bpo-46311: Clean up PyLong_FromLong and PyLong_FromLongLong (GH-30496)

Author: mdickinson

Misc/NEWS.d/next/Core and Builtins/2022-01-09-11-59-04.bpo-30496.KvuuGT.rst

@@ -0,0 +1,3 @@
+Fixed a minor portability issue in the implementation of
+:c:func:`PyLong_FromLong`, and added a fast path for single-digit integers
+to :c:func:`PyLong_FromLongLong`.

Objects/longobject.c

@@ -272,44 +272,40 @@ _PyLong_Negate(PyLongObject **x_p)
 }
 
 /* Create a new int object from a C long int */
+
 PyObject *
 PyLong_FromLong(long ival)
 {
+    PyLongObject *v;
+    unsigned long abs_ival, t;
+    int ndigits;
+
+    /* Handle small and medium cases. */
     if (IS_SMALL_INT(ival)) {
         return get_small_int((sdigit)ival);
     }
-    unsigned long abs_ival;
-    int sign;
-    if (ival < 0) {
-        /* negate: can't write this as abs_ival = -ival since that
-           invokes undefined behaviour when ival is LONG_MIN */
-        abs_ival = 0U-(twodigits)ival;
-        sign = -1;
-    }
-    else {
-        abs_ival = (unsigned long)ival;
-        sign = 1;
-    }
-    /* Fast path for single-digit ints */
-    if (!(abs_ival >> PyLong_SHIFT)) {
+    if (-(long)PyLong_MASK <= ival && ival <= (long)PyLong_MASK) {
         return _PyLong_FromMedium((sdigit)ival);
     }
-    /* Must be at least two digits.
-     * Do shift in two steps to avoid undefined behavior. */
-    unsigned long t = (abs_ival >> PyLong_SHIFT) >> PyLong_SHIFT;
-    Py_ssize_t ndigits = 2;
+
+    /* Count digits (at least two - smaller cases were handled above). */
+    abs_ival = ival < 0 ? 0U-(unsigned long)ival : (unsigned long)ival;
+    /* Do shift in two steps to avoid possible undefined behavior. */
+    t = abs_ival >> PyLong_SHIFT >> PyLong_SHIFT;
+    ndigits = 2;
     while (t) {
         ++ndigits;
         t >>= PyLong_SHIFT;
     }
-    PyLongObject *v = _PyLong_New(ndigits);
+
+    /* Construct output value. */
+    v = _PyLong_New(ndigits);
     if (v != NULL) {
         digit *p = v->ob_digit;
-        Py_SET_SIZE(v, ndigits * sign);
+        Py_SET_SIZE(v, ival < 0 ? -ndigits : ndigits);
         t = abs_ival;
         while (t) {
-            *p++ = Py_SAFE_DOWNCAST(
-                t & PyLong_MASK, unsigned long, digit);
+            *p++ = (digit)(t & PyLong_MASK);
             t >>= PyLong_SHIFT;
         }
     }
@@ -1105,38 +1101,32 @@ PyObject *
 PyLong_FromLongLong(long long ival)
 {
     PyLongObject *v;
-    unsigned long long abs_ival;
-    unsigned long long t;  /* unsigned so >> doesn't propagate sign bit */
-    int ndigits = 0;
-    int negative = 0;
+    unsigned long long abs_ival, t;
+    int ndigits;
 
+    /* Handle small and medium cases. */
     if (IS_SMALL_INT(ival)) {
         return get_small_int((sdigit)ival);
     }
-
-    if (ival < 0) {
-        /* avoid signed overflow on negation;  see comments
-           in PyLong_FromLong above. */
-        abs_ival = (unsigned long long)(-1-ival) + 1;
-        negative = 1;
-    }
-    else {
-        abs_ival = (unsigned long long)ival;
+    if (-(long long)PyLong_MASK <= ival && ival <= (long long)PyLong_MASK) {
+        return _PyLong_FromMedium((sdigit)ival);
     }
 
-    /* Count the number of Python digits.
-       We used to pick 5 ("big enough for anything"), but that's a
-       waste of time and space given that 5*15 = 75 bits are rarely
-       needed. */
-    t = abs_ival;
+    /* Count digits (at least two - smaller cases were handled above). */
+    abs_ival = ival < 0 ? 0U-(unsigned long long)ival : (unsigned long long)ival;
+    /* Do shift in two steps to avoid possible undefined behavior. */
+    t = abs_ival >> PyLong_SHIFT >> PyLong_SHIFT;
+    ndigits = 2;
     while (t) {
         ++ndigits;
         t >>= PyLong_SHIFT;
     }
+
+    /* Construct output value. */
     v = _PyLong_New(ndigits);
     if (v != NULL) {
         digit *p = v->ob_digit;
-        Py_SET_SIZE(v, negative ? -ndigits : ndigits);
+        Py_SET_SIZE(v, ival < 0 ? -ndigits : ndigits);
         t = abs_ival;
         while (t) {
             *p++ = (digit)(t & PyLong_MASK);