RFR 8170348: Appendable.appendN(char, int) method to append multiple copies of char

[Ivan Gerasimov]

Hello!

There are several places in JDK where the same character is appended to 
a StringBuilder object multiple times (usually padding).
With each append there are a few routine checks performed.
They could have been done only once, if we had a method for appending 
multiple copies at a time.
A simple benchmark shows that such method may save us a few machine 
cycles (see the results below).

In the benchmark, three approaches were compared:
0) Using the new appendN(char, int) method to append several chars at once,
1) Calling append(char) in a loop,
2) Appending a prepared-in-advance string

On my machine, the new method demonstrates better or comparable 
performance for all sizes up to 20.

In the webrev, there are two changesets included:
- the new default Appendable.appendN(char, int) method, its overrides in 
StringBuilder/Buffer and a basic test,
- several applications of the new method across JDK.

Would you please help review?
Comments, suggestions are welcome.

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8170348
WEBREV: http://cr.openjdk.java.net/~igerasim/8170348/00/webrev/
Benchmark: http://cr.openjdk.java.net/~igerasim/8170348/00/MyBenchmark.java


Benchmark                 (size)   Mode  Cnt Score          Error  Units
MyBenchmark.test_0_New         0  thrpt   70  331922128.215 ± 
16399254.452  ops/s
MyBenchmark.test_0_New         1  thrpt   70  209207932.893 ± 
14955800.231  ops/s
MyBenchmark.test_0_New         5  thrpt   70   72926671.621 ±  
4841791.555  ops/s
MyBenchmark.test_0_New        10  thrpt   70   67779575.053 ±  
3234366.239  ops/s
MyBenchmark.test_0_New        20  thrpt   70   59731629.661 ±  
2769497.288  ops/s
MyBenchmark.test_1_Old         0  thrpt   70  333467628.860 ± 
15981678.430  ops/s
MyBenchmark.test_1_Old         1  thrpt   70  156126381.967 ±  
9619653.294  ops/s
MyBenchmark.test_1_Old         5  thrpt   70   46550204.382 ±  
2009987.637  ops/s
MyBenchmark.test_1_Old        10  thrpt   70   23309297.849 ±  
1268874.282  ops/s
MyBenchmark.test_1_Old        20  thrpt   70   13143637.821 ±   
662265.103  ops/s
MyBenchmark.test_2_Solid       0  thrpt   70  138548108.540 ±  
6408775.462  ops/s
MyBenchmark.test_2_Solid       1  thrpt   70   63890936.132 ±  
3918274.970  ops/s
MyBenchmark.test_2_Solid       5  thrpt   70   65838879.075 ±  
2701493.698  ops/s
MyBenchmark.test_2_Solid      10  thrpt   70   65387238.993 ±  
3131562.548  ops/s
MyBenchmark.test_2_Solid      20  thrpt   70   57528150.828 ±  
3171453.716  ops/s


With kind regards,
Ivan