RFR: 8224986: (str) optimize StringBuilder.append(CharSequence, int, int) for String arguments

[Claes Redestad]

Hi,

it's been pointed out[1] that append(string.substring(start, end)) can
outperform append(string, start, end) in some circumstances. In part due
the former being intrinsified, but also I think due it tickling some
string concat optimizations that allow for eliding the StringBuilder
itself completely when there's only one non-constant argument.

While not ruling out intrinsification of append(String, int, int), I was
able to achieve a reasonable speed-up by peeling off String arguments
to append(CharSequence, int, int) and dropping into a routine that uses
System.arraycopy when appropriate.

This seems like a reasonable improvement in the interim, and an
opportunity to incorporate the relevant microbenchmarks.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8224986
Webrev: http://cr.openjdk.java.net/~redestad/8224986/open.00/

I extended and incorporated the microbenchmark provided in [1] to also 
include variants with UTF16 Strings and mixes of arguments, ensuring
speedups on all appendBounds* variants[2].

Thanks!

/Claes

[1] 
http://mail.openjdk.java.net/pipermail/core-libs-dev/2019-January/057834.html
[2]
Baseline:

Benchmark            (length)  Mode  Cnt     Score     Error   Units
appendBounds             1000  avgt   10   576.939 ±  21.125   ns/op
  ·gc.alloc.rate.norm     1000  avgt   10  2144.000 ±   0.001    B/op
appendBoundsMix          1000  avgt   10   855.308 ±  27.071   ns/op
  ·gc.alloc.rate.norm     1000  avgt   10  3160.000 ±   0.001    B/op
appendBoundsUtf16        1000  avgt   10  1424.518 ±  33.319   ns/op
  ·gc.alloc.rate.norm     1000  avgt   10  5192.000 ±   0.001    B/op

Patch:

Benchmark            (length)  Mode  Cnt     Score     Error   Units
appendBounds             1000  avgt   10   466.640 ±  15.069   ns/op
  ·gc.alloc.rate.norm     1000  avgt   10  2120.000 ±   0.001    B/op
appendBoundsMix          1000  avgt   10   758.886 ±  22.983   ns/op
  ·gc.alloc.rate.norm     1000  avgt   10  3160.000 ±   0.001    B/op
appendBoundsUtf16        1000  avgt   10  1320.360 ±  49.097   ns/op
  ·gc.alloc.rate.norm     1000  avgt   10  5192.000 ±   0.001    B/op