The result of Math.log(3.0) is different on x86_64 and aarch64?

[Tianhua huang]

In fact, we found the differences when we took tests of apache/spark, there
are some related tests failed.

And till now we also found the results for 'power' function is different
between x86_64 and aarch64 for some cases, for an example:

on x86_64:

scala> import java.lang.Math._ import java.lang.Math._ scala>
abs(-83028485) res3: Int = 83028485 scala> var a = -83028485 a: Int =
-83028485 scala> abs(a) res4: Int = 83028485 scala> math.log(abs(a)) res5:
Double = 18.234694299654787 scala> pow(10, math.log(abs(a))) res6: Double =
*1.71669957511859584E18*

on aarch64:
scala> var a = -83028485 a: Int = -83028485 scala> abs(a) res38: Int =
83028485 scala> math.log(abs(a)) res39: Double = 18.234694299654787 scala>
pow(10, math.log(abs(a))) res40: Double = *1.71669957511859558E18*

So the algorithm of these math functions(power, log, maybe there are
others) is different on platforms? How or where I can find these
information? Could anyone help us? Thank you very much.
I don't know the behavior is correct or not, or maybe the tests of spark is
not appropriate?




On Thu, Jul 25, 2019 at 4:11 PM Tianhua huang <huangtianhua223 at gmail.com>
wrote:

> Sorry to disturb you again, I took some tests of java.lang.Math.log
> function on aarch64 server, I met a strange case, the result of
> Math.log(3.0) is different with x86_64:
> on x86_64:
> scala> Math.log(3.0) res50: Double = 1.0986122886681098
>
> but on aarch64:
> scala> Math.log(3.0) res19: Double = 1.0986122886681096
>
> Then I tried  Math.log(4.0), Math.log(5.0) and some other numbers, the
> results on x86_64 and aarch64 are same, why Math.log(3.0) is so special?
> this confused me much, I think the result of
> Math.log(Double) should be same across platforms, right?
>
>