[OE-core] libm accuracy, eglibc compared to glibc

[Burton, Ross]

On 12 March 2014 15:30, Mats Kärrman <Mats.Karrman at tritech.se> wrote:
> Does anyone know if this is a known "feature" of eglibc or know any other reason
> for this difference?

I expect the standard response from upstream here would be that
floating point mathematics isn't exact and if you want exact answers
then you should use a different representation. The differences start
at the first sqrt():

Test 1 and 2:
sqrt(2.00000000000000000000) = 1.41421356237309514547
Test 3:
sqrt(2.00000000000000000000) = 1.41421356237309492343
Test using bc and scale=30 (bc is fixed-point not floating-point):
sqrt(2.00000000000000000000) = 1.414213562373095048801688724209

The output from bc is a truncation of the "real" value of sqrt(2)
whereas both values from C's sqrt() function using floats are
approximations, both equally close.  Why does your test case expect
the answer to be 2.0?  That's a massive assumption to make, floats can
trivially lose accuracy rapidly as
http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems
explains.

Ross