A "RecursiveConstants" attribute

[Dan Smith]

> On Feb 17, 2018, at 4:01 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
>> Most classes should naturally use backward references exclusively, since entries tend to be generated bottom-up.
>> (In theory. If there are counter examples produced by any mainstream bytecode generators, that would be good to know.)
> 
> Currently, javac and ecj tends to generate their constants in the "wrong order".
> ecj:
>  #15 = Methodref          #16.#18        // java/lang/Class.desiredAssertionStatus:()Z
>  #16 = Class              #17            // java/lang/Class
>  #17 = Utf8               java/lang/Class
>  #18 = NameAndType        #19:#20        // desiredAssertionStatus:()Z
>  #19 = Utf8               desiredAssertionStatus
>  #20 = Utf8               ()Z
> 
> javac:
>  ...
>  #20 = Methodref          #65.#66        // java/lang/Class.desiredAssertionStatus:()Z
>  ...
>  #65 = Class              #75            // java/lang/Class
>  #66 = NameAndType        #76:#77        // desiredAssertionStatus:()Z
>  ...
>  #75 = Utf8               java/lang/Class
>  #76 = Utf8               desiredAssertionStatus
>  #77 = Utf8               ()Z
> 
> It means nothing now because we only cares about constants that can introduce cycles so CONSTANT_Dynamic, it may be an issue in the future if by example a NameAndType can reference a constant with cycle, the code of javac and ecj will have to be changed.

Yep, looks like my depth-first assumption is wrong.

Here's a javac constant pool for an empty class:

   #1 = Methodref          #3.#10         // java/lang/Object."<init>":()V
   #2 = Class              #11            // ConstantPool
   #3 = Class              #12            // java/lang/Object
   #4 = Utf8               <init>
   #5 = Utf8               ()V
   #6 = Utf8               Code
   #7 = Utf8               LineNumberTable
   #8 = Utf8               SourceFile
   #9 = Utf8               ConstantPool.java
  #10 = NameAndType        #4:#5          // "<init>":()V
  #11 = Utf8               ConstantPool
  #12 = Utf8               java/lang/Object

I can kind of squint and see the logic, but there's no simple pattern here. Forward and backward references both occur.

I'll ask our code gen experts about what they'd expect from Dynamic, and whether it's easy to assume they always point in one direction or the other.

—Dan
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