RFR: 8330465: Stable Values and Collections (Internal) [v5]

[Chen Liang]

On Thu, 16 May 2024 06:54:26 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> Maybe the `state == NULL` check should be moved before `v != null`, as the **JIT** doesn’t constant‑fold `null` [`@Stable`] values:
>> https://github.com/openjdk/jdk/blob/8a4315f833f3700075d65fae6bc566011c837c07/src/java.base/share/classes/jdk/internal/vm/annotation/Stable.java#L41-L44  https://github.com/openjdk/jdk/blob/8a4315f833f3700075d65fae6bc566011c837c07/src/java.base/share/classes/jdk/internal/vm/annotation/Stable.java#L64-L71
>> 
>> [`@Stable`]: https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/vm/annotation/Stable.java
>
> It seems reasonable to assume `null` values are not constant-folded. For straight-out-of-the-box usage, there is no apparent significant difference as indicated by a new benchmark I just added:
> 
> 
> Benchmark                                  Mode  Cnt      Score      Error   Units
> StableStaticBenchmark.atomic              thrpt   10   5729.683 ?  502.023  ops/us
> StableStaticBenchmark.dcl                 thrpt   10   6069.222 ?  951.784  ops/us
> StableStaticBenchmark.dclHolder           thrpt   10   5502.102 ? 1630.627  ops/us
> StableStaticBenchmark.stable              thrpt   10  12737.158 ? 1746.456  ops/us <- Non-null benchmark
> StableStaticBenchmark.stableHolder        thrpt   10  12053.978 ? 1421.527  ops/us
> StableStaticBenchmark.stableList          thrpt   10  12443.870 ? 2084.607  ops/us
> StableStaticBenchmark.stableNull          thrpt   10  13164.232 ?  591.284  ops/us <- Added null benchmark
> StableStaticBenchmark.stableRecordHolder  thrpt   10  13638.893 ? 1250.895  ops/us
> StableStaticBenchmark.staticCHI           thrpt   10  13639.220 ? 1190.922  ops/us
> 
> 
> If the `null` value participates in a much larger constant-folding tree, there might be a significant difference. I am afraid moving the order would have detrimental effects on instance performance:
> 
> Checking value first:
> 
> 
> Benchmark                           Mode  Cnt     Score      Error   Units
> StableBenchmark.atomic             thrpt   10   246.460 ?   75.417  ops/us
> StableBenchmark.dcl                thrpt   10   243.481 ?   35.021  ops/us
> StableBenchmark.stable             thrpt   10  4977.693 ?  675.926  ops/us  <- Non-null
> StableBenchmark.stableHoldingList  thrpt   10  3614.460 ?  275.140  ops/us
> StableBenchmark.stableList         thrpt   10  3328.155 ?  898.202  ops/us
> StableBenchmark.stableListStored   thrpt   10  3842.174 ?  535.902  ops/us
> StableBenchmark.stableNull         thrpt   10  6217.737 ?  840.376  ops/us <- null
> StableBenchmark.supplier           thrpt   10  9369.934 ? 1449.182  ops/us
> 
> 
> Checking null first:
> 
> 
> Benchmark                           Mode  Cnt     Score      Error   Units
> StableBenchmark.atomic             thrpt   10   287.640 ?   17.858  ops/us
> StableBenchmark.dcl                thrpt   10   250.398 ?   20.874  ops/us
> StableBenchmark.stable             thrpt   10  3745.885 ? 1040.534  ops/us <- Non-null
> StableBenchmark.stableHoldingList  thrpt   10  2982.129 ?  503.492  ops/us
> StableBenchmark.stableList         thrpt   10  3125.045 ?  416.792  ops/us
> StableBenchmark.sta...

I think the result would be more convincing if the stable case is changed to `sum += (stable.orThrow() == null ? 0 : 1) + (stable2.orThrow() == null ? 0 : 1);` as adding by 1 might be somewhat better optimized by JIT.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18794#discussion_r1603148915