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

[STINNER Victor]

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

> Can't we simply use (approximation to 1/log(2)) * log(x)?
> Is it worse than reimplementing it using log(x)/log(2) in Python?

Hum. With a x86 and the right compiler optimization level, log(x)/log(2) in C can be more accurate than log(x)/log(2) in Python, because the FPU works with 80 bits float internally, and the result is only "truncated" to 64 bits float at the end. In Python, the result is truncated to 64 bits on each Python instruction.

I don't know if it should be called a feature or a bug. In PHP world, it would be called a bug :-D http://bugs.php.net/bug.php?id=53632

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<http://bugs.python.org/issue11888>
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