On Mon, 17 Apr 2023 13:27:43 GMT, olivergillespie <d...@openjdk.org> wrote:
>> Improve the speed of Enum.hashCode by caching the identity hashcode on first >> use. I've seen an application where Enum.hashCode is a hot path, and this is >> fairly simple speedup. The memory overhead is low; in enums with no extra >> fields there is already a 4-byte space due to alignment so this new field >> can slot in 'for free'. In other cases, the singleton nature of enum values >> means that the number of total instances is typically very low, so a small >> per-instance overhead is not a concern. >> >> Please see more discussion/explanation in the [original enhancement >> request](https://bugs.openjdk.org/browse/JDK-8306075). >> >> ### Benchmark >> >> >> >> Before: >> >> Benchmark Mode Cnt Score Error Units >> # Intel Cascade lake >> EnumHashCode.constant avgt 15 1.602 ± 0.011 ns/op >> EnumHashCode.field avgt 15 1.681 ± 0.014 ns/op >> # Arm Neoverse N1 >> EnumHashCode.constant avgt 15 1.642 ± 0.033 ns/op >> EnumHashCode.field avgt 15 1.717 ± 0.059 ns/op >> >> >> >> After: >> >> Benchmark Mode Cnt Score Error Units >> # Intel Cascade lake >> EnumHashCode.constant avgt 15 0.479 ± 0.001 ns/op >> EnumHashCode.field avgt 15 0.799 ± 0.002 ns/op >> # Arm Neoverse N1 >> EnumHashCode.constant avgt 15 0.802 ± 0.002 ns/op >> EnumHashCode.field avgt 15 1.059 ± 0.056 ns/op >> >> >> Using `-prof perfasm` on the benchmark, we can compare the generated code >> for x86_64: >> >> Before: >> >> │ 0x00007fae4868dd17: lea (%r12,%r10,8),%rsi ;*getfield e >> {reexecute=0 rethrow=0 return_oop=0} >> │ ; - >> org.sample.EnumHashCode::field@1 (line 24) >> │ ; - >> org.sample.jmh_generated.EnumHashCode_field_jmhTest::field_avgt_jmhStub@17 >> (line 186) >> │ 0x00007fae4868dd1b: mov (%rsi),%r10 >> │ 0x00007fae4868dd1e: mov %r10,%r11 >> │ 0x00007fae4868dd21: and $0x3,%r11 >> │ 0x00007fae4868dd25: cmp $0x1,%r11 >> │ 0x00007fae4868dd29: jne 0x00007fae4868dcc6 >> │ 0x00007fae4868dd2b: shr $0x8,%r10 >> │ 0x00007fae4868dd2f: mov %r10d,%eax >> │ 0x00007fae4868dd32: and $0x7fffffff,%eax >> │ 0x00007fae4868dd37: test %eax,%eax >> │ 0x00007fae4868dd39: je 0x00007fae4868dcc6 ;*invokespecial >> hashCode {reexecute=0 rethrow=0 return_oop=0} >> │ ; - >> java.lang.Enum::hashCode@1 (line 175) >> >> >> This is the normal Object.hashCode intrinsic, which involves reading the >> object header, extracting the hash code and handling two slow-path cases >> (displaced object header, hash not initialized). >> >> After: >> >> >> │ 0x00007f550068e3b4: mov 0x10(%r12,%r10,8),%r8d <-- read the hash >> field >> │ 0x00007f550068e3b9: test %r8d,%r8d <-- if (hash == 0) >> │ 0x00007f550068e3bc: je 0x00007f550068e413 <-- slow init >> path, only taken on first use >> >> >> Thanks @shipilev for help with the implementation and interpreting the >> generated code. > > olivergillespie has refreshed the contents of this pull request, and previous > commits have been removed. Incremental views are not available. The pull > request now contains one commit: > > 8306075: Micro-optimize Enum.hashCode src/java.base/share/classes/java/lang/Enum.java line 181: > 179: */ > 180: @Stable > 181: private int hash; Should not we hide the field from reflection? I understand this is an implementation detail, however, it makes a big difference that you add a private field to the *public* class that is *meant* to be inherited by user classes. Although I'm not aware of a particular example, this can be a breaking change for user code that does introspection on enums. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/13491#discussion_r1168724994
