[issue11888] Add C99's log2() function to the math library

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

> we should check that it's not pow that's at fault here

Some tests on Mac OS X Tiger:

>>> (2.0 ** -255).hex()
'0x1.0000000000000p-255'

=> pow is correct

>>> import ctypes; import ctypes.util, math
>>> libc = ctypes.cdll.LoadLibrary(ctypes.util.find_library('c'))
>>> clog2=libc.log2
>>> clog2.restype=ctypes.c_double
>>> clog2.argtypes=(ctypes.c_double,)
>>> clog2(2.0**-255)
-254.99999999999997
>>> math.log(2.0**-255) / math.log(2.0)
-255.0

>>> math.log(2.0**-255)
-176.75253104278605
>>> math.log(2.0**-255).hex()
'-0x1.61814bbfb3fb5p+7'
>>> math.log(2.0)
0.6931471805599453
>>> math.log(2.0).hex()
'0x1.62e42fefa39efp-1'

>>> clog2(2.0**-255).hex()
'-0x1.fdfffffffffffp+7'
>>> (math.log(2.0**-255) / math.log(2.0)).hex()
'-0x1.fe00000000000p+7'

clog2() is wrong for 2^-255.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11888>
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