Array equality, comparison and mismatch

[Paul Sandoz]

> On 13 Oct 2015, at 12:03, Andrew Haley <aph at redhat.com> wrote:
> 
> On 13/10/15 10:22, Paul Sandoz wrote:
>> Analysis so far indicate big gains are to be had on larger arrays with better or no impact on small arrays if i do the following instead:
>> 
>>  if (Double.doubleToRawLongBits(a[i]) !=
>>      Double.doubleToRawLongBits(b[i])) {
>>      int c = Double.compare(a[i], b[i]);
>>      if (c != 0) return c;
>>  }
> 
> I was about to make a similar comment.  My experiment was with
> 
>        if (Double.doubleToRawLongBits(a[i]) != Double.doubleToRawLongBits(b[i])
>                && (Double.doubleToLongBits(a[i]) !=
>                    Double.doubleToLongBits(b[i])))
>                return Double.compare(a[i], b[i]);
>        }
> 
> which is about twice as fast as the original version, as is yours.
> But yours is more elegant.  :-)
> 

Thanks :-) updated:

  http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8033148-Arrays-lexico-compare/webrev/


> It's a shame that HotSpot doesn't see through the load of a
> double and then the conversion through doubleToRawLongBits:
> it could just load directly into the integer registers.
> 

The following webrev explicitly does what you mention above and is faster for mismatching double arrays (with no differing NaN values):

  http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8136924-arrays-mismatch-vectorized-unsafe/webrev/

I would be very interested in your opinion on being able to make intrinsic on ARM the method ArraysSupport.vectorizedMismatch [1].

Paul.

[1] http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8136924-arrays-mismatch-vectorized-unsafe/webrev/src/java.base/share/classes/java/util/ArraysSupport.java.html