[aarch64-port-dev ] RFR: 8215100: AArch64: fix compareTo intrinsic with four-character Latin/Unicode

[Nick Gasson (Arm Technology China)]

Hi,

Please help me review this patch to fix an intermittent failure of
java/util/Calendar/Bug8167273.java on AArch64.

Webrev: http://cr.openjdk.java.net/~njian/8215100/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8215100
(also https://bugs.openjdk.java.net/browse/JDK-8209402)

The calendar test failure reported in JDK-8209402 happens because it
can't find a particular key in a TreeMap, but if you dump out the
contents of that TreeMap, the key is present but the tree is not
sorted correctly. 

It's caused by a bug in the compareTo intrinsic that's triggered when
both arguments are four characters long but one is Latin1 encoded and
one is Unicode encoded. In that case the "Compare longwords" loop
header (isLU or else branches) will branch to the TAIL_CHECK label
with tmp1 (isLU) or tmp2 (isUL) uninitialized. The instructions after
TAIL_CHECK assume tmp1 and tmp2 contain the final 64-bit words of each
string, and use these to compute the difference between the final
characters. In the LU/UL case this result is garbage.

Fix by handling four-character mixed U/L strings in the SHORT_STRING
loop (currently this is done for length < 4). Also extended the
existing string intrinsics test to catch this.

Thanks,
Nick