From b7b7d7ad6c97456d6d2c1dc9808ddb737a2a0713 Mon Sep 17 00:00:00 2001 From: Sergey B Kirpichev Date: Mon, 26 May 2025 05:44:33 +0300 Subject: [PATCH] gh-132876: workaround broken ldexp() on Windows 10 (GH-133135) * 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 cf8941c60356acdd00055e5583a2d64761c34af4) Co-authored-by: Sergey B Kirpichev Co-authored-by: Tim Peters --- Lib/test/test_math.py | 6 ++++++ ...-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst | 4 ++++ Modules/mathmodule.c | 21 +++++++++++++++++++ 3 files changed, 31 insertions(+) create mode 100644 Misc/NEWS.d/next/Library/2025-04-29-11-48-46.gh-issue-132876.lyTQGZ.rst 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; }