RFR: 8306075: Micro-optimize Enum.hashCode

[olivergillespie]

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 at 1 (line 24)
│                                                           ; - org.sample.jmh_generated.EnumHashCode_field_jmhTest::field_avgt_jmhStub at 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 at 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