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Merged revisions 76295 via svnmerge from 

svn+ssh://pythondev@svn.python.org/python/trunk

........
  r76295 | mark.dickinson | 2009-11-15 12:31:13 +0000 (Sun, 15 Nov 2009) | 5 lines

  Avoid signed overflow in some xrange calculations, and extend
  xrange tests to cover some special cases that caused problems
  in py3k.  This is a partial backport of r76292-76293 (see
  issue #7298.)
........
This commit is contained in:
Mark Dickinson 2009-11-15 12:34:12 +00:00
parent 10c858ac4d
commit cc83afd803
2 changed files with 113 additions and 34 deletions
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69
Lib/test/test_xrange.py
View File

@ -3,12 +3,49 @@
import test.test_support, unittest
import sys
import pickle
import itertools
import warnings
warnings.filterwarnings("ignore", "integer argument expected",
DeprecationWarning, "unittest")
# pure Python implementations (3 args only), for comparison
def pyrange(start, stop, step):
if (start - stop) // step < 0:
# replace stop with next element in the sequence of integers
# that are congruent to start modulo step.
stop += (start - stop) % step
while start != stop:
yield start
start += step
def pyrange_reversed(start, stop, step):
stop += (start - stop) % step
return pyrange(stop - step, start - step, -step)
class XrangeTest(unittest.TestCase):
def assert_iterators_equal(self, xs, ys, test_id, limit=None):
# check that an iterator xs matches the expected results ys,
# up to a given limit.
if limit is not None:
xs = itertools.islice(xs, limit)
ys = itertools.islice(ys, limit)
sentinel = object()
pairs = itertools.izip_longest(xs, ys, fillvalue=sentinel)
for i, (x, y) in enumerate(pairs):
if x == y:
continue
elif x == sentinel:
self.fail('{0}: iterator ended unexpectedly '
'at position {1}; expected {2}'.format(test_id, i, y))
elif y == sentinel:
self.fail('{0}: unexpected excess element {1} at '
'position {2}'.format(test_id, x, i))
else:
self.fail('{0}: wrong element at position {1};'
'expected {2}, got {3}'.format(test_id, i, y, x))
def test_xrange(self):
self.assertEqual(list(xrange(3)), [0, 1, 2])
self.assertEqual(list(xrange(1, 5)), [1, 2, 3, 4])
@ -67,6 +104,38 @@ class XrangeTest(unittest.TestCase):
self.assertEquals(list(pickle.loads(pickle.dumps(r, proto))),
list(r))
def test_range_iterators(self):
# see issue 7298
limits = [base + jiggle
for M in (2**32, 2**64)
for base in (-M, -M//2, 0, M//2, M)
for jiggle in (-2, -1, 0, 1, 2)]
test_ranges = [(start, end, step)
for start in limits
for end in limits
for step in (-2**63, -2**31, -2, -1, 1, 2)]
for start, end, step in test_ranges:
try:
iter1 = xrange(start, end, step)
except OverflowError:
pass
else:
iter2 = pyrange(start, end, step)
test_id = "xrange({0}, {1}, {2})".format(start, end, step)
# check first 100 entries
self.assert_iterators_equal(iter1, iter2, test_id, limit=100)
try:
iter1 = reversed(xrange(start, end, step))
except OverflowError:
pass
else:
iter2 = pyrange_reversed(start, end, step)
test_id = "reversed(xrange({0}, {1}, {2}))".format(
start, end, step)
self.assert_iterators_equal(iter1, iter2, test_id, limit=100)
def test_main():
test.test_support.run_unittest(XrangeTest)

78
Objects/rangeobject.c
View File

@ -9,33 +9,32 @@ typedef struct {
long len;
} rangeobject;
/* Return number of items in range/xrange (lo, hi, step). step > 0
* required. Return a value < 0 if & only if the true value is too
* large to fit in a signed long.
/* Return number of items in range (lo, hi, step). step != 0
* required. The result always fits in an unsigned long.
*/
static long
static unsigned long
get_len_of_range(long lo, long hi, long step)
{
/* -------------------------------------------------------------
If lo >= hi, the range is empty.
Else if n values are in the range, the last one is
lo + (n-1)*step, which must be <= hi-1. Rearranging,
n <= (hi - lo - 1)/step + 1, so taking the floor of the RHS gives
the proper value. Since lo < hi in this case, hi-lo-1 >= 0, so
the RHS is non-negative and so truncation is the same as the
floor. Letting M be the largest positive long, the worst case
for the RHS numerator is hi=M, lo=-M-1, and then
hi-lo-1 = M-(-M-1)-1 = 2*M. Therefore unsigned long has enough
precision to compute the RHS exactly.
---------------------------------------------------------------*/
long n = 0;
if (lo < hi) {
unsigned long uhi = (unsigned long)hi;
unsigned long ulo = (unsigned long)lo;
unsigned long diff = uhi - ulo - 1;
n = (long)(diff / (unsigned long)step + 1);
}
return n;
/* -------------------------------------------------------------
If step > 0 and lo >= hi, or step < 0 and lo <= hi, the range is empty.
Else for step > 0, if n values are in the range, the last one is
lo + (n-1)*step, which must be <= hi-1. Rearranging,
n <= (hi - lo - 1)/step + 1, so taking the floor of the RHS gives
the proper value. Since lo < hi in this case, hi-lo-1 >= 0, so
the RHS is non-negative and so truncation is the same as the
floor. Letting M be the largest positive long, the worst case
for the RHS numerator is hi=M, lo=-M-1, and then
hi-lo-1 = M-(-M-1)-1 = 2*M. Therefore unsigned long has enough
precision to compute the RHS exactly. The analysis for step < 0
is similar.
---------------------------------------------------------------*/
assert(step != 0);
if (step > 0 && lo < hi)
return 1UL + (hi - 1UL - lo) / step;
else if (step < 0 && lo > hi)
return 1UL + (lo - 1UL - hi) / (0UL - step);
else
return 0UL;
}
static PyObject *
@ -43,7 +42,7 @@ range_new(PyTypeObject *type, PyObject *args, PyObject *kw)
{
rangeobject *obj;
long ilow = 0, ihigh = 0, istep = 1;
long n;
unsigned long n;
if (!_PyArg_NoKeywords("xrange()", kw))
return NULL;
@ -64,11 +63,8 @@ range_new(PyTypeObject *type, PyObject *args, PyObject *kw)
PyErr_SetString(PyExc_ValueError, "xrange() arg 3 must not be zero");
return NULL;
}
if (istep > 0)
n = get_len_of_range(ilow, ihigh, istep);
else
n = get_len_of_range(ihigh, ilow, -istep);
if (n < 0) {
n = get_len_of_range(ilow, ihigh, istep);
if (n > (unsigned long)LONG_MAX || (long)n > PY_SSIZE_T_MAX) {
PyErr_SetString(PyExc_OverflowError,
"xrange() result has too many items");
return NULL;
@ -78,7 +74,7 @@ range_new(PyTypeObject *type, PyObject *args, PyObject *kw)
if (obj == NULL)
return NULL;
obj->start = ilow;
obj->len = n;
obj->len = (long)n;
obj->step = istep;
return (PyObject *) obj;
}
@ -98,7 +94,9 @@ range_item(rangeobject *r, Py_ssize_t i)
"xrange object index out of range");
return NULL;
}
return PyInt_FromSsize_t(r->start + i * r->step);
/* do calculation entirely using unsigned longs, to avoid
undefined behaviour due to signed overflow. */
return PyInt_FromLong((long)(r->start + (unsigned long)i * r->step));
}
static Py_ssize_t
@ -304,9 +302,21 @@ range_reverse(PyObject *seq)
len = ((rangeobject *)seq)->len;
it->index = 0;
it->start = start + (len-1) * step;
it->step = -step;
it->len = len;
/* the casts below guard against signed overflow by turning it
into unsigned overflow instead. The correctness of this
code still depends on conversion from unsigned long to long
wrapping modulo ULONG_MAX+1, which isn't guaranteed (see
C99 6.3.1.3p3) but seems to hold in practice for all
platforms we're likely to meet.
If step == LONG_MIN then we still end up with LONG_MIN
after negation; but this works out, since we've still got
the correct value modulo ULONG_MAX+1, and the range_item
calculation is also done modulo ULONG_MAX+1.
*/
it->start = (long)(start + (unsigned long)(len-1) * step);
it->step = (long)(-(unsigned long)step);
return (PyObject *)it;
}