RFR: 8013395 StringBuffer.toString performance regression impacting embedded benchmarks

[David Holmes]

Short version:

Cache the value returned by toString and use it to copy-construct a new 
String on subsequent calls to toString(). Clear the cache on any 
mutating operation.

webrev: http://cr.openjdk.java.net/~dholmes/8013395/webrev.v2/

Testing: microbenchmark for toString performance; new regression test 
for correctness; JPRT testset core as a sanity check

Still TBD - full SE benchmark (?)

Thanks,
David
---------

Long version:

One of the goals for JDK8 is to provide a path from Java ME CDC to Java 
SE (or SE Embedded). In the embedded space some pretty old benchmarks 
still get used for doing comparisons between JRE's. One of which makes 
heavy use of StringBuffer.toString, without modifying the StringBuffer 
in between.

Up to Java 1.4.2 a StringBuffer and a String could share the underlying 
char[]. This meant that toString simply needed to create a new String 
that referenced the StringBuffer's char[] with no copying of the array 
needed. In Java 5 the String/StringBuffer implementations were 
completely revised: StringBuilder was introduced for non-synchronized 
use, and a new AbstractStringBuilder base class added for it and 
StringBuffer. In that implementation toString now has to copy the 
StringBuffer's char[]. This resulted in a significant performance 
regression for toString() and a bug - 6219959 - was opened. There is 
quite an elaborate evaluation in that bug report but bottom line was 
that "real code doesn't depend on this - won't fix".

At some stage ME also updated to the new Java 5 code and they also 
noticed the problem. As a result CDC6 included a variation of the 
caching strategy that is proposed here.

Going forward because we want people to be able to compare ME and SE 
with their familiar benchmarks, we would like to address this corner 
case and fix it using the caching strategy outlined. As a data point an 
8K StringBuffer that takes ~1ms to be converted to a String initially, 
can process subsequent toString() calls in a few microseconds. So that 
performance issue is addressed.

However we've added a write to a field in all the mutating methods which 
obviously adds some additional computational effort - though I have no 
doubt it is lost in the noise for all but the smallest of mutating 
methods. Even so this should be run against regular SE benchmarks to 
ensure there are no performance regressions there - so if anyone has a 
suggestion as to the best benchmark to run to exercise StringBuffer (if 
it exists), please let me know.

Thanks for reading this far :)