RFR 8199843 : Optimize Integer/Long.highestOneBit()

[Ivan Gerasimov]

Hello!

The hightestOneBit function doesn't have an intrinsic and is currently 
implemented with a dozen of instructions.
Alternatively, it could be implemented as MIN_VALUE >>> 
numberOfLeadingZeros(i), which works for all integers but zero.
The former function gets intrisified by hotspot, which results in +27% 
of throughput (see the jmh results below).

Would you please help review this simple fix?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8199843
WEBREV: http://cr.openjdk.java.net/~igerasim/8199843/00/webrev/
Benchmark: http://cr.openjdk.java.net/~igerasim/8199843/00/MyBenchmark.java

Benchmark results:

Benchmark                        (arg)   Mode  Cnt Score          Error  
Units
MyBenchmark.int_testMethod_new       0  thrpt   35 323430664.593 ±  
7492044.171  ops/s
MyBenchmark.int_testMethod_new      42  thrpt   35 298526237.078 ±  
5978291.689  ops/s
MyBenchmark.int_testMethod_new     -42  thrpt   35 302903562.073 ±  
7984723.721  ops/s
MyBenchmark.int_testMethod_org       0  thrpt   35 236245042.891 ±  
3635990.596  ops/s
MyBenchmark.int_testMethod_org      42  thrpt   35 237903410.753 ±  
3437684.390  ops/s
MyBenchmark.int_testMethod_org     -42  thrpt   35 238472580.618 ±  
2654886.010  ops/s
MyBenchmark.long_testMethod_new      0  thrpt   35 282646114.501 ± 
48028366.305  ops/s
MyBenchmark.long_testMethod_new     42  thrpt   35 282382228.405 ±  
5781529.307  ops/s
MyBenchmark.long_testMethod_new    -42  thrpt   35 276724858.286 ±  
6529561.227  ops/s
MyBenchmark.long_testMethod_org      0  thrpt   35 198500211.972 ± 
15096862.367  ops/s
MyBenchmark.long_testMethod_org     42  thrpt   35 215854630.194 ±  
3112930.563  ops/s
MyBenchmark.long_testMethod_org    -42  thrpt   35 217992805.521 ±  
2622877.082  ops/s

Thanks in advance!

-- 
With kind regards,
Ivan Gerasimov