[aarch64-port-dev ] The result of Math.log(3.0) is different on x86_64 and aarch64?

[Joe Darcy]

On 7/29/2019 7:44 AM, Andrew Dinn wrote:
> On 29/07/2019 15:25, Martin Buchholz wrote:
>> I was surprised to see the doc of Math.log make it explicit:
>>
>> https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Math.html#log(double)
>>
>> """The computed result must be within 1 ulp of the exact result. Results
>> must be semi-monotonic."""
> Yes, indeed. I had to look up semi-monotonic even though I really ought
> to have known what it meant :-)
>
> Anyway, there is no surprise really once you correlate the thoroughness
> of that documentation (and the rest ...) with the inestimably efficient
> purring of the engine that is Joe Darcy's brain. This is one area that
> really can be nailed down by spec and it is very nice to see that it has
> been.

To give credit where it is due, before I joined the JDK group way back 
in JDK 1.3 Tim Lindholm added the changes to address

     JDK-4245947: "Resolve tension between performance and 
reproducibility in Math class"

I had input on the changes and IIRC Prof. Kahan provided some guidance 
as well. The "Improving Java for Numerical Computation" report [1] from 
the Java Grande numerics working group circa 1998 also called for 
updates to the math library to allow high-performance results as well as 
reproducible results. The solution was to introduce a new class, 
java.lang.StrictMath to provide operational reproduciblity and to 
re-define the java.lang.Math class to allow any algorithm that met the 
stated quality of implementation criteria. A consequence of that 
implementation flexibility is that for many methods of java.lang.Math it 
is *not* required that the same value be returned for the same input 
across different implementations.

HTH,

-Joe

[1] 
https://math.nist.gov/javanumerics/reports/jgfnwg-01.htmlhttps://math.nist.gov/javanumerics/reports/jgfnwg-01.html