optimise 2-digit PyLong case for float +/- operator

--HG--
extra : amend_source : 7d848b95b73159432b1eb94e238006dda0bd6e32

Cython/Utility/Optimize.c

@@ -600,6 +600,12 @@ static PyObject* __Pyx_PyFloat_{{op}}{{order}}(PyObject *op1, PyObject *op2, dou
             case -1: {{fval}} = -(double)((PyLongObject*){{pyval}})->ob_digit[0]; break;
             case  0: {{fval}} = 0.0; break;
             case  1: {{fval}} = (double)((PyLongObject*){{pyval}})->ob_digit[0]; break;
+            case  2:
+                if (8 * sizeof(unsigned long) > 2 * PyLong_SHIFT) {
+                    {{fval}} = (double) ((((unsigned long)((PyLongObject*){{pyval}})->ob_digit[1]) << PyLong_SHIFT) | ((PyLongObject*){{pyval}})->ob_digit[0]);
+                    break;
+                }
+                // fall through if two platform digits don't fit into a double
             default: {{fval}} = PyLong_AsDouble({{pyval}});
                 if (unlikely({{fval}} == -1 && PyErr_Occurred())) return NULL;
                 break;

tests/run/addop.pyx

@@ -131,6 +131,12 @@ def add_1f_x(x):
     2.0
     >>> add_1f_x(-1)
     0.0
+    >>> 1.0 + 2**52
+    4503599627370497.0
+    >>> add_1f_x(2**52)
+    4503599627370497.0
+    >>> add_1f_x(2**60) == 1.0 + 2**60 or add_1f_x(2**60)
+    True
     >>> add_1f_x(1.5)
     2.5
     >>> add_1f_x(-1.5)

tests/run/subop.pyx

@@ -71,10 +71,12 @@ def sub_x_1f(x):
     0.0
     >>> sub_x_1f(-1)
     -2.0
-    >>> 2**50 - 1.0
-    1125899906842623.0
-    >>> sub_x_1f(2**50)
-    1125899906842623.0
+    >>> 2**52 - 1.0
+    4503599627370495.0
+    >>> sub_x_1f(2**52)
+    4503599627370495.0
+    >>> sub_x_1f(2**60) == 2**60 - 1.0 or sub_x_1f(2**60)
+    True
     >>> sub_x_1f(1.5)
     0.5
     >>> sub_x_1f(-1.5)
@@ -146,6 +148,12 @@ def sub_1f_x(x):
     2.0
     >>> sub_1f_x(1)
     0.0
+    >>> 1.0 - 2**52
+    -4503599627370495.0
+    >>> sub_1f_x(2**52)
+    -4503599627370495.0
+    >>> sub_1f_x(2**60) == 1.0 - 2**60 or sub_1f_x(2**60)
+    True
     >>> sub_1f_x(1.5)
     -0.5
     >>> sub_1f_x(-1.5)