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. ------------- Commit messages: - 8306075: Micro-optimize Enum.hashCode Changes: https://git.openjdk.org/jdk/pull/13491/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13491&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8306075 Stats: 74 lines in 2 files changed: 73 ins; 0 del; 1 mod Patch: https://git.openjdk.org/jdk/pull/13491.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/13491/head:pull/13491 PR: https://git.openjdk.org/jdk/pull/13491
