RFR: 8322292: Rearrange comparison of fields in Record.equals() [v4]

[ExE Boss]

On Wed, 20 Dec 2023 10:11:58 GMT, Sergey Tsypanov <stsypanov at openjdk.org> wrote:

>> Currently if we create a record it's fields are compared in their declaration order. This might be ineffective in cases when two objects have "heavy" fields equals to each other, but different "lightweight" fields (heavy and lightweight in terms of comparison) e.g. primitives, enums, nullable/non-nulls etc.
>> 
>> If we have declared a record like
>> 
>> public record MyRecord(String field1, int field2) {}
>> 
>> It's equals() looks like:
>> 
>>   public final equals(Ljava/lang/Object;)Z
>>    L0
>>     LINENUMBER 3 L0
>>     ALOAD 0
>>     ALOAD 1
>>     INVOKEDYNAMIC equals(Lcom/caspianone/openbanking/productservice/controller/MyRecord;Ljava/lang/Object;)Z [
>>       // handle kind 0x6 : INVOKESTATIC
>>       java/lang/runtime/ObjectMethods.bootstrap(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/TypeDescriptor;Ljava/lang/Class;Ljava/lang/String;[Ljava/lang/invoke/MethodHandle;)Ljava/lang/Object;
>>       // arguments:
>>       com.caspianone.openbanking.productservice.controller.MyRecord.class,
>>       "field1;field2",
>>       // handle kind 0x1 : GETFIELD
>>       com/caspianone/openbanking/productservice/controller/MyRecord.field1(Ljava/lang/String;),
>>       // handle kind 0x1 : GETFIELD
>>       com/caspianone/openbanking/productservice/controller/MyRecord.field2(I)
>>     ]
>>     IRETURN
>>    L1
>>     LOCALVARIABLE this Lcom/caspianone/openbanking/productservice/controller/MyRecord; L0 L1 0
>>     LOCALVARIABLE o Ljava/lang/Object; L0 L1 1
>>     MAXSTACK = 2
>>     MAXLOCALS = 2
>> 
>> This can be improved by rearranging the comparison order of the fields moving enums and primitives upper, and collections/arrays lower.
>
> Sergey Tsypanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8322292: Tiny improvement

src/java.base/share/classes/java/lang/runtime/ObjectMethods.java line 227:

> 225:             }
> 226:             return rt1.isPrimitive() || rt1.isEnum() || rt1.isArray() ? 1 : Iterable.class.isAssignableFrom(rt1) ? -1 : 0;
> 227:         });

The way this comparator is currently implemented, the `signum(c.compare(mh1, mh2)) == -signum(c.compare(mh2, mh1))` assertion won’t hold, which will cause the sorting to be unstable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17143#discussion_r1434166820