On Tue, 11 Nov 2025 16:48:58 GMT, Jorn Vernee <[email protected]> wrote:
>> I've added two new benchmarks:
>>
>>
>> private static final LazyConstant<Optional<Integer>> OPTIONAL_42 =
>> LazyConstant.of(() -> Optional.of(42));
>> private static final LazyConstant<Optional<Integer>> OPTIONAL_42_2 =
>> LazyConstant.of(() -> Optional.of(42));
>> private static final LazyConstant<Optional<Integer>> OPTIONAL_EMPTY =
>> LazyConstant.of(Optional::empty);
>> private static final LazyConstant<Optional<Integer>> OPTIONAL_EMPTY2 =
>> LazyConstant.of(Optional::empty);
>>
>> ...
>>
>> @Benchmark
>> public int staticOptional42() {
>> return OPTIONAL_42.get().orElseThrow() +
>> OPTIONAL_42_2.get().orElseThrow();
>> }
>>
>> @Benchmark
>> public boolean staticOptionalEmpty() {
>> return OPTIONAL_EMPTY.get().isEmpty() ^
>> OPTIONAL_EMPTY2.get().isEmpty();
>> }
>>
>>
>>
>>
>> Which gives:
>>
>>
>> Benchmark Mode Cnt Score Error Units
>> StableValueBenchmark.staticOptional42 avgt 10 0.354 ± 0.045 ns/op
>>
>> Benchmark Mode Cnt Score Error Units
>> StableValueBenchmark.staticOptionalEmpty avgt 10 0.370 ± 0.030 ns/op
>>
>>
>> So, both `Optional` variants appears to support constant folding.
>
> I don't think we can drawn that conclusion just based on the speed. For the
> non-constant folded cases, this would be 2 loads and an xor.
>
> I don't see how C2 would be allowed to constant fold the loads if the value
> is `null`, since that's explicitly forbidden by `@Stable` (and there's no
> easy way around that AFAIK, since the JIT thread may see a partially
> initialized object, so it conservatively ignores default values).
Yes. I think you are right @JornVernee. I've explored several ways to fix this
and the simplest one might be to have a sentinel value instead of `null` to
flag emptiness. Splitting the class into a bimorphic implementation would not
be good for `value` classes (at least not for now), even though the C2 might
otherwise be able to optimize the two variants.
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PR Review Comment: https://git.openjdk.org/jdk/pull/27605#discussion_r2527523848
