Re: mauve results posted nightly

[Eric Blake]

Stephen Crawley wrote:
> > Perhaps japize 
> > should use Double.doubleToLongBits() to compare the bitwise value of 
> > double constants (likewise Float.floatToIntBits()).
>
> That would be a bit safer, considering that even JDK 1.4.x has minor
> bugs in its implementations of double -> string translation!  However,
> even comparing the bits returned Double.doubleToLongBits() is not
> strictly correct, because the Java VM spec says that VMs are allowed to
> use a range of bit patterns to represent NaN values.  

Double.doubleToLongBits is specified to flatten all doubles to the 
canonical version of NaN, 0x7ff8000000000000L.  You're thinking of 
Double.doubleToRawLongBits where NaN can be from a range of values. 
Mauve tests whether an implementation actually obeys this rule.

Of all the compilers and VMs I have seen for Java, the only one that 
currently gets double->String and float->String conversion 100% correct 
is jikes, version 1.16 or later.  There are several tests in the Jacks 
test suite for corner cases of string conversion; but I ought to port 
them to Mauve as well, to have runtime as well as compile-time tests.

(BTW, Sun JDK 1.4.x also has bugs in the string -> double translation, 
and I know at least one string which sends Sun's Double.parseDouble, and 
thus javac, into an infinite loop).

> The safest way for for japize to compare Java doubles (and floats) is to 
> do the following:
>
>    double d1, d2;
>    ...
>    boolean sameValue = (Double.isNaN(d1) && Double.isNaN(d2)) || (d1 == d2)
>
> Note: you have to test NaNs separately because the Java VM spec says that
> (NaN == NaN) always gives false!!

Not just the JVM - IEEE 754 also specified this relation!  But your test 
above does not work for 0.0 vs. -0.0.  The best comparison is 
(Double.compare(d1, d2) == 0), added in JDK 1.4; or (new 
Double(d1).compareTo(new Double(d2)) == 0) in JDK 1.2.  If you want 
japize to work with JDK 1.1, just copy the implementation of these 
methods from Classpath or libgcj.  These methods are tested by mauve.

--
This signature intentionally left boring.

Eric Blake             address@hidden
  BYU student, free software programmer