JEP draft: Efficient array comparison intrinsics

(rough draft; needs formatting; see http://cr.openjdk.java.net/~mr/jep/jep-2.0-02.html)

There are efficient operations (both safe and unsafe) for copying arrays, but none for comparing them. Let's fix this so users don't have to roll their own.

At a minimum, the following intrinsic would allow suitable higher-level comparison operations to be implemented:

/** Find the index of a mismatch between src and dest, or -1 if none. */
System.arrayMismatch((Object src, int srcPos,
    Object dest, int destPos, int length, boolean fromEnd);

• Returns an index in src of an element which differs from the corresponding element in dest, if such an element exists.
• If no such element exists, returns -1.
• If fromEnd is true, returns the index of the last differing element, else returns the first.
• Correspondences between elements are as for System.arraycopy.
• The result will be either -1 or a value between srcPos and srcPos+length-1, inclusive.
• Exceptions are as for System.arraycopy.
• The arrays must have compatible element types, either both the same primitive type or both reference types.
• Comparisons are done as if by the Java == operator.
• In particular references to different objects differ, even if Object.equals would return true.
• Also, floating point NaNs always differ, even from themselves.

This mechanism is easy to code and easy to vectorize. Most higher-level array comparison operations can readily be built on top of it.

Basic comparison operations (at least lexicographic compareTo) using it should be added to java.util.Arrays.

String.compareTo can be re-implemented on top of this intrinsic. Methods of the form String.mismatchTo should be added.

As a possible follow-up, System.arrayIndexOf could be added to perform a vectorized non-anchored search of one array inside another. This loop is less interesting, however, since it would be too narrow for some use cases involving NaNs, structural object comparisons, or case-insensitive searches.