RFR: JDK-8286666: JEP 429: Implementation of Scoped Values (Incubator) [v10]

[Andrew Haley]

On Fri, 4 Nov 2022 09:50:10 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> src/jdk.incubator.concurrent/share/classes/jdk/incubator/concurrent/ScopedValue.java line 481:
>> 
>>> 479:         }
>>> 480:  */
>>> 481:         return findBinding() != Snapshot.NIL;
>> 
>> This should probably call `Cache.put(this, value)` when `findBinding()` isn’t `Snapshot.NIL`, since it’s likely that `isBound()` will most commonly be used in the form of:
>> 
>> if (SCOPED_VALUE.isBound()) {
>> 	final var value = SCOPED_VALUE.get();
>> 	// do something with `value`
>> }
>> 
>> 
>> --------------------------------------------------------------------------------
>> 
>> Suggestion:
>> 
>>         var value = findBinding();
>>         if (value == Snapshot.NIL) {
>>             return false;
>>         }
>>         Cache.put(this, value);
>>         return true;
>
> Probably so, yes. I'll have a look at that along with caching failure.

So I just did the experiment of caching failures and the result of `isBound()`.

This test:


    @Benchmark
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int thousandMaybeGets(Blackhole bh) throws Exception {
        int result = 0;
        for (int i = 0; i < 1_000; i++) {
            if (ScopedValuesData.sl1.isBound()) {
                result += ScopedValuesData.sl1.get();
            }
        }
        return result;
    }



Before and after:


ScopedValues.thousandMaybeGets       avgt   10  13436.112 ± 20.885  ns/op
ScopedValues.thousandMaybeGets       avgt   10     56.315 ± 0.583  ns/op


You may have a point. The experiment is on a branch  called `JDK-8286666-cache-queries` in [My personal repo](https://github.com/theRealAph/jdk).

I'd push it now but it's getting a bit late to make such changes now. WDYT?

-------------

PR: https://git.openjdk.org/jdk/pull/10952