Math.log(17197) produces inconsistent results between javac, jit, Math and StrictMath on various architectures.

[Andrew John Hughes]

On 24 May 2010 13:55, Xerxes Rånby <xerxes at zafena.se> wrote:
> Hi
>
> Im hoping to find someone who can enlighten me how to resolve a bug
> where some hotspot JITs
> fails the hotspot/test/compiler/6539464 jtreg regression test.
>
> The testcase looks like:
>
> public class Test {
>    static double log_value = 17197;
>    static double log_result = Math.log(log_value);
>
>    public static void main(String[] args) throws Exception {
>        for (int i = 0; i < 1000000; i++) {
>            double log_result2 = Math.log(log_value);
>            if (log_result2 != log_result) {
>                throw new InternalError("Math.log produces inconsistent results: " + log_result2 + " != " + log_result);
>            }
>        }
>    }
> }
>
> When running this testcase using various jvms / architectures i get varying results of the calculated log_result2 and log_result which
>
> ARCH+JVM combination                                         log_result (javac)      log_result2 (jit)       passes regression test?
> ia32+OpenJDK Server VM (build 14.0-b16, mixed mode)          9.752490228984199       9.752490228984199       yes
> arm+OpenJDK Shark VM (build 16.0-b13, mixed mode)            9.75249022898412        9.752490228984199       no
> x86_64+OpenJDK 64-Bit Server VM (build 14.0-b16, mixed mode) 9.75249022898412        9.752490228984199       no
>
> While trying to figure out whats the correct result for the log(17197) calculation i tested what StrictMath.log outputs:
> StrictMath.log(17197) = 9.75249022898412
>
> And in order to get an independent mathematically correct opinion of what log(17197) equals i asked wolfram alpha:
> http://www.wolframalpha.com/input/?i=log%2817197%29
> log(17197) = 9.7524902289841994797298917760120602447583441794533189705223...
> or in short 9.752490228984199
>
> So we have a situation where javac, jit, Math and StrictMath generates inconsistent results.
> How do we resolve this situation?
>
> Quoting the Math package java doc:
> "If a method always has an error less than 0.5 ulps, the method always
>  returns the floating-point number nearest the exact result; such a
>  method is correctly rounded.  A correctly rounded method is
>  generally the best a floating-point approximation can be; however,
>  it is impractical for many floating-point methods to be correctly
>  rounded.  Instead, for the Math class, a larger error
>  bound of 1 or 2 ulps is allowed for certain methods"
>
> Would this quotation imply that both log(17197) calculation results above are correct ( 9.752490228984199 and 9.75249022898412 )?
> If both calculations are correct then I propose that we should change the hotspot/test/compiler/6539464 jtreg regression test to make all
> JVM's pass the test when they are within +- 1 ulps from the mathematically correct answer of:
> 9.7524902289841994797298917760120602447583441794533189705223...
>
> Cheers and have a great day!
> Xerxes
>
>

Do x86 and x86_64 give the same results if you actually use the same
version of HotSpot as you do on ARM?
-- 
Andrew :-)

Free Java Software Engineer
Red Hat, Inc. (http://www.redhat.com)

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