Some minor cleanups in PyLong_FromLong:

 - fast path wasn't being properly taken for negative ints;
   thanks Victor Stinner for pointing this out.
 - use Py_SAFE_DOWNCAST instead of direct casting to digit
   (it's safer, especially if we ever consider moving to 30-bit
   digits)
 - cleaner way to write negation

diff --git a/Objects/longobject.c b/Objects/longobject.c
index 46efe5f291e031c6367c3d7d777d21b6203382ea..a932df7edaaeada4df016a42b7249f83a3b0e549 100644 (file)
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -175,7 +175,7 @@ PyObject *
 PyLong_FromLong(long ival)
 {
        PyLongObject *v;
-        unsigned long abs_ival;
+       unsigned long abs_ival;
        unsigned long t;  /* unsigned so >> doesn't propagate sign bit */
        int ndigits = 0;
        int sign = 1;
@@ -183,33 +183,35 @@ PyLong_FromLong(long ival)
        CHECK_SMALL_INT(ival);
 
        if (ival < 0) {
-               /* if LONG_MIN == -LONG_MAX-1 (true on most platforms) then
-                  ANSI C says that the result of -ival is undefined when ival
-                  == LONG_MIN.  Hence the following workaround. */
-               abs_ival = (unsigned long)(-1-ival) + 1;
+               /* negate: can't write this as abs_ival = -ival since that
+                  invokes undefined behaviour when ival is LONG_MIN */
+               abs_ival = 0U-(unsigned long)ival;
                sign = -1;
        }
        else {
                abs_ival = (unsigned long)ival;
        }
 
-       /* Fast path for single-digits ints */
-       if (!(ival>>PyLong_SHIFT)) {
+       /* Fast path for single-digit ints */
+       if (!(abs_ival >> PyLong_SHIFT)) {
                v = _PyLong_New(1);
                if (v) {
                        Py_SIZE(v) = sign;
-                       v->ob_digit[0] = (digit)ival;
+                       v->ob_digit[0] = Py_SAFE_DOWNCAST(
+                               abs_ival, unsigned long, digit);
                }
                return (PyObject*)v;
        }
 
        /* 2 digits */
-       if (!(ival >> 2*PyLong_SHIFT)) {
+       if (!(abs_ival >> 2*PyLong_SHIFT)) {
                v = _PyLong_New(2);
                if (v) {
                        Py_SIZE(v) = 2*sign;
-                       v->ob_digit[0] = (digit)ival & PyLong_MASK;
-                       v->ob_digit[1] = (digit)(ival >> PyLong_SHIFT);
+                       v->ob_digit[0] = Py_SAFE_DOWNCAST(
+                               abs_ival & PyLong_MASK, unsigned long, digit);
+                       v->ob_digit[1] = Py_SAFE_DOWNCAST(
+                             abs_ival >> PyLong_SHIFT, unsigned long, digit);
                }
                return (PyObject*)v;
        }
@@ -226,7 +228,8 @@ PyLong_FromLong(long ival)
                Py_SIZE(v) = ndigits*sign;
                t = abs_ival;
                while (t) {
-                       *p++ = (digit)(t & PyLong_MASK);
+                       *p++ = Py_SAFE_DOWNCAST(
+                               t & PyLong_MASK, unsigned long, digit);
                        t >>= PyLong_SHIFT;
                }
        }