Java 8 RFR 8010430: Math.round has surprising behavior for odd values of ulp 1

[Joseph Darcy]

Hello,

On 8/23/2013 1:36 PM, Guy Steele wrote:
> The specification of java.lang.Math.round in the first edition of the
> Java Language Specification is quite clear:
>
> public static int round(float a)
>    The result is rounded to an integer by adding 1/2, taking the floor of
>    the result, and casting the result to type int.
>    In other words, the result is equal to the value of the expression
> 	(int)Math.floor(a + 0.5f)
>
> and a similar statement for the case where the type of the argument is double.
>
> This does not correspond to "rounding away from zero" as in IEEE754.
>
> The phrase "ties rounding up" entered the Java documentation later on
> as a (perhaps unfortunately worded) shorthand for the original specification.

To give some context, the original specification was changed in JDK 7 
under bug

     JDK-6430675 Math.round has surprising behavior for 0x1.fffffffffffffp-2
     http://bugs.sun.com/view_bug.do?bug_id=6430675

At the time, it was making the rounds that Math.round gave a "wrong" 
answer for an input value equal to the largest floating-point value less 
than 0.5: Math.round(0.49999999999999994) returned 1 rather than 0. The 
result is wrong in terms of being unexpected; it was the result mandated 
by the operational definition of the specification.

I addressed JDK-6430675 by changing the implementation and removing the 
operational definition of Math.round. While I thought I had ruled out 
unexpected round-off behavior at the other end of the range, mea culpa, 
I did not account for the issues reported in 8010430.

-Joe

>
> --Guy Steele
>
>
> On Aug 23, 2013, at 4:24 PM, Dmitry Nadezhin <dmitry.nadezhin at gmail.com> wrote:
>
>> The specification of java.lang.Math.round() says
>>     * Returns the closest {@code int} to the argument, with ties
>>      * rounding up.
>>
>> It is not clarified what is "ties rounding up".
>> I guess that it should correspond to the direction "roundTiesToAway" of
>> IEEE 754-2008
>> and to the java.math.RoundingMode.HALF_UP .
>>
>> They round
>> +0.5 -> +1
>> -0.5 -> -1
>>
>> The current implementation of java.lang.Math.round() rounds
>> +0.5 -> +1
>> -0.5 -> 0
>>
>> "ties rounding up" should match IEEE754 standard and other JDK math class,
>> shouldn't it ?
>>
>>
>> On Fri, Aug 23, 2013 at 10:32 PM, Brian Burkhalter <
>> brian.burkhalter at oracle.com> wrote:
>>
>>> This message follows the RFC
>>> http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-August/019560.htmlposted on August 2.
>>>
>>> The issue is http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8010430.
>>>
>>> The proposed patch http://cr.openjdk.java.net/~bpb/8010430/ has the
>>> effect of option (A) in the aforementioned RFC.
>>>
>>> Thanks,
>>>
>>> Brian