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

Xerxes Rånby xerxes at
Mon May 24 12:55:02 UTC 2010


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:
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: 

Cheers and have a great day!

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