[issue7117] Backport py3k float repr to trunk

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

Some key revision numbers from the py3k short float repr, for reference:

r71663:  include Gay's code, build and configure fixes
r71723:  backout SSE2 detection code added in r71663
r71665:  rewrite of float formatting code to use Gay's code

Backported most of r71663 and r71723 to trunk in:

r75651: Add dtoa.c, dtoa.h, update license.
r75658: configuration changes - detect float endianness,
        add functions to get and set x87 control word, and
        determine when short float repr can be used.

Significant changes from r71663 not yet included:

* Misc/NEWS update
* Lib/test/formatfloat_testcases.txt needs updating to match py3k.

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