the End of History in the constant pool

[John Rose]

(I'm moving an amber-dev conversation about condy to mlvm-dev.)

We are working on a condy JEP and spec. as well as a prototype, which is
good progress.  I'll post some info about that in a moment.

On May 18, 2017, at 12:16 PM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> I would prefer 17 to 21, because 21 is already used internally by ASM :)

I don't think anyone is objecting to 17 for CONSTANT_ConstantDynamic,
and I expect 17 is what we will land on.  It's been held open for (approximately)
this purpose.

Fun facts from History:  CONSTANT_17 was used by a prototype version of
invokedynamic which was discarded.  (The EG wisely discarded a couple of designs,
including a design without method handles!)  The prototype in that case overloaded
the invokeinterface instruction, in ways which are useless to recall.  In order to make
a clean break, we helped ourselves to another constant tag.  Soon after that,
I realized a future need for expression-like structures threaded through the constant
pool and bootstrap specifiers, although it was a bridge too far at the time.  So
we made no effort to "compact" our constant pool tag usage, knowing there might
be followup work in the future.

Also:  From the primordial days of Java there is CONSTANT_Unicode (tag 2)
which AFAIK has never been used from JDK 1 forward.  I think modern "take" on
character sets is to have one format for text (usually UTF8) and one for binary
octets.  (This is exemplified, for example, in CBOR.)  I expect some day to use
the constant tag 2 for such a purpose.  Basically, it would amount to giving class
files the power to "swallow" resource files (or smaller random byte snippets).
It has an obvious multiplicative effect on condy, but we don't need it yet, so
we are going with the minimal proposal.

I think the Ultimate, End-of-History CP tags are CONSTANT_ConstantDynamic,
CONSTANT_Data, and CONSTANT_Group.

The Group is simply a subsequence of CP values (probably of limited set of types).
It would be used for packing array constants and other aggregate types.
Today we use bootstrap specifiers, which can be as long as 2^16-1 items,
so there's no immediate motivation for a new grouping construct.  But the main point
of a Group would be to lift the restriction that all CP constants are defined in one
space of 2^16-1 code points.  Instead, a group would contain serialized CP
entries that have no absolute CP index, but rather are loaded as part of the group.

The group's size limit could also be raised to a u4 from u2.  I think the octet data
size limit should be u8 but that requires further API work in the JDK.  My hope is that
both Data and Group can serve at a wide range of length scales, O(10) to O(10^10).

In the interests of incremental delivery, the forthcoming JEP only deals with a limited
subset of this stuff.   The bug JDK-8161256 is a "kitchen sink" description of proposals
(both live and abandoned) for futures in this direction.

— John

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