bpo-37986: Improve perfomance of PyLong_FromDouble() (GH-15611)
* bpo-37986: Improve perfomance of PyLong_FromDouble()
* Use strict bound check for safety and symmetry
* Remove possibly outdated performance claims
Co-authored-by: Mark Dickinson <dickinsm@gmail.com>
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 11fc75b..0ff0e80 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -416,6 +416,21 @@
PyObject *
PyLong_FromDouble(double dval)
{
+ /* Try to get out cheap if this fits in a long. When a finite value of real
+ * floating type is converted to an integer type, the value is truncated
+ * toward zero. If the value of the integral part cannot be represented by
+ * the integer type, the behavior is undefined. Thus, we must check that
+ * value is in range (LONG_MIN - 1, LONG_MAX + 1). If a long has more bits
+ * of precision than a double, casting LONG_MIN - 1 to double may yield an
+ * approximation, but LONG_MAX + 1 is a power of two and can be represented
+ * as double exactly (assuming FLT_RADIX is 2 or 16), so for simplicity
+ * check against [-(LONG_MAX + 1), LONG_MAX + 1).
+ */
+ const double int_max = (unsigned long)LONG_MAX + 1;
+ if (-int_max < dval && dval < int_max) {
+ return PyLong_FromLong((long)dval);
+ }
+
PyLongObject *v;
double frac;
int i, ndig, expo, neg;
@@ -435,8 +450,7 @@
dval = -dval;
}
frac = frexp(dval, &expo); /* dval = frac*2**expo; 0.0 <= frac < 1.0 */
- if (expo <= 0)
- return PyLong_FromLong(0L);
+ assert(expo > 0);
ndig = (expo-1) / PyLong_SHIFT + 1; /* Number of 'digits' in result */
v = _PyLong_New(ndig);
if (v == NULL)