On Mon, 17 Apr 2023 14:15:39 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 updated the pull request incrementally with one > additional commit since the last revision: > > Fix two typos src/java.base/share/classes/java/lang/Enum.java line 177: > 175: * HotSpot's identity hash code generation also never returns zero > 176: * as the identity hash code. This makes zero a convenient marker > 177: * for the un-initialized value for both {@link @Stable} and the lazy I'm sorry, but `@Stable` won't work with `@link`. `@Stable` is not a class, interface, member name, etc. See https://docs.oracle.com/en/java/javase/20/docs/specs/javadoc/doc-comment-spec.html#references ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/13491#discussion_r1168783477
