https://github.com/python/cpython/commit/cf8941c60356acdd00055e5583a2d64761c34af4
commit: cf8941c60356acdd00055e5583a2d64761c34af4
branch: main
author: Sergey B Kirpichev <skirpic...@gmail.com>
committer: tim-one <tim.pet...@gmail.com>
date: 2025-05-25T21:44:33-05:00
summary:
gh-132876: workaround broken ldexp() on Windows 10 (#133135)
* gh-132876: workaround broken ldexp() on Windows 10
ldexp() fails to round subnormal results before Windows 11,
so hide their bug.
Co-authored-by: Tim Peters <tim.pet...@gmail.com>
files:
A Misc/NEWS.d/next/Library/2025-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst
M Lib/test/test_math.py
M Modules/mathmodule.c
diff --git a/Lib/test/test_math.py b/Lib/test/test_math.py
index 913a60bf9e04e3..d14336f8bac498 100644
--- a/Lib/test/test_math.py
+++ b/Lib/test/test_math.py
@@ -1214,6 +1214,12 @@ def testLdexp(self):
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)
diff --git
a/Misc/NEWS.d/next/Library/2025-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst
b/Misc/NEWS.d/next/Library/2025-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst
new file mode 100644
index 00000000000000..cb3ca3321e3d26
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst
@@ -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.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 40abd69f0a6600..71d9c1387f5780 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -2161,6 +2161,27 @@ 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 (isinf(r))
errno = ERANGE;
}
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