VM model and aconst_init

[Brian Goetz]

Both value and primitive classes use the aconst_init / withfield 
initialization protocol.  The former is an L-type (QOptional is 
illegal); the latter uses Q types for initialization.

Value classes
  -> are L types
  -> which are references
  -> references can be null
  -> VM manages where the nullity bits are

so for value classes, there has to be some way of starting the ball 
rolling with an uninitialized, but non-null, value.  We can then modify 
the fields (but not the nullity) with `withfield`.

On 1/12/2022 7:31 AM, Remi Forax wrote:
> I've some troubles to wrap my head around those two sentences
>
> """
> aconst_init is the analogue of new for value objects; it leaves a reference to the initial value for a value class on the stack. This initial value is guaranteed to not be equal to null. The sole operand of this bytecode is a reference to a CONSTANT_Class item giving the internal binary name of the value class (not its Q descriptor).
> """
>
> and
> """
> Both withfield and aconst_init return a Q type if and only if their class is a primitive class.
> """
>
> The second is ambiguous because it's not clear if aconst_init can return a L-type. I suppose it can not but this is not clear at all.
>
> If this is the case, what is the use case for withfield taking a L-type as parameter ??
>
> regards,
> Rémi