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Mikael's ideas on a new type of Scheme interpreter

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***
*** These notes about the design of a new type of Scheme interpreter
*** "Ior" are cut out from various emails from early spring 2000.
***
*** MDJ 000817 <djurfeldt@nada.kth.se>
***
Generally, we should try to make a design which is clean and
minimalistic in as many respects as possible. For example, even if we
need more primitives than those in R5RS internally, I don't think
these should be made available to the user in the core, but rather be
made available *through* libraries (implementation in core,
publication via library).
The suggested working name for this project is "Ior" (Swedish name for
the donkey in "Winnie the Pooh" :). If, against the odds, we really
would succeed in producing an Ior, and we find it suitable, we could
turn it into a Guile 2.0 (or whatever). (The architecture still
allows for support of the gh interface and uses conservative GC (Hans
Böhm's, in fact).)
Beware now that I'm just sending over my original letter, which is
just a sketch of the more detailed, but cryptic, design notes I made
originally, which are, in turn, not as detailed as the design has
become now. :)
Please also excuse the lack of structure. I shouldn't work on this at
all right now. Choose for yourselves if you want to read this
unstructured information or if you want to wait until I've structured
it after end of January.
But then I actually have to blurt out the basic idea of my
architecture already now. (I had hoped to present you with a proper
and fairly detailed spec, but I won't be able to complete such a spec
quickly.)
The basic idea is this:
* Don't waste time on non-computation!
Why waste a lot of time on type-checks, unboxing and boxing of data?
Neither of these actions do any computations!
I'd like both interpreter and compiled code to work directly with data
in raw, native form (integers represented as 32bit longs, inexact
numbers as doubles, short strings as bytes in a word, longer strings
as a normal pointer to malloced memory, bignums are just pointers to a
gmp (GNU MultiPrecision library) object, etc.)
* Don't we need to dispatch on type to know what to do?
But don't we need to dispatch on the type in order to know how to
compute with the data? E.g., `display' does entirely different
computations on a <fixnum> and a <string>. (<fixnum> is an integer
between -2^31 and 2^31-1.)
The answer is *no*, not in 95% of all cases. The main reason is that
the interpreter does type analysis while converting closures to
bytecode, and knows already when _calling_ `display' what type it's
arguments has. This means that the bytecode compiler can choose a
suitable _version_ of `display' which handles that particular type.
This type analysis is greatly simplified by the fact that just as the
type analysis _results_ in the type of the argument in the call to
`display', and, thus, we can select the correct _version_ of
`display', the closure byte-code itself will only be one _version_ of
the closure with the types of its arguments fixed at the start of the
analysis.
As you already have understood by now, the basic architecture is that
all procedures are generic functions, and the "versions" I'm speaking
about is a kind of methods. Let's call them "branches" by now.
For example:
(define foo
(lambda (x)
...
(display x)
...)
may result in the following two branches:
1. [<fixnum>-foo] =
(branch ((x <fixnum>))
...
([<fixnum>-display] x)
...)
2. [<string>-foo] =
(branch ((x <string>))
...
([<string>-display] x)
...)
and a new closure
(define bar
(lambda (x y)
...
(foo x)
...))
results in
[<fixnum>-<fixnum>-bar] =
(branch ((x <fixnum>) (y <fixnum>))
...
([<fixnum>-foo] x)
...)
Note how all type dispatch is eliminated in these examples.
As a further reinforcement to the type analysis, branches will not
only have typed parameters but also have return types. This means
that the type of a branch will look like
<type 1> x ... x <type n> --> <type r>
In essence, the entire system will be very ML-like internally, and we
can benefit from the research done on ML-compilation.
However, we now get three major problems to confront:
1. In the Scheme language not all situations can be completely type
analyzed.
2. In particular, for some operations, even if the types of the
parameters are well defined, we can't determine the return type
generically. For example, [<fixnum>-<fixnum>-+] may have return
type <fixnum> _or_ <bignum>.
3. Even if we can do a complete analysis, some closures will generate
a combinatoric explosion of branches.
Problem 1: Incomplete analysis
We introduce a new type <boxed>. This data type has type <boxed> and
contents
struct ior_boxed_t {
ior_type *type; /* pointer to class struct */
void *data; /* generic field, may also contain immediate objects
*/
}
For example, a boxed fixnum 4711 has type <boxed> and contents
{ <fixnum>, 4711 }. The boxed type essentially corresponds to Guile's
SCM type. It's just that the 1 or 3 or 7 or 16-bit type tag has been
replaced with a 32-bit type tag (the pointer to the class structure
describing the type of the object).
This is more inefficient than the SCM type system, but it's no problem
since it won't be used in 95% of all cases. The big advantage
compared to SCM's type system is that it is so simple and uniform.
I should note here that while SCM and Guile are centered around the
cell representation and all objects either _are_ cells or have a cell
handle, objects in ior will more look like mallocs. This is the
reason why I planned to start with B<><42>öhm's GC which has C pointers as
object handles. But it is of course still possible to use a heap, or,
preferably several heaps for different kinds of objects. (B<><42>öhm's GC
has multiple heaps for different sizes of objects.) If we write a
custom GC, we can increase speed further.
Problem 3 (yes, I skipped :) Combinatoric explosion
We simply don't generate all possible branches. In the interpreter we
generate branches "just-too-late" (well, it's normally called "lazy
compilation" or "just-in-time", but if it was "in-time", the procedure
would already be compiled when it was needed, right? :) as when Guile
memoizes or when a Java machine turns byte-codes into machine code, or
as when GOOPS turns methods into cmethods for that matter.
Have noticed that branches (although still without return type
information) already exist in GOOPS? They are currently called
"cmethods" and are generated on demand from the method code and put
into the GF cache during evaluation of GOOPS code. :-) (I have not
utilized this fully yet. I plan to soon use this method compilation
(into branches) to eliminate almost all type dispatch in calls to
accessors.)
For the compiler, we use profiling information, just as the modern GCC
scheduler, or else relies on some type analysis (if a procedure says
(+ x y), x is not normally a <string> but rather some subclass of
<number>) and some common sense (it's usually more important to
generate <fixnum> branches than <foobar> branches).
The rest of the cases can be handled by <boxed>-branches. We can, for
example, have a:
[<boxed>-<boxed>-bar] =
(branch ((x <boxed>) (y <boxed>))
...
([<boxed>-foo] x)
...)
[<boxed>-foo] will use an efficient type dispatch mechanism (for
example akin to the GOOPS one) to select the right branch of
`display'.
Problem 2: Ambiguous return type
If the return type of a branch is ambiguous, we simply define the
return type as <boxed>, and box data at the point in the branch where
it can be decided which type of data we will return. This is how
things can be handled in the general case. However, we might be able
to handle things in a more neat way, at least in some cases:
During compilation to byte code, we'll probably use an intermediate
representation in continuation passing style. We might even use a
subtype of branches reprented as continuations (not a heavy
representation, as in Guile and SCM, but probably not much more than a
function pointer). This is, for example, one way of handling tail
recursion, especially mutual tail recursion.
One case where we would like to try really hard not to box data is
when fixnums "overflow into" bignums.
Let's say that the branch [<fixnum>-<fixnum>-bar] contains a form
(+ x y)
where the type analyzer knows that x and y are fixnums. We then split
the branch right after the form and let it fork into two possible
continuation branches bar1 and bar2:
[The following is only pseudo code. It can be made efficient on the C
level. We can also use the asm compiler directive in conditional
compilation for GCC on i386. We could even let autoconf/automake
substitute an architecture specific solution for multiple
architectures, but still support a C level default case.]
(if (sum-over/underflow? x y)
(bar1 (fixnum->bignum x) (fixnum->bignum y) ...)
(bar2 x y ...))
bar1 begins with the evaluation of the form
([<bignum>-<bignum>-+] x y)
while bar 2 begins with
([<fixnum>-<fixnum>-+] x y)
Note that the return type of each of these forms is unambiguous.
Now some random points from the design:
* The basic concept in Ior is the class. A type is a concrete class.
Classes which are subclasses of <object> are concrete, otherwise they
are abstract.
* A procedure is a collection of methods. Each method can have
arbitrary number of parameters of arbitrary class (not type).
* The type of a method is the tuple of it's argument classes.
* The type of a procedure is the set of it's method types.
But the most important new concept is the branch.
Regard the procedure:
(define (half x)
(quotient x 2))
The procedure half will have the single method
(method ((x <top>))
(quotient x 2))
When `(half 128)' is called the Ior evaluator will create a new branch
during the actual evaluation. I'm now going to extend the branch
syntax by adding a second list of formals: the continuations of the
branch.
* The type of a branch is namely the tuple of the tuple of it's
argument types (not classes!) and the tuple of it's continuation
argument types. The branch generated above will be:
(branch ((x <fixnum>) ((c <fixnum>))
(c (quotient x 2)))
If the method
(method ((x <top>) (y <top>))
(quotient (+ x 1) y))
is called with arguments 1 and 2 it results in the branch
(branch ((x <fixnum>) (y <fixnum>)) ((c1 <fixnum>) (c2 <bignum>))
(quotient (+ x 1 c3) 2))
where c3 is:
(branch ((x <fixnum>) (y <fixnum>)) ((c <bignum>))
(quotient (+ (fixnum->bignum x) 1) 2)
The generated branches are stored in a cache in the procedure object.
But wait a minute! What about variables and data structures?
In essence, what we do is that we fork up all data paths so that they
can be typed: We put the type tags on the _data paths_ instead of on
the data itself. You can look upon the "branches" as tubes of
information where the type tag is attached to the tube instead of on
what passes through it.
Variables and data structures are part of the "tubes", so they need to
be typed. For example, the generic pair looks like:
(define-class <pair> ()
car-type
car
cdr-type
cdr)
But note that since car and cdr are generic procedures, we can let
more efficient pairs exist in parallel, like
(define-class <immutable-fixnum-list> ()
(car (class <fixnum>))
(cdr (class <immutable-fixnum-list>)))
Note that instances of this last type only takes two words of memory!
They are easy to use too. We can't use `cons' or `list' to create
them, since these procedures can't assume immutability, but we don't
need to specify the type <fixnum> in our program. Something like
(const-cons 1 x)
where x is in the data flow path tagged as <immutable-fixnum-list>, or
(const-list 1 2 3)
Some further notes:
* The concepts module and instance are the same thing. Using other
modules means 1. creating a new module class which inherits the
classes of the used modules and 2. instantiating it.
* Module definitions and class definitions are equivalent but
different syntactic sugar adapted for each kind of use.
* (define x 1) means: create an instance variable which is itself a
subclass of <boxed> with initial value 1 (which is an instance of
<fixnum>).
The interpreter is a mixture between a stack machine and a register
machine. The evaluator looks like this... :)
/* the interpreter! */
if (!setjmp (ior_context->exit_buf))
#ifndef i386_GCC
while (1)
#endif
(*ior_continue) (IOR_MICRO_OP_ARGS);
The branches are represented as an array of pointers to micro
operations. In essence, the evaluator doesn't exist in itself, but is
folded out over the entire implementation. This allows for an extreme
form of modularity!
The i386_GCC is a machine specific optimization which avoids all
unnecessary popping and pushing of the CPU stack (which is different
from the Ior data stack).
The execution environment consists of
* a continue register similar to the program counter in the CPU
* a data stack (where micro operation arguments and results are stored)
* a linked chain of environment frames (but look at exception below!)
* a dynamic context
I've written a small baby Ior which uses Guile's infrastructure.
Here's the context from that baby Ior:
typedef struct ior_context_t {
ior_data_t *env; /* rest of environment frames */
ior_cont_t save_continue; /* saves or represents continuation */
ior_data_t *save_env; /* saves or represents environment */
ior_data_t *fluids; /* array of fluids (use GC_malloc!) */
int n_fluids;
int fluids_size;
/* dynwind chain is stored directly in the environment, not in context */
jmp_buf exit_buf;
IOR_SCM guile_protected; /* temporary */
} ior_context_t;
There's an important exception regarding the lowest environment
frame. That frame isn't stored in a separate block on the heap, but
on Ior's data stack. Frames are copied out onto the heap when
necessary (for example when closures "escape").
Now a concrete example:
Look at:
(define sum
(lambda (from to res)
(if (= from to)
res
(sum (+ 1 from) to (+ from res)))))
This can be rewritten into CPS (which captures a lot of what happens
during flow analysis):
(define sum
(lambda (from to res c1)
(let ((c2 (lambda (limit?)
(let ((c3 (lambda ()
(c1 res)))
(c4 (lambda ()
(let ((c5 (lambda (from+1)
(let ((c6 (lambda (from+res)
(sum from+1 to from+res c1))))
(_+ from res c6)))))
(_+ 1 from c5)))))
(_if limit? c3 c4)))))
(_= from to c2))))
Finally, after branch expansion, some optimization, code generation,
and some optimization again, we end up with the byte code for the two
branches (here marked by labels `sum' and `sumbig'):
c5
(ref -3)
(shift -1)
(+ <fixnum> <fixnum> c4big)
;; c4
(shift -2)
(+ <fixnum> 1 sumbig)
;; c6
sum
(shift 3)
(ref2 -3)
;; c2
(if!= <fixnum> <fixnum> c5)
;; c3
(ref -1)
;; c1
(end)
c5big
(ref -3)
(shift -1)
(+ <bignum> <bignum>)
c4big
(shift -2)
(+ <bignum> 1)
;; c6
sumbig
(shift 3)
(ref2 -3)
;; c2
(= <bignum> <bignum>)
(if! c5big)
;; c3
(ref -1)
;; c1
(end)
Let's take a closer look upon the (+ <fixnum> 1 sumbig) micro
operation. The generated assembler from the Ior C source + machine
specific optimizations for i386_GCC looks like this (with some rubbish
deleted):
ior_int_int_sum_intbig:
movl 4(%ebx),%eax ; fetch arg 2
addl (%ebx),%eax ; fetch arg 1 and do the work!
jo ior_big_sum_int_int ; dispatch to other branch on overflow
movl %eax,(%ebx) ; store result in first environment frame
addl $8,%esi ; increment program counter
jmp (%esi) ; execute next opcode
ior_big_sum_int_int:
To clearify: This is output from the C compiler. I added the comments
afterwards.
The source currently looks like this:
IOR_MICRO_BRANCH_2_2 ("+", int, big, sum, int, int, 1, 0)
{
int res = IOR_ARG (int, 0) + IOR_ARG (int, 1);
IOR_JUMP_OVERFLOW (res, ior_big_sum_int_int);
IOR_NEXT2 (z);
}
where the macros allow for different definitions depending on if we
want to play pure ANSI or optimize for a certain machine/compiler.
The plan is actually to write all source in the Ior language and write
Ior code to translate the core code into bootstrapping C code.
Please note that if i386_GCC isn't defined, we run plain portable ANSI C.
Just one further note:
In Ior, there are three modes of evaluation
1. evaluating and type analyzing (these go in parallel)
2. code generation
3. executing byte codes
It is mode 3 which is really fast in Ior.
You can look upon your program as a web of branch segments where one
branch segment can be generated from fragments of many closures. Mode
switches doesn't occur at the procedure borders, but at "growth
points". I don't have time to define them here, but they are based
upon the idea that the continuation together with the type signature
of the data flow path is unique.
We normally run in mode 3. When we come to a source growth point
(essentially an apply instruction) for uncompiled code we "dive out"
of mode 3 into mode 1 which starts to eval/analyze code until we come
to a "sink". When we reach the "sink", we have enough information
about the data path to do code generation, so we backtrack to the
source growth point and grow the branch between source and sink.
Finally, we "dive into" mode 3!
So, code generation doesn't respect procedure borders. We instead get
a very neat kind of inlining, which, e.g., means that it is OK to use
closures instead of macros in many cases.
----------------------------------------------------------------------
Ior and module system
=====================
How, exactly, should the module system of Ior look like?
There is this general issue of whether to have a single-dispatch or
multi-dispatch system. Personally, I see that Scheme already use
multi-dispatch. Compare (+ 1.0 2) and (+ 1 2.0).
As you've seen if you've read the notes about Ior design, efficiency
is not an issue here, since almost all dispatch will be eliminated
anyway.
Also, note an interesting thing: GOOPS actually has a special,
implicit, argument to all of it's methods: the lexical environment.
It would be very ugly to add a second, special, argument to this.
Of course, the theoreticians have already recognised this, and in many
systems, the implicit argument (the object) and the environment for
the method is the same thing.
I think we should especially take impressions from Matthias Blume's
module/object system.
The idea, now, for Ior (remember that everything about Ior is
negotiable between us) is that a module is a type, as well as an
instance of that type. The idea is that we basically keep the GOOPS
style of methods, with the implicit argument being the module object
(or some other lexical environment, in a chain with the module as
root).
Let's say now that module C uses modules A and B. Modules A and B
both exports the procedure `foo'. But A:foo and B:foo as different
sets of methods.
What does this mean? Well, it obviously means that the procedure
`foo' in module C is a subtype of A:foo and B:foo. Note how this is
similar in structure to slot inheritance: When class C is created with
superclasses A and B, the properties of a slot in C are created
through slot inheritance. One way of interpreting variable foo in
module A is as a slot with init value foo. Through the MOP, we can
specify that procedure slot inheritance in a module class implies
creation of new init values through inheritance.
This may look like a kludge, and perhaps it is, and, sure, we are not
going to accept any kludges in Ior. But, it might actually not be a
kludge...
I think it is commonly accepted by computer scientists that a module,
and/or at least a module interface is a type. Again, this type can be
seen as the set of types of the functions in the interface. The types
of our procedures are the set of branch types the provide. It is then
natural that a module using two other modules create new procedure
types by folding.
This thing would become less cloudy (yes, this is a cloudy part of my
reasoning; I meant previously that the interpreter itself is now
clear) if module interfaces were required to be explicitly types.
Actually, this would fit much better together with the rest of Ior's
design. On one hand, we might be free to introduce such a restriction
(compiler writers would applaud it), since R5RS hasn't specified any
module system. On the other hand, it might be strange to require
explicit typing when Scheme is fundamentally implicitly types...
We also have to consider that a module has an "inward" face, which is
one type, and possibly many "outward" faces, which are different
types. (Compare the idea of "interfaces" in Scheme48.)
It thus, seems that, while a module can truly be an Ior class, the
reverse should probably not hold in the general case...
Unless
instance <-> module proper
class of the instance <-> "inward interface"
superclasses <-> "outward interfaces + inward uses"
...hmm, is this possible to reconcile with Rees' object system?
Please think about these issues. We should try to end up with a
beautiful and consistent object/module system.
----------------------------------------------------------------------
Here's a difficult problem in Ior's design:
Let's say that we have a mutable data structure, like an ordinary
list. Since, in Ior, the type tag (which is really a pointer to a
class structure) is stored separately from the data, it is thinkable
that another thread modifies the location in the list between when our
thread reads the type tag and when it reads the data.
The reading of type and data must be made atomic in some way.
Probably, some kind of locking of the heap is required. It's just
that it may cause a lot of overhead to look the heap at every *read*
from a mutable data structure.
Look how much trouble those set!-operations cause! Not only does it
force us to store type tags for each car and cdr in the list, but it
also forces a lot of explicit dispatch to be done, and causes troubles
in a threaded system...
----------------------------------------------------------------------
Jim Blandy <jimb@red-bean.com> writes:
> We also should try to make less work for the GC, by avoiding consing
> up local environments until they're closed over.
Did the texts which I sent to you talk about Ior's solution?
It basically is: Use *two* environment "arguments" to the evaluator
(in Ior, they aren't arguments but registers):
* One argument is a pointer to the "top" of an environment stack.
This is used in the "inner loop" for very efficient access to
in-between results. The "top" segment of the environment stack is
also regarded as the first environment frame in the lexical
environment. ("top" is bottom on a stack which grows downwards)
* The other argument points to a structure holding the evaluation
context. In this context, there is a pointer to the chain of the
rest of the environment frames. Note that since frames are just
blocks of SCM values, you can very efficiently "release" a frame
into the heap by block copying it (remember that Ior uses Boehms GC;
this is how we allocate the block).