RFR: 8196334: Optimize UUID#fromString

[Peter Levart]

Sorry, bash this.

I just noticed that the units used in benchmark are ops/us and that the 
best performing code is the one which is in place now.

I usually use ns/op as a unit in JMH benchmarks, so this led me to false 
belief that alternatives are better performing where in fact are worse.

Sorry for noise.

Regards, Peter


On 3/8/20 12:46 PM, Peter Levart wrote:
> Hi,
>
> When I noticed this optimization, I had some ideas how to push this 
> even further, but only now had some time to try them out.
>
> Here are some incremental improvements [1] to the already brilliant 
> idea with the following JMH results (the baseline is JDK build with 
> patch for 8196334 already applied):
>
> Benchmark                     (size)   Mode  Cnt   Score   Error Units
> UUIDBench.uuidFromString       20000  thrpt    5  37.344 ± 0.257 ops/us
> UUIDBench.uuidFromStringAlt1   20000  thrpt    5  29.069 ± 0.137 ops/us
> UUIDBench.uuidFromStringAlt2   20000  thrpt    5  27.308 ± 0.204 ops/us
> UUIDBench.uuidFromStringAlt3   20000  thrpt    5  23.195 ± 0.885 ops/us
>
> As can be seen, the trick was to eliminate branches (alt1, alt2) and 
> to reduce the number of operations (shifts) in alt3. One last trick in 
> alt3 was to use the array length in a check that is then fused with 
> array index bounds check (I think) so that we get one of them for free.
>
> What do you think? Is this good enough to warrant another change? As 
> you can see, I haven't prepared the patch yet,  just benchmark.
>
> Regards, Peter
>
> [1] 
> http://cr.openjdk.java.net/~plevart/jdk-dev/8196334_UUID.fromString/UUIDBench.java
>
>
> On 3/2/20 8:26 AM, Claes Redestad wrote:
>> On 2020-03-02 08:14, Alan Bateman wrote:
>>>>
>>>> http://cr.openjdk.java.net/~redestad/8196334/open.02/
>>> Looks good.
>>
>> Thanks - pushed!
>>
>> /Claes
>