The Glorious New Update Mechanism
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Simon & Jim Dec 93

Return convention
~~~~~~~~~~~~~~~~~
When a constructor returns it makes sure that

	R2 contains the info pointer for the constructor
	R1,R3.. contain the components (if return in regs)
	R1 points to the constructor object itself (if return in heap)

The info table for a constructor contains a pointer to the
constructor's update code.  If a constructor returns to an
update frame, the update frame's code just jumps direct to the
constructor's update code, via the info pointer in R2.

This penalises slightly the return of a new constructor,
because we have to load R2 with the info ptr.  [Fact: in runs
of the compiler, 20-30% of all returns are of a new constructor;
70-80% are existing constructors.]

Info tables
~~~~~~~~~~~
Each dynamic-heap-allocated constructor has *two* info tables:

* the "NewCon" info table is put into R2 when returning a new
  constructor, which does not yet exist in the heap; R1 is dead!
  The "NewCon" info table has no GC entries, because it's only ever used 
  when returning in regs, never installed in a real constructor.

  The NewCon table also needs a valid tag field (see taggery below)

* the "ExistingCon" info table is used for all constructors allocated
  in the heap.  

The update code for a constructor
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The update code for a constructor actually performs the update
right away.  [At present, the update is deferred until we get
back to the case expression.]  It knows how to do the update
because the update code is constructor-specific.

Once it's done the update, it makes R1 point to the constructor object
in the heap (which'll either be freshly-allocated, if big, or the
updated thing itself), and (for non-niladic constructors) makes R2 point 
to the "ExistingCon" info table for the constructor.  (Of course the new 
constructor will also have an ExistingCon info ptr.)  For niladic 
constructors, we do *not* use the "ExistingCon" info table.  We continue
to overwrite updatees in-place, because this saves us an indirection
prior to garbage collection (and the extra niladic constructors disappear
during the next garbage collection anyway).

The update code in the ExistingCon info table simply updates with an
indirection, using R1.  I *think* this can be one standard piece of
code.  The only doubt here concerns GC; if updating with an
indirection can cause GC (possible on GRIP?  or generational GC?),
then we need to know which regs are live.  We can solve this by
putting a liveness mask in the info table too.  [Arguably we want
that anyway; consider returning to the bottom of a stack object.]
So a liveness mask in the info table is probably a good idea.

Constructors which return in heap return with an ExistingCon info 
ptr.  They don't need a NewCon info table at all.

Notice that this means that when we return an *existing* constructor,
to an update frame, the update is done with an indirection, rather
than [as now] copying the constructor afresh.  This solves the space duplication
problem which shows up in "clausify".

GC: R1 might be dead; R2 is a non-ptr.  So this return convention
relies on using liveness masks for GC reg-liveness info, not the
old no-of-live-ptrs info.

Taggery 
~~~~~~~ 

  [Current: For unvectored returns with more than one constructor, we
  currently load TagReg, and scrutinise it in the case expression.
  Worse, we also have to scrutinise TagReg in the update entry of the
  return vector.]

In the new world, updates are handled without any nonsense.  No need
to look at any register, becase we just jump to the constructor
specific update code.

Since we have an info ptr in R2, we can get the tag out of the info
table, thus getting rid of TagReg altogether.  (This could conceivably
be a *lose* on a machine with lots of regs, because it replaces a
immediate small-const load by a memory fetch of the tag from the info
table.  

Not clear whether this is worth trying to improve.  Could 

	a) #define TagReg to be a register or an offset from R2
	b) issue a SET_TAG macro in the entry code for a constructor,
	   which usually expands to nothing

[NB 75-95% of all returns are vectored in runs of the compiler itself.]

The entry code for a constructor
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The real reason the registers are assigned as above is to make the
entry code for a constructor simple.  When the entry code is executed,
we have a new entry convention:

	R1 points to the object
	R2 is its info pointer

(Why? because we usually enter it by indirecting through its info
table, so it seems a shame to load the info ptr from memory twice.)

So all the entry code has to do is to return (perhaps vectored-ly).
(Maybe load TagReg, usually not --- see above.)

NB this entry convention applies, of course, to all thunks as
well as constructors -- whenever we enter an unknown object via R1 (Node).

Case expressions
~~~~~~~~~~~~~~~~
Return vectors no longer need update code.

Unvectored returns can therefore be *direct* to the code,
rather than *indirect* via a 2-entry vector.

Penalty for this improvement: "polymorphic" return vectors,
notably that in an update frame, needs to accomodate either 
a direct or a vectored return.  So it has to look like:

	UpdVec:	jmp UnvectoredUpd
		.word UpdVec0
		.word UpdVec1
		...

that is, the return vector is offset by some fixed amount
from the pointer put on the stack.  Or, it could be done
backwards:

		...
		.word UpdVec1
		.word UpdVec0
	UpdVec:	...code for UnvectoredUpd...

and then vectored returns would use negative offsets.

This grunge is necessary *only* for a fixed set of polymorphic return
vectors, part of the runtime system:

	- update frames
	- restore cost centres
	- seq, par
	- thread base
	- stack object base

Case expressions generate either a direct return, or a vector,
but never a combination.

Update Frames
~~~~~~~~~~~~~

Standard update frames are still required if we don't know the type of
the constructor being returned.  However, we often do know the type.  In
this case, we can generate a type-specific updating return-vector to place 
in the update frame rather than the StdUpdRetVector.  This saves us one
level of indirection.

Partial applications
~~~~~~~~~~~~~~~~~~~~
PAPs are basically handled just the same way as at present.

Changes from now
~~~~~~~~~~~~~~~~
* UpdReg dies.
* TagReg dies.
* RetVecReg dies.  (Previously needed to do return after update.)
* Return vectors have half the number of entries.
* Unvectored returns go direct.
* Polymorphic seq/par and friends.
* No space duplication problem (cf clausify)


Glosses
~~~~~~~
Tag and update code are needed only for constructor info tables.
It seems a shame to take up space in other info tables (ie 99% of them).

Possibilities:

- use an indirection to GC code, so the vari-sized gc stuff becomes
  fixed
- put the tag/upd code ptrs before the start of the info table.  (or
  between the info table and code when reversing info tables...)
 

Looks tricky to me.