RFR: 8332249: Micro-optimize Method.hashCode [v2]

[Aleksey Shipilev]

On Tue, 11 Jun 2024 08:58:34 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Sean Gwizdak has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
>> 
>>  - Remove trailing whitespace.
>>  - Move hashCode benchmark into the newly created MethodBenchmark file
>>  - Merge branch 'master' into method-hashcode-JDK-8332249
>>  - Remove changes to JavaDoc per guidance.
>>  - Fix whitespace issues pointed by the bot
>>  - Micro-optimize Method.hashCode
>
> As usual in these cases, we need to make a footprint argument as well. `Method` is a special class with lots of injected fields, so the only "true" source of layout information is `-XX:+PrintFieldLayout`. It tells me there is a 4-byte tail due to object alignment in compressed oops mode. This can accommodate a new `int` field. There seems to be no space when compressed oops are disabled.
> 
> 
> # Out of the box, compressed oops enabled
> Layout of class java/lang/reflect/Method
> Instance fields:
>  @0 12/- RESERVED
>  @12 "override" Z 1/1 INHERITED
>  @13 "callerSensitive" B 1/1 REGULAR
>  @14 2/1 EMPTY
>  @16 "accessCheckCache" Ljava/lang/Object; 4/4 INHERITED
>  @20 "parameterData" Ljava/lang/reflect/Executable$ParameterData; 4/4 INHERITED
>  @24 "declaredAnnotations" Ljava/util/Map; 4/4 INHERITED
>  @28 "slot" I 4/4 REGULAR
>  @32 "modifiers" I 4/4 REGULAR
>  @36 "clazz" Ljava/lang/Class; 4/4 REGULAR
>  @40 "name" Ljava/lang/String; 4/4 REGULAR
>  @44 "returnType" Ljava/lang/Class; 4/4 REGULAR
>  @48 "parameterTypes" [Ljava/lang/Class; 4/4 REGULAR
>  @52 "exceptionTypes" [Ljava/lang/Class; 4/4 REGULAR
>  @56 "signature" Ljava/lang/String; 4/4 REGULAR
>  @60 "genericInfo" Lsun/reflect/generics/repository/MethodRepository; 4/4 REGULAR
>  @64 "annotations" [B 4/4 REGULAR
>  @68 "parameterAnnotations" [B 4/4 REGULAR
>  @72 "annotationDefault" [B 4/4 REGULAR
>  @76 "methodAccessor" Ljdk/internal/reflect/MethodAccessor; 4/4 REGULAR
>  @80 "root" Ljava/lang/reflect/Method; 4/4 REGULAR
> Instance size = 88 bytes
> 
> # -XX:-UseCompressedOops
> Layout of class java/lang/reflect/Method
> Instance fields:
>  @0 12/- RESERVED
>  @12 "override" Z 1/1 INHERITED
>  @13 "callerSensitive" B 1/1 REGULAR
>  @14 2/1 EMPTY
>  @16 "accessCheckCache" Ljava/lang/Object; 8/8 INHERITED
>  @24 "parameterData" Ljava/lang/reflect/Executable$ParameterData; 8/8 INHERITED
>  @32 "declaredAnnotations" Ljava/util/Map; 8/8 INHERITED
>  @40 "slot" I 4/4 REGULAR
>  @44 "modifiers" I 4/4 REGULAR
>  @48 "clazz" Ljava/lang/Class; 8/8 REGULAR
>  @56 "name" Ljava/lang/String; 8/8 REGULAR
>  @64 "returnType" Ljava/lang/Class; 8/8 REGULAR
>  @72 "parameterTypes" [Ljava/lang/Class; 8/8 REGULAR
>  @80 "exceptionTypes" [Ljava/lang/Class; 8/8 REGULAR
>  @88 "signature" Ljava/lang/String; 8/8 REGULAR
>  @96 "genericInfo" Lsun/reflect/generics/repository/MethodRepository; 8/8 REGULAR
>  @104 "annotations" [B 8/8 REGULAR
>  @112 "parameterAnnotations" [B 8/8 REGULAR
>  @120 "annotationDefault" [B 8/8 REGULAR
>  @128 "methodAccessor" Ljdk/internal/reflect/MethodAccessor; 8/8 REGULAR
>  @136 "root" Ljava/...

> @shipilev Any recommendations on reducing footprint size?

I see two ways from here:

1. There seems to be a 2-byte gap in the object, so if we can squeeze out 2 additional bytes, it would allow us to put another `int` in. It looks like `slot` does not have to be `int`: on VM side, it is `u2` (unsigned 2-byte), which might be just `short` or `char` with proper precautions about signs and conversions. That would probably be a bit ugly.

2. Ignore this regression and move on. It would have been a non-issue if `Method`-s were canonicalized/de-duplicated in some way. But AFAICS, `Method`-s are getting copied from `Class.getDeclaredMethods`, so it is probable we have quite a few instances floating around, and footprint regressions would stack up.

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PR Comment: https://git.openjdk.org/jdk/pull/19433#issuecomment-2222532168