Parametric VM class file format

[John Rose]

On Apr 20, 2021, at 9:40 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> Hi all,
> at least as an exercise to understand the proposed class file format for the parametric VM, i will update ASM soon (in a branch) to see how things work.

Thank you!

> As usual with ASM, there is the question of sharing the same index in the constant pool,
> i.e. should two anchors (SpecializationAnchor) that have the same kind and the same BSM + constant arguments share the same constant pool index ?

Yes, I don’t see why not.  For a Class-level SA it would
be a bug if there were two of them in one classfile.

> And same question with two linkages (SpecializationLinkage).

Definitely.  Just like you’d want to unique-ify
CONSTANT_Class items.

> I believe the answer is yes for both, the index is shared.
> 
> The other questions are about "ldc Linkage", first, why do we need a ldc Linkage with a Linkage that reference a class given that we already have Condy ?

Condy can do a lot of this stuff, but it shouldn’t duplicate
work that the JVM is already likely to have done.
If you have code using a C_Linkage[C_Class,] the JVM
has probably resolved it to a species.  In that case,
a condy is wasted work.

We did without ldc C_Class for a while but added it
for similar reasons.  Once we added it we had no
desire to go back to the workarounds.  I predict the
same for “C_Species” which is C_Linkage[C_Class,].

> And second question, is there a ldc Linkage with the linkage referencing something else than a class ?

No!  It would be possible to find a meaning for such
a thing, but it would probably interfere with ldc
of C_Linkage[C_Class,].  (I tried.)  The workaround
is not bad:  Just wrap the C_Linkage[C_Methodref]
in a C_MethodHandle, et voilà.

An earlier draft made ldc of a C_Linkage recover
the SpecializationAnchor object, with the theory
that from there condy gets you everything.  But
I turned away from that design because (a) it was
clunky to use, and (b) it exposed SA objects which,
as I came to understand, should really be encapsulated
and private to their defining class.

— John