RFR 8209171 : Simplify Java implementation of Integer/Long.numberOfTrailingZeros()

[Ivan Gerasimov]

Okay, your last variant is the winner :)

Here's the updated webrev, benchmark and the graphs:

http://cr.openjdk.java.net/~igerasim/8209171/02/webrev/
http://cr.openjdk.java.net/~igerasim/8209171/02/bench/int/MyBenchmark.java
http://cr.openjdk.java.net/~igerasim/8209171/02/bench/int/bench-int-04-client.png
http://cr.openjdk.java.net/~igerasim/8209171/02/bench/int/bench-int-04-server.png

With kind regards,
Ivan

On 8/13/18 10:10 AM, Ivan Gerasimov wrote:
> Hi Martin!
>
> Good point about the command line flags, thanks!
>
> These variants are close to numberOfTrailingZeros_07 that I've already 
> tested, though you did better by saving one arithmetic operation at 
> the return line!
>
> I'll rerun the benchmarks.
>
> With kind regards,
>
> Ivan
>
>
> On 8/13/18 7:56 AM, Martin Buchholz wrote:
>> The number of plausible variants is astonishing!
>>
>> ---
>>
>> Your use of -client and -server is outdated, which explains why you 
>> get the same results for both (-client is ignored).
>>
>> I'm not sure what's blessed by hotspot team, but for C1 I use 
>> -XX:+TieredCompilation -XX:TieredStopAtLevel=1 and for C2 I use 
>> -XX:-TieredCompilation -server
>>
>> ---
>>
>> Now I understand the advantage of using ~i & (i - 1): the subsequent 
>> zero check is a short-circuit for all odd numbers, better than i & 
>> -i, which explains your results - they depend on being able to 
>> short-circuit.
>>
>> So just use a more faithful inlining of nlz without trying to improve 
>> on it.
>>
>>     static int ntz_inlineNlz5(int i) {
>>         i = ~i & (i - 1);
>>         if (i <= 0)
>>             return (i == 0) ? 0 : 32;
>>         int n = 1;
>>         if (i >= 1 << 16) { n += 16; i >>>= 16; }
>>         if (i >= 1 <<  8) { n +=  8; i >>>= 8; }
>>         if (i >= 1 <<  4) { n +=  4; i >>>= 4; }
>>         if (i >= 1 <<  2) { n +=  2; i >>>= 2; }
>>         return n + (i >>> 1);
>>     }
>>
>> But it's hard to resist the urge to optimize out a branch:
>>
>>     static int ntz_inlineNlz6(int i) {
>>         i = ~i & (i - 1);
>>         if (i <= 0) return i & 32;
>>         int n = 1;
>>         if (i >= 1 << 16) { n += 16; i >>>= 16; }
>>         if (i >= 1 <<  8) { n +=  8; i >>>= 8; }
>>         if (i >= 1 <<  4) { n +=  4; i >>>= 4; }
>>         if (i >= 1 <<  2) { n +=  2; i >>>= 2; }
>>         return n + (i >>> 1);
>>     }
>>
>

-- 
With kind regards,
Ivan Gerasimov