mirrors/cpython
1
0
Fork 0
mirror of https://github.com/python/cpython.git synced 2025-08-04 17:08:35 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 13

[3.13] gh-132876: workaround broken ldexp() on Windows 10 (GH-133135) (#134685)

Browse source 
* gh-132876: workaround broken ldexp() on Windows 10

ldexp() fails to round subnormal results before Windows 11,
so hide their bug.
(cherry picked from commit cf8941c603)

Co-authored-by: Tim Peters <tim.peters@gmail.com>
This commit is contained in:
Sergey B Kirpichev 2025-05-26 06:39:34 +03:00 • committed by GitHub
parent f331d03e88
commit e483dcf19e
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
3 changed files with 31 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

6
Lib/test/test_math.py
View file

@ -1202,6 +1202,12 @@ class MathTests(unittest.TestCase):
self.assertEqual(math.ldexp(NINF, n), NINF)
self.assertTrue(math.isnan(math.ldexp(NAN, n)))
@requires_IEEE_754
def testLdexp_denormal(self):
# Denormal output incorrectly rounded (truncated)
# on some Windows.
self.assertEqual(math.ldexp(6993274598585239, -1126), 1e-323)
def testLog(self):
self.assertRaises(TypeError, math.log)
self.assertRaises(TypeError, math.log, 1, 2, 3)

4
Misc/NEWS.d/next/Library/2025-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst Normal file
View file

@ -0,0 +1,4 @@
``ldexp()`` on Windows doesn't round subnormal results before Windows 11,
but should. Python's :func:`math.ldexp` wrapper now does round them, so
results may change slightly, in rare cases of very small results, on
Windows versions before 11.

21
Modules/mathmodule.c
View file

@ -2166,6 +2166,27 @@ math_ldexp_impl(PyObject *module, double x, PyObject *i)
} else {
errno = 0;
r = ldexp(x, (int)exp);
#ifdef _MSC_VER
if (DBL_MIN > r && r > -DBL_MIN) {
/* Denormal (or zero) results can be incorrectly rounded here (rather,
truncated). Fixed in newer versions of the C runtime, included
with Windows 11. */
int original_exp;
frexp(x, &original_exp);
if (original_exp > DBL_MIN_EXP) {
/* Shift down to the smallest normal binade. No bits lost. */
int shift = DBL_MIN_EXP - original_exp;
x = ldexp(x, shift);
exp -= shift;
}
/* Multiplying by 2**exp finishes the job, and the HW will round as
appropriate. Note: if exp < -DBL_MANT_DIG, all of x is shifted
to be < 0.5ULP of smallest denorm, so should be thrown away. If
exp is so very negative that ldexp underflows to 0, that's fine;
no need to check in advance. */
r = x*ldexp(1.0, (int)exp);
}
#endif
if (Py_IS_INFINITY(r))
errno = ERANGE;
}