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

[Paul Sandoz]

> On 18 May 2016, at 14:09, Aleksey Shipilev <aleksey.shipilev at oracle.com> wrote:
> 
> 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)
> 

+1

I think this is reasonable as this execution mode has been soaking for a while with existing tests, and is easy to revert if there are issues.

Paul.

> 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
>