RFR (M) 8148604: JEP 280, Switch to more optimal concatenation strategy

[Aleksey Shipilev]

Hi,

We have been waiting for this for at least 8 weeks, waiting for Jigsaw
to settle in, and hs-main -> jdk9/dev integration to happen that brings
a few testbug fixes into the jdk9/dev.

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8148604

Webrev:
  http://cr.openjdk.java.net/~shade/8148604/webrev.01/
 (the change is XS, but impact is M)

Targeted benchmarks show significant improvements with the
MH_INLINE_SIZED_EXACT strategy, see the bug for the test results.
Performance improvements are due to these reasons:

 *) it does not rely on OptimizeStringConcat in C2, but instead does
lift up the concatenation into the Java code, spelling it out with
java.lang.invoke building blocks. This also helps to improve C1
performance significantly;

 *) it uniformly covers the cases that OptimizeStringConcat does not
cover, e.g. non-int/char arguments where OptimizeStringConcat bails, etc;

 *) it uniformly covers the Compact Strings (JEP 254) changes, e.g.
selecting the final coder for the concatenated String, and calling
appropriate coder-specific routines from JDK. Doing this in Java has the
apparent benefit of piggy-backing on normal Java code profiling to
figure out the hot branches -- a luxury OptimizeStringConcat-constructed
code lacks.

Unfortunately, we cannot test this with our regular performance testing,
because it requires tests compiled with -target 9. The only easy target
we have now is javac, and switching the concat strategy does not regress
it (see JDK-8148605).

MH_INLINE_SIZED_EXACT is functionally tested routinely with
java/lang/String/concat/ jtregs on all platforms.

Testing: java/lang/String/ jtregs; JPRT -testset hotspot

Thanks,
-Aleksey