On Tue, 11 Jun 2024 08:58:34 GMT, Aleksey Shipilev <sh...@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. ------------- PR Comment: https://git.openjdk.org/jdk/pull/19433#issuecomment-2222532168
