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

[Xerxes Rånby]

On 2010-05-24 15:44, Andrew John Hughes wrote:
> 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?
>   
Im only able to test x86:

But no javac still produces different ressult when compiling

static double log_result = Math.log(log_value);

and running javac on x86 and arm using the same version of hotspot:

ARCH+JVM combination                                log_result (javac)      log_result2 (jit)       passes regression test?
ia32+OpenJDK Shark VM (build 16.0-b13, mixed mode)  9.752490228984199       9.752490228984199       yes
arm+OpenJDK Shark VM (build 16.0-b13, mixed mode)   9.75249022898412        9.752490228984199       no