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

2 files changed, 45 insertions, 0 deletions

diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 4928d13..51c85d4 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -266,6 +266,19 @@ always available.
       The information in the table is simplified.
 
 
+.. data:: float_repr_style
+
+   A string indicating how the :func:`repr` function behaves for
+   floats.  If the string has value ``'short'`` then for a finite
+   float ``x``, ``repr(x)`` aims to produce a short string with the
+   property that ``float(repr(x)) == x``.  This is the usual behaviour
+   in Python 3.1 and later.  Otherwise, ``float_repr_style`` has value
+   ``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
+   versions of Python prior to 3.1.
+
+   .. versionadded:: 3.1
+
+
 .. function:: getcheckinterval()
 
    Return the interpreter's "check interval"; see :func:`setcheckinterval`.
diff --git a/Doc/license.rst b/Doc/license.rst
index 7ae9f56..eb266a7 100644
--- a/Doc/license.rst
+++ b/Doc/license.rst
@@ -657,3 +657,35 @@ The :mod:`select` and contains the following notice for the kqueue interface::
   LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   SUCH DAMAGE.
+
+
+strtod and dtoa
+---------------
+
+The file :file:`Python/dtoa.c`, which supplies C functions dtoa and
+strtod for conversion of C doubles to and from strings, is derived
+from the file of the same name by David M. Gay, currently available
+from http://www.netlib.org/fp/.  The original file, as retrieved on
+March 16, 2009, contains the following copyright and licensing
+notice::
+
+   /****************************************************************
+    *
+    * The author of this software is David M. Gay.
+    *
+    * Copyright (c) 1991, 2000, 2001 by Lucent Technologies.
+    *
+    * Permission to use, copy, modify, and distribute this software for any
+    * purpose without fee is hereby granted, provided that this entire notice
+    * is included in all copies of any software which is or includes a copy
+    * or modification of this software and in all copies of the supporting
+    * documentation for such software.
+    *
+    * THIS SOFTWARE IS BEING PROVIDED "AS IS", WITHOUT ANY EXPRESS OR IMPLIED
+    * WARRANTY.  IN PARTICULAR, NEITHER THE AUTHOR NOR LUCENT MAKES ANY
+    * REPRESENTATION OR WARRANTY OF ANY KIND CONCERNING THE MERCHANTABILITY
+    * OF THIS SOFTWARE OR ITS FITNESS FOR ANY PARTICULAR PURPOSE.
+    *
+    ***************************************************************/
+
+