RFR (S) 8148869: StringConcatFactory MH_INLINE_SIZED_EXACT strategy does not work with -XX:-CompactStrings

[Aleksey Shipilev]

Hi,

Please review another simple fix in String concat:
  https://bugs.openjdk.java.net/browse/JDK-8148869

The failure happens because MH_INLINE_SIZED_EXACT strategy has to figure
out the coder for the String concat result, as the binary "or" of
initial coder and all argument's (possible) coders. And it mistakenly
starts with coder=0, which is String.LATIN1.

Under -XX:+CompactStrings (default), if all concat arguments can be
encoded in LATIN1, it should produce LATIN1 result; and if at least one
argument is UTF16, then the result should be UTF16.

The trouble comes with -XX:-CompactStrings, which should always produce
UTF16 concat result. Then, even if all arguments can be encoded into
LATIN1, the result should be UTF16. But it isn't, because the initial
coder is always LATIN1!

The fix is to make initial coder depend on Strings.COMPACT_STRINGS:
   http://cr.openjdk.java.net/~shade/8148869/webrev.01/

Since the failure is predicated on -XX:-CompactStrings, our current
tests fail only on platforms where CompactStrings are turned off by
default, e.g. AArch64. Added a new regression test that concats
differently encodeable chars under +|-CompactStrings.

Cheers,
-Aleksey