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

[Hannes Greule]

On Mon, 18 Dec 2023 13:42:35 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.

Generally, this is a good idea. A few comments:

1. Arrays are compared by reference rather than element-wise, so they should/could be at the beginning similar to enums, not at the end.
2. You are sorting the array passed to the bootstrap method. This might be observable to callers, potentially causing problems, especially when e.g. creating the method handle for equals first, and then for toString with the same array.
3. What's the test situation here? Is there enough coverage for things like that already?

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PR Comment: https://git.openjdk.org/jdk/pull/17143#issuecomment-1862179369