
@@ -60,12 +59,6 @@ maybe_small_long(PyLongObject *v)
     }
     return v;
 }
-#else
-#define IS_SMALL_INT(ival) 0
-#define IS_SMALL_UINT(ival) 0
-#define get_small_int(ival) (Py_UNREACHABLE(), NULL)
-#define maybe_small_long(val) (val)
-#endif
 
 /* If a freshly-allocated int is already shared, it must
    be a small integer, so negating it must go to PyLong_FromLong */
@@ -2559,8 +2552,9 @@ long_divrem(PyLongObject *a, PyLongObject *b,
         if (*prem == NULL) {
             return -1;
         }
-        Py_INCREF(_PyLong_Zero);
-        *pdiv = (PyLongObject*)_PyLong_Zero;
+        PyObject *zero = _PyLong_GetZero();
+        Py_INCREF(zero);
+        *pdiv = (PyLongObject*)zero;
         return 0;
     }
     if (size_b == 1) {
@@ -3669,7 +3663,7 @@ l_divmod(PyLongObject *v, PyLongObject *w,
             Py_DECREF(div);
             return -1;
         }
-        temp = (PyLongObject *) long_sub(div, (PyLongObject *)_PyLong_One);
+        temp = (PyLongObject *) long_sub(div, (PyLongObject *)_PyLong_GetOne());
         if (temp == NULL) {
             Py_DECREF(mod);
             Py_DECREF(div);
@@ -4078,7 +4072,7 @@ long_invmod(PyLongObject *a, PyLongObject *n)
 
     Py_DECREF(c);
     Py_DECREF(n);
-    if (long_compare(a, (PyLongObject *)_PyLong_One)) {
+    if (long_compare(a, (PyLongObject *)_PyLong_GetOne())) {
         /* a != 1; we don't have an inverse. */
         Py_DECREF(a);
         Py_DECREF(b);
@@ -4313,7 +4307,7 @@ long_invert(PyLongObject *v)
     PyLongObject *x;
     if (Py_ABS(Py_SIZE(v)) <=1)
         return PyLong_FromLong(-(MEDIUM_VALUE(v)+1));
-    x = (PyLongObject *) long_add(v, (PyLongObject *)_PyLong_One);
+    x = (PyLongObject *) long_add(v, (PyLongObject *)_PyLong_GetOne());
     if (x == NULL)
         return NULL;
     _PyLong_Negate(&x);
@@ -5105,7 +5099,8 @@ _PyLong_DivmodNear(PyObject *a, PyObject *b)
 
     /* compare twice the remainder with the divisor, to see
        if we need to adjust the quotient and remainder */
-    twice_rem = long_lshift((PyObject *)rem, _PyLong_One);
+    PyObject *one = _PyLong_GetOne();  // borrowed reference
+    twice_rem = long_lshift((PyObject *)rem, one);
     if (twice_rem == NULL)
         goto error;
     if (quo_is_neg) {
@@ -5122,9 +5117,9 @@ _PyLong_DivmodNear(PyObject *a, PyObject *b)
     if ((Py_SIZE(b) < 0 ? cmp < 0 : cmp > 0) || (cmp == 0 && quo_is_odd)) {
         /* fix up quotient */
         if (quo_is_neg)
-            temp = long_sub(quo, (PyLongObject *)_PyLong_One);
+            temp = long_sub(quo, (PyLongObject *)one);
         else
-            temp = long_add(quo, (PyLongObject *)_PyLong_One);
+            temp = long_add(quo, (PyLongObject *)one);
         Py_DECREF(quo);
         quo = (PyLongObject *)temp;
         if (quo == NULL)
@@ -5406,7 +5401,7 @@ int_as_integer_ratio_impl(PyObject *self)
     if (numerator == NULL) {
         return NULL;
     }
-    ratio_tuple = PyTuple_Pack(2, numerator, _PyLong_One);
+    ratio_tuple = PyTuple_Pack(2, numerator, _PyLong_GetOne());
     Py_DECREF(numerator);
     return ratio_tuple;
 }
@@ -5712,7 +5707,6 @@ PyLong_GetInfo(void)
 int
 _PyLong_Init(PyThreadState *tstate)
 {
-#if NSMALLNEGINTS + NSMALLPOSINTS > 0
     for (Py_ssize_t i=0; i < NSMALLNEGINTS + NSMALLPOSINTS; i++) {
         sdigit ival = (sdigit)i - NSMALLNEGINTS;
         int size = (ival < 0) ? -1 : ((ival == 0) ? 0 : 1);
@@ -5727,7 +5721,6 @@ _PyLong_Init(PyThreadState *tstate)
 
         tstate->interp->small_ints[i] = v;
     }
-#endif
 
     if (_Py_IsMainInterpreter(tstate)) {
         _PyLong_Zero = PyLong_FromLong(0);
@@ -5759,9 +5752,7 @@ _PyLong_Fini(PyThreadState *tstate)
         Py_CLEAR(_PyLong_Zero);
     }
 
-#if NSMALLNEGINTS + NSMALLPOSINTS > 0
     for (Py_ssize_t i = 0; i < NSMALLNEGINTS + NSMALLPOSINTS; i++) {
         Py_CLEAR(tstate->interp->small_ints[i]);
     }
-#endif
 }

PCbuild/pythoncore.vcxproj

@@ -185,6 +185,7 @@
     <ClInclude Include="..\Include\internal\pycore_initconfig.h" />
     <ClInclude Include="..\Include\internal\pycore_interp.h" />
     <ClInclude Include="..\Include\internal\pycore_list.h" />
+    <ClInclude Include="..\Include\internal\pycore_long.h" />
     <ClInclude Include="..\Include\internal\pycore_object.h" />
     <ClInclude Include="..\Include\internal\pycore_pathconfig.h" />
     <ClInclude Include="..\Include\internal\pycore_pyerrors.h" />

PCbuild/pythoncore.vcxproj.filters

@@ -537,6 +537,9 @@
     <ClInclude Include="..\Include\internal\pycore_list.h">
       <Filter>Include\internal</Filter>
     </ClInclude>
+    <ClInclude Include="..\Include\internal\pycore_long.h">
+      <Filter>Include\internal</Filter>
+    </ClInclude>
     <ClInclude Include="..\Include\internal\pycore_object.h">
       <Filter>Include\internal</Filter>
     </ClInclude>