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Issue #1580: use short float repr where possible.
 - incorporate and adapt David Gay's dtoa and strtod into the Python core - on platforms where we can use Gay's code (almost all!), repr(float) is based on the shortest sequence of decimal digits that rounds correctly. - add sys.float_repr_style attribute to indicate whether we're using Gay's code or not - add autoconf magic to detect and enable SSE2 instructions on x86/gcc - slight change to repr and str: repr switches to exponential notation at 1e16 instead of 1e17, str switches at 1e11 instead of 1e12
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‎Doc/library/sys.rst‎

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The information in the table is simplified.
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.. data:: float_repr_style
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A string indicating how the :func:`repr` function behaves for
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floats. If the string has value ``'short'`` then for a finite
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float ``x``, ``repr(x)`` aims to produce a short string with the
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property that ``float(repr(x)) == x``. This is the usual behaviour
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in Python 3.1 and later. Otherwise, ``float_repr_style`` has value
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``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
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versions of Python prior to 3.1.
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.. versionadded:: 3.1
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.. function:: getcheckinterval()
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Return the interpreter's "check interval"; see :func:`setcheckinterval`.

‎Doc/license.rst‎

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@@ -657,3 +657,35 @@ The :mod:`select` and contains the following notice for the kqueue interface::
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LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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SUCH DAMAGE.
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strtod and dtoa
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---------------
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The file :file:`Python/dtoa.c`, which supplies C functions dtoa and
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strtod for conversion of C doubles to and from strings, is derived
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from the file of the same name by David M. Gay, currently available
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from http://www.netlib.org/fp/. The original file, as retrieved on
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March 16, 2009, contains the following copyright and licensing
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notice::
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/****************************************************************
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*
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* The author of this software is David M. Gay.
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*
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* Copyright (c) 1991, 2000, 2001 by Lucent Technologies.
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose without fee is hereby granted, provided that this entire notice
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* is included in all copies of any software which is or includes a copy
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* or modification of this software and in all copies of the supporting
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* documentation for such software.
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*
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* THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED
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* WARRANTY. IN PARTICULAR, NEITHER THE AUTHOR NOR LUCENT MAKES ANY
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* REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE MERCHANTABILITY
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* OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR PURPOSE.
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*
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***************************************************************/
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‎Include/Python.h‎

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#include "pystrtod.h"
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#include "pystrcmp.h"
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#include "dtoa.h"
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/* _Py_Mangle is defined in compile.c */
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PyAPI_FUNC(PyObject*) _Py_Mangle(PyObject *p, PyObject *name);

‎Include/dtoa.h‎

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#ifndef PY_NO_SHORT_FLOAT_REPR
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#ifdef __cplusplus
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extern "C" {
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#endif
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PyAPI_FUNC(double) _Py_dg_strtod(const char *str, char **ptr);
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PyAPI_FUNC(char *) _Py_dg_dtoa(double d, int mode, int ndigits,
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int *decpt, int *sign, char **rve);
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PyAPI_FUNC(void) _Py_dg_freedtoa(char *s);
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#ifdef __cplusplus
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}
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#endif
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#endif

‎Include/pymacconfig.h‎

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# undef SIZEOF_VOID_P
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# undef SIZEOF__BOOL
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# undef WORDS_BIGENDIAN
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# undef DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754
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# undef DOUBLE_IS_BIG_ENDIAN_IEEE754
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# undef DOUBLE_IS_LITTLE_ENDIAN_IEEE754
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# undef VA_LIST_IS_ARRAY
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# if defined(__LP64__) && defined(__x86_64__)
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#ifdef __BIG_ENDIAN__
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#define WORDS_BIGENDIAN 1
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#define DOUBLE_IS_BIG_ENDIAN_IEEE754
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#else
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#define DOUBLE_IS_LITTLE_ENDIAN_IEEE754
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#endif /* __BIG_ENDIAN */
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‎Include/pymath.h‎

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# endif
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#endif
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#ifdef HAVE_GCC_ASM_FOR_X87
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PyAPI_FUNC(unsigned short) _Py_get_387controlword(void);
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PyAPI_FUNC(void) _Py_set_387controlword(unsigned short);
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#endif
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/* Py_IS_NAN(X)
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* Return 1 if float or double arg is a NaN, else 0.
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* Caution:

‎Include/pyport.h‎

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errno = 0; \
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} while(0)
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/* The functions _Py_dg_strtod and _Py_dg_dtoa in Python/dtoa.c require that
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the FPU is using 53-bit precision. Here are macros that force this. See
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Python/pystrtod.c for an example of their use. */
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#ifdef USING_X87_FPU
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#define _Py_SET_53BIT_PRECISION_HEADER \
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unsigned short old_387controlword, new_387controlword
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#define _Py_SET_53BIT_PRECISION_START \
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do { \
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old_387controlword = _Py_get_387controlword(); \
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new_387controlword = (old_387controlword & ~0x0f00) | 0x0200; \
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if (new_387controlword != old_387controlword) \
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_Py_set_387controlword(new_387controlword); \
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} while (0)
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#define _Py_SET_53BIT_PRECISION_END \
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if (new_387controlword != old_387controlword) \
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_Py_set_387controlword(old_387controlword)
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#else
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#define _Py_SET_53BIT_PRECISION_HEADER
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#define _Py_SET_53BIT_PRECISION_START
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#define _Py_SET_53BIT_PRECISION_END
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#endif
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/* If we can't guarantee 53-bit precision, don't use the code
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in Python/dtoa.c, but fall back to standard code. This
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means that repr of a float will be long (17 sig digits).
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Realistically, there are two things that could go wrong:
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(1) doubles aren't IEEE 754 doubles, or
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(2) we're on x86 with the rounding precision set to 64-bits
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(extended precision), and we don't know how to change
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the rounding precision.
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*/
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#if !defined(DOUBLE_IS_LITTLE_ENDIAN_IEEE754) && \
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!defined(DOUBLE_IS_BIG_ENDIAN_IEEE754) && \
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!defined(DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754)
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#define PY_NO_SHORT_FLOAT_REPR
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#endif
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/* double rounding is symptomatic of use of extended precision on x86 */
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#ifdef X87_DOUBLE_ROUNDING
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#define PY_NO_SHORT_FLOAT_REPR
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#endif
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/* Py_DEPRECATED(version)
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* Declare a variable, type, or function deprecated.
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* Usage:

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