[Python-checkins] python/dist/src/Objects floatobject.c, 2.132,
2.133
tim_one at users.sourceforge.net
tim_one at users.sourceforge.net
Thu Sep 23 10:06:44 CEST 2004
Update of /cvsroot/python/python/dist/src/Objects
In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv28365/Objects
Modified Files:
floatobject.c
Log Message:
SF bug #513866: Float/long comparison anomaly.
When an integer is compared to a float now, the int isn't coerced to float.
This avoids spurious overflow exceptions and insane results. This should
compute correct results, without raising spurious exceptions, in all cases
now -- although I expect that what happens when an int/long is compared to
a NaN is still a platform accident.
Note that we had potential problems here even with "short" ints, on boxes
where sizeof(long)==8. There's #ifdef'ed code here to handle that, but
I can't test it as intended. I tested it by changing the #ifdef to
trigger on my 32-bit box instead.
I suppose this is a bugfix candidate, but I won't backport it. It's
long-winded (for speed) and messy (because the problem is messy). Note
that this also depends on a previous 2.4 patch that introduced
_Py_SwappedOp[] as an extern.
Index: floatobject.c
===================================================================
RCS file: /cvsroot/python/python/dist/src/Objects/floatobject.c,v
retrieving revision 2.132
retrieving revision 2.133
diff -u -d -r2.132 -r2.133
--- floatobject.c 8 Jul 2004 01:55:58 -0000 2.132
+++ floatobject.c 23 Sep 2004 08:06:40 -0000 2.133
@@ -354,38 +354,236 @@
return PyString_FromString(buf);
}
+/* Comparison is pretty much a nightmare. When comparing float to float,
+ * we do it as straightforwardly (and long-windedly) as conceivable, so
+ * that, e.g., Python x == y delivers the same result as the platform
+ * C x == y when x and/or y is a NaN.
+ * When mixing float with an integer type, there's no good *uniform* approach.
+ * Converting the double to an integer obviously doesn't work, since we
+ * may lose info from fractional bits. Converting the integer to a double
+ * also has two failure modes: (1) a long int may trigger overflow (too
+ * large to fit in the dynamic range of a C double); (2) even a C long may have
+ * more bits than fit in a C double (e.g., on a a 64-bit box long may have
+ * 63 bits of precision, but a C double probably has only 53), and then
+ * we can falsely claim equality when low-order integer bits are lost by
+ * coercion to double. So this part is painful too.
+ */
+
static PyObject*
float_richcompare(PyObject *v, PyObject *w, int op)
{
double i, j;
int r = 0;
- CONVERT_TO_DOUBLE(v, i);
- CONVERT_TO_DOUBLE(w, j);
+ assert(PyFloat_Check(v));
+ i = PyFloat_AS_DOUBLE(v);
+
+ /* Switch on the type of w. Set i and j to doubles to be compared,
+ * and op to the richcomp to use.
+ */
+ if (PyFloat_Check(w))
+ j = PyFloat_AS_DOUBLE(w);
+
+ else if (Py_IS_INFINITY(i)) {
+ /* XXX If we had a reliable way to check whether i is a
+ * XXX NaN, it would belong in this branch too.
+ */
+ if (PyInt_Check(w) || PyLong_Check(w))
+ /* The magnitude of i exceeds any finite integer,
+ * so it doesn't matter which int we compare i with.
+ */
+ j = 0.0;
+ else
+ goto Unimplemented;
+ }
+
+ else if (PyInt_Check(w)) {
+ long jj = PyInt_AS_LONG(w);
+ /* In the worst realistic case I can imagine, C double is a
+ * Cray single with 48 bits of precision, and long has 64
+ * bits.
+ */
+#if SIZEOF_LONG > 4
+ unsigned long abs = (unsigned long)(jj < 0 ? -jj : jj);
+ if (abs >> 48) {
+ /* Needs more than 48 bits. Make it take the
+ * PyLong path.
+ */
+ PyObject *result;
+ PyObject *ww = PyLong_FromLong(jj);
+
+ if (ww == NULL)
+ return NULL;
+ result = float_richcompare(v, ww, op);
+ Py_DECREF(ww);
+ return result;
+ }
+#endif
+ j = (double)jj;
+ assert((long)j == jj);
+ }
+
+ else if (PyLong_Check(w)) {
+ int vsign = i == 0.0 ? 0 : i < 0.0 ? -1 : 1;
+ int wsign = _PyLong_Sign(w);
+ size_t nbits;
+ double mant;
+ int exponent;
+
+ if (vsign != wsign) {
+ /* Magnitudes are irrelevant -- the signs alone
+ * determine the outcome.
+ */
+ i = (double)vsign;
+ j = (double)wsign;
+ goto Compare;
+ }
+ /* The signs are the same. */
+ /* Convert w to a double if it fits. In particular, 0 fits. */
+ nbits = _PyLong_NumBits(w);
+ if (nbits == (size_t)-1 && PyErr_Occurred()) {
+ /* This long is so large that size_t isn't big enough
+ * to hold the # of Python digits. Replace with
+ * little doubles that give the same outcome --
+ * w is so large that its magnitude must exceed
+ * the magnitude of any finite float.
+ */
+ PyErr_Clear();
+ i = (double)vsign;
+ assert(wsign != 0);
+ j = wsign * 2.0;
+ goto Compare;
+ }
+ if (nbits <= 48) {
+ j = PyLong_AsDouble(w);
+ /* It's impossible that <= 48 bits overflowed. */
+ assert(j != -1.0 || ! PyErr_Occurred());
+ goto Compare;
+ }
+ assert(wsign != 0); /* else nbits was 0 */
+ assert(vsign != 0); /* if vsign were 0, then since wsign is
+ * not 0, we would have taken the
+ * vsign != wsign branch at the start */
+ /* We want to work with non-negative numbers. */
+ if (vsign < 0) {
+ /* "Multiply both sides" by -1; this also swaps the
+ * comparator.
+ */
+ i = -i;
+ op = _Py_SwappedOp[op];
+ }
+ assert(i > 0.0);
+ mant = frexp(i, &exponent);
+ /* exponent is the # of bits in v before the radix point;
+ * we know that nbits (the # of bits in w) > 48 at this point
+ */
+ if (exponent < 0 || (size_t)exponent < nbits) {
+ i = 1.0;
+ j = 2.0;
+ goto Compare;
+ }
+ if ((size_t)exponent > nbits) {
+ i = 2.0;
+ j = 1.0;
+ goto Compare;
+ }
+ /* v and w have the same number of bits before the radix
+ * point. Construct two longs that have the same comparison
+ * outcome.
+ */
+ {
+ double fracpart;
+ double intpart;
+ PyObject *result = NULL;
+ PyObject *one = NULL;
+ PyObject *vv = NULL;
+ PyObject *ww = w;
+
+ if (wsign < 0) {
+ ww = PyNumber_Negative(w);
+ if (ww == NULL)
+ goto Error;
+ }
+ else
+ Py_INCREF(ww);
+ fracpart = modf(i, &intpart);
+ vv = PyLong_FromDouble(intpart);
+ if (vv == NULL)
+ goto Error;
+
+ if (fracpart != 0.0) {
+ /* Shift left, and or a 1 bit into vv
+ * to represent the lost fraction.
+ */
+ PyObject *temp;
+
+ one = PyInt_FromLong(1);
+ if (one == NULL)
+ goto Error;
+
+ temp = PyNumber_Lshift(ww, one);
+ if (temp == NULL)
+ goto Error;
+ Py_DECREF(ww);
+ ww = temp;
+
+ temp = PyNumber_Lshift(vv, one);
+ if (temp == NULL)
+ goto Error;
+ Py_DECREF(vv);
+ vv = temp;
+
+ temp = PyNumber_Or(vv, one);
+ if (temp == NULL)
+ goto Error;
+ Py_DECREF(vv);
+ vv = temp;
+ }
+
+ r = PyObject_RichCompareBool(vv, ww, op);
+ if (r < 0)
+ goto Error;
+ result = PyBool_FromLong(r);
+ Error:
+ Py_XDECREF(vv);
+ Py_XDECREF(ww);
+ Py_XDECREF(one);
+ return result;
+ }
+ } /* else if (PyLong_Check(w)) */
+
+ else /* w isn't float, int, or long */
+ goto Unimplemented;
+
+ Compare:
PyFPE_START_PROTECT("richcompare", return NULL)
switch (op) {
case Py_EQ:
- r = i==j;
+ r = i == j;
break;
case Py_NE:
- r = i!=j;
+ r = i != j;
break;
case Py_LE:
- r = i<=j;
+ r = i <= j;
break;
case Py_GE:
- r = i>=j;
+ r = i >= j;
break;
case Py_LT:
- r = i<j;
+ r = i < j;
break;
case Py_GT:
- r = i>j;
+ r = i > j;
break;
}
PyFPE_END_PROTECT(r)
return PyBool_FromLong(r);
+
+ Unimplemented:
+ Py_INCREF(Py_NotImplemented);
+ return Py_NotImplemented;
}
static long
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