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bpo-46640: Py_NAN now uses the C99 NAN constant #31134

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Feb 6, 2022
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bpo-46640: Py_NAN now uses the C99 NAN constant #31134

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bpo-46640: Py_NAN now uses the C99 NAN constant
vstinner Feb 4, 2022
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Update Py_NAN comment
vstinner Feb 6, 2022
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6 changes: 6 additions & 0 deletions Doc/whatsnew/3.11.rst
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Expand Up @@ -604,6 +604,12 @@ Build Changes
``isinf()``, ``isnan()``, ``round()``.
(Contributed by Victor Stinner in :issue:`45440`.)

* Building Python now requires a C99 ``<math.h>`` header file providing
a ``NAN`` constant, or the ``__builtin_nan()`` built-in function. If a
platform does not support Not-a-Number (NaN), the ``Py_NO_NAN`` macro can be
defined in the ``pyconfig.h`` file.
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@mdickinson mdickinson Feb 6, 2022

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It turns out that building under Py_NO_NAN doesn't actually work right now. I've opened an issue: https://bugs.python.org/issue46656

(Contributed by Victor Stinner in :issue:`46640`.)

* Freelists for object structs can now be disabled. A new :program:`configure`
option :option:`!--without-freelists` can be used to disable all freelists
except empty tuple singleton.
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26 changes: 8 additions & 18 deletions Include/pymath.h
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Expand Up @@ -56,24 +56,14 @@
* doesn't support NaNs.
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@mdickinson mdickinson Feb 6, 2022

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With this change, the comment about INF*0 or INF/INF working is out of date; that sentence should probably be removed.

*/
#if !defined(Py_NAN) && !defined(Py_NO_NAN)
# if !defined(__INTEL_COMPILER)
# define Py_NAN (Py_HUGE_VAL * 0.)
# else /* __INTEL_COMPILER */
# if defined(ICC_NAN_STRICT)
#pragma float_control(push)
#pragma float_control(precise, on)
#pragma float_control(except, on)
Py_NO_INLINE static double __icc_nan()
{
return sqrt(-1.0);
}
#pragma float_control (pop)
# define Py_NAN __icc_nan()
# else /* ICC_NAN_RELAXED as default for Intel Compiler */
static const union { unsigned char buf[8]; double __icc_nan; } __nan_store = {0,0,0,0,0,0,0xf8,0x7f};
# define Py_NAN (__nan_store.__icc_nan)
# endif /* ICC_NAN_STRICT */
# endif /* __INTEL_COMPILER */
# if _Py__has_builtin(__builtin_nan)
// Built-in implementation of the ISO C99 function nan(): quiet NaN.
# define Py_NAN (__builtin_nan(""))
#else
// Use C99 NAN constant: quiet Not-A-Number (when supported).
// NAN is a float, Py_NAN is a double: cast to double.
# define Py_NAN ((double)NAN)
# endif
#endif

#endif /* Py_PYMATH_H */
4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Build/2022-02-04-21-26-50.bpo-46640.HXUmQp.rst
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@@ -0,0 +1,4 @@
Building Python now requires a C99 ``<math.h>`` header file providing a ``NAN``
constant, or the ``__builtin_nan()`` built-in function. If a platform does not
support Not-a-Number (NaN), the ``Py_NO_NAN`` macro can be defined in the
``pyconfig.h`` file. Patch by Victor Stinner.