RFR 8199843 : Optimize Integer/Long.highestOneBit()

[Peter Levart]

Hi Ivan,

What about branch-less variant?

public static int highestOneBit(int i) {
     return i & (MIN_VALUE >>> numberOfLeadingZeros(i));
}

Would it be any better for call sites that vary 0 and non-0 argument?

Regards, Peter

On 03/20/2018 09:58 AM, Ivan Gerasimov wrote:
> 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!
>