| author |
Steve Losh <steve@stevelosh.com> |
| date |
Mon, 18 Apr 2016 18:43:12 +0000 |
| parents |
fb3a334a14f3 |
| children |
5085c5254515 |
(in-package #:bones.wam)
(named-readtables:in-readtable :fare-quasiquote)
;;;; Registers
(deftype register-type ()
'(member :argument :local :permanent))
(deftype register-number ()
'(integer 0))
(defclass register ()
((type
:initarg :type
:reader register-type
:type register-type)
(number
:initarg :number
:reader register-number
:type register-number)))
(defun* make-register ((type register-type) (number register-number))
(:returns register)
(make-instance 'register :type type :number number))
(defun* make-temporary-register ((number register-number) (arity arity))
(:returns register)
(make-register (if (< number arity) :argument :local)
number))
(defun* make-permanent-register ((number register-number) (arity arity))
(:returns register)
(declare (ignore arity))
(make-register :permanent number))
(defun* register-to-string ((register register))
(format nil "~A~D"
(ecase (register-type register)
(:argument #\A)
(:local #\X)
(:permanent #\Y))
(+ (register-number register)
(if *off-by-one* 1 0))))
(defmethod print-object ((object register) stream)
(print-unreadable-object (object stream :identity nil :type nil)
(format stream (register-to-string object))))
(defun* register-temporary-p ((register register))
(member (register-type register) '(:argument :local)))
(defun* register-permanent-p ((register register))
(eql (register-type register) :permanent))
(defun* register= ((r1 register) (r2 register))
(:returns boolean)
(ensure-boolean
(and (eql (register-type r1)
(register-type r2))
(= (register-number r1)
(register-number r2)))))
(defun* register≈ ((r1 register) (r2 register))
(:returns boolean)
(ensure-boolean
(and (or (eql (register-type r1)
(register-type r2))
;; local and argument registers are actually the same register,
;; just named differently
(and (register-temporary-p r1)
(register-temporary-p r2)))
(= (register-number r1)
(register-number r2)))))
;;;; Register Assignments
(deftype register-assignment ()
;; A register assignment represented as a cons of (register . contents).
'(cons register t))
(deftype register-assignment-list ()
'(trivial-types:association-list register t))
(defun* pprint-assignments ((assignments register-assignment-list))
(format t "~{~A~%~}"
(loop :for (register . contents) :in assignments :collect
(format nil "~A <- ~S" (register-to-string register) contents))))
(defun* find-assignment ((register register)
(assignments register-assignment-list))
(:returns register-assignment)
"Find the assignment for the given register number in the assignment list."
(assoc register assignments))
(defun* variable-p (term)
(:returns boolean)
(ensure-boolean (keywordp term)))
(defun* variable-assignment-p ((assignment register-assignment))
"Return whether the register assigment is a simple variable assignment.
E.g. `X1 = Foo` is simple, but `X2 = f(...)` is not.
Note that register assignments actually look like `(1 . contents)`, so
a simple variable assignment would be `(1 . :foo)`.
"
(:returns boolean)
(variable-p (cdr assignment)))
(defun* variable-register-p ((register register)
(assignments register-assignment-list))
(:returns boolean)
"Return whether the given register contains a variable assignment."
(variable-assignment-p (find-assignment register assignments)))
(defun* register-assignment-p ((assignment register-assignment))
(:returns boolean)
"Return whether the register assigment is a register-to-register assignment.
E.g. `A1 = X2`.
Note that this should only ever happen for argument registers.
"
(typep (cdr assignment) 'register))
(defun* structure-assignment-p ((assignment register-assignment))
(:returns boolean)
"Return whether the given assignment pair is a structure assignment."
(listp (cdr assignment)))
(defun* structure-register-p ((register register)
(assignments register-assignment-list))
(:returns boolean)
"Return whether the given register contains a structure assignment."
(structure-assignment-p (find-assignment register assignments)))
;;;; Parsing
;;; You might want to grab a coffee for this one.
;;;
;;; Consider this simple Prolog example: `p(A, q(A, r(B)))`. We're going to get
;;; this as a Lisp list: `(p :a (q :a (r b)))`.
;;;
;;; The goal is to turn this list into a set of register assignments. The book
;;; handwaves around how to do this, and it turns out to be pretty complicated.
;;; This example will (maybe, read on) be turned into:
;;;
;;; A0 <- X2
;;; A1 <- (q X2 X3)
;;; X2 <- :a
;;; X3 <- (r X4)
;;; X4 <- :b
;;;
;;; There are a few things to note here. First: like the book says, the
;;; outermost predicate is stripped off and returned separately (later it'll be
;;; used to label the code for a program, or to figure out the procedure to call
;;; for a query).
;;;
;;; The first N registers are designated as argument registers. Structure
;;; assignments can live directly in the argument registers, but variables
;;; cannot. In the example above we can see that A1 contains a structure
;;; assignment. However, the variable `:a` doesn't live in A0 -- it lives in
;;; X2, which A0 points at. The books neglects to explain this little fact.
;;;
;;; The next edge case is permanent variables, which the book does talk about.
;;; Permanent variables are allocated to stack registers, so if `:b` was
;;; permanent in our example we'd get:
;;;
;;; A0 <- X2
;;; A1 <- (q X2 X3)
;;; X2 <- :a
;;; X3 <- (r Y0)
;;; Y0 <- :b
;;;
;;; Note that the mapping of permanent variables to stack register numbers has
;;; to be consistent as we compile all the terms in a clause, so we cheat a bit
;;; here and just always add them all, in order, to the register assignment
;;; produced when parsing. They'll get flattened away later anyway -- it's the
;;; USES that we actually care about. In our example, the `Y0 <- :b` will get
;;; flattened away, but the USE of Y0 in X3 will remain).
;;;
;;; We're almost done, I promise, but there's one more edge case to deal with.
;;;
;;; When we've got a clause with a head and at least one body term, we need the
;;; head term and the first body term to share argument/local registers. For
;;; example, if we have the clause `p(Cats) :- q(A, B, C, Cats)` then when
;;; compiling the head `(p :cats)` we want to get:
;;;
;;; A0 <- X4
;;; A1 <- ???
;;; A2 <- ???
;;; A3 <- ???
;;; X4 <- :cats
;;;
;;; And when compiling `(q :a :b :c :cats)` we need:
;;;
;;; A0 <- X5
;;; A1 <- X6
;;; A2 <- X7
;;; A3 <- X4
;;; X4 <- :cats
;;; X5 <- :a
;;; X6 <- :b
;;; X7 <- :c
;;;
;;; What the hell are those empty argument registers in p? And why did we order
;;; the X registers of q like that?
;;;
;;; The book does not bother to mention this important fact at all, so to find
;;; out that you have to handle this you need to do the following:
;;;
;;; 1. Implement it without this behavior.
;;; 2. Notice your results are wrong.
;;; 3. Figure out the right bytecode on a whiteboard.
;;; 4. Try to puzzle out why that bytecode isn't generated when you do exactly
;;; what the book says.
;;; 5. Scour IRC and the web for scraps of information on what the hell you need
;;; to do here.
;;; 6. Find the answer in a comment squirreled away in a source file somewhere
;;; in a language you don't know.
;;; 7. Drink.
;;;
;;; Perhaps you're reading this comment as part of step 6 right now. If so:
;;; welcome aboard. Email me and we can swap horror stories about this process
;;; over drinks some time.
;;;
;;; Okay, so the clause head and first body term need to share argument/local
;;; registers. Why? To understand this, we need to go back to what Prolog
;;; clauses are supposed to do.
;;;
;;; Imagine we have:
;;;
;;; p(f(X)) :- q(X), ...other goals.
;;;
;;; When we want to check if `p(SOMETHING)` is true, we need to first unify
;;; SOMETHING with `f(X)`. Then we search all of the goals in the body, AFTER
;;; substituting in any X's in those goals with the X from the result of the
;;; unification.
;;;
;;; This substitution is why we need the head and the first term in the body to
;;; share the same argument/local registers. By sharing the registers, when the
;;; body term builds a representation of itself on the stack before calling its
;;; predicate any references to X will be point at the (unified) results instead
;;; of fresh ones (because they'll be compiled as `put_value` instead of
;;; `put_variable`).
;;;
;;; But wait: don't we need to substitute into ALL the body terms, not just the
;;; first one? Yes we do, but the trick is that any variables in the REST of
;;; the body that would need to be substituted must, by definition, be permanent
;;; variables! So the substitution process for the rest of the body is handled
;;; automatically with the stack machinery.
;;;
;;; In theory, you could eliminate this edge case by NOT treating the head and
;;; first goal as a single term when searching for permanent variables. Then
;;; all substitution would happen elegantly through the stack. But this
;;; allocates more variables on the stack than you really need (especially for
;;; rules with just a single term in the body (which is many of them)), so we
;;; have this extra corner case to optimize it away.
;;;
;;; We now return you to your regularly scheduled Lisp code.
(defun parse-term (term permanent-variables
;; JESUS TAKE THE WHEEL
&optional reserved-variables reserved-arity)
"Parse a term into a series of register assignments.
Returns:
* The assignment list
* The root functor
* The root functor's arity
"
(let* ((predicate (first term))
(arguments (rest term))
(arity (length arguments))
;; Preallocate enough registers for all of the arguments. We'll fill
;; them in later. Note that things are more complicated in the head
;; and first body term of a clause (see above).
(local-registers (make-array 64
:fill-pointer (or reserved-arity arity)
:adjustable t
:initial-element nil))
;; We essentially "preallocate" all the permanent variables up front
;; because we need them to always be in the same stack registers across
;; all the terms of our clause.
;;
;; The ones that won't get used in this term will end up getting
;; flattened away anyway.
(stack-registers (make-array (length permanent-variables)
:initial-contents permanent-variables)))
(loop :for variable :in reserved-variables :do
(vector-push-extend variable local-registers))
(labels
((find-variable (var)
(let ((r (position var local-registers))
(s (position var stack-registers)))
(cond
(r (make-temporary-register r arity))
(s (make-permanent-register s arity))
(t nil))))
(store-variable (var)
(make-temporary-register
(vector-push-extend var local-registers)
arity))
(parse-variable (var)
;; If we've already seen this variable just return the register it's
;; in, otherwise allocate a register for it and return that.
(or (find-variable var)
(store-variable var)))
(parse-structure (structure reg)
(destructuring-bind (functor . arguments) structure
;; If we've been given a register to hold this structure (i.e.
;; we're parsing a top-level argument) use it. Otherwise allocate
;; a fresh one. Note that structures always live in local
;; registers, never permanent ones.
(let ((reg (or reg (vector-push-extend nil local-registers))))
(setf (aref local-registers reg)
(cons functor (mapcar #'parse arguments)))
(make-temporary-register reg arity))))
(parse (term &optional register)
(cond
((variable-p term) (parse-variable term))
((symbolp term) (parse (list term) register)) ; f -> f/0
((listp term) (parse-structure term register))
(t (error "Cannot parse term ~S." term))))
(make-assignment-list (registers register-maker)
(loop :for i :from 0
:for contents :across registers
:when contents :collect ; don't include unused reserved regs
(cons (funcall register-maker i arity)
contents))))
;; Arguments are handled specially. We parse the children as normal,
;; and then fill in the argument registers after each child.
(loop :for argument :in arguments
:for i :from 0
:for parsed = (parse argument i)
;; If the argument didn't fill itself in (structure), do it.
:when (not (aref local-registers i))
:do (setf (aref local-registers i) parsed))
(values (append
(make-assignment-list local-registers #'make-temporary-register)
(make-assignment-list stack-registers #'make-permanent-register))
predicate
arity))))
;;;; Flattening
;;; "Flattening" is the process of turning a series of register assignments into
;;; a sorted sequence appropriate for turning into a series of instructions.
;;;
;;; The order depends on whether we're compiling a query term or a program term.
;;;
;;; It's a stupid name because the assignments are already flattened as much as
;;; they ever will be. "Sorting" would be a better name. Maybe I'll change it
;;; once I'm done with the book.
;;;
;;; Turns:
;;;
;;; X0 <- p(X1, X2)
;;; X1 <- A
;;; X2 <- q(X1, X3)
;;; X3 <- B
;;;
;;; into something like:
;;;
;;; X2 <- q(X1, X3), X0 <- p(X1, X2)
(defun find-dependencies (assignments)
"Return a list of dependencies amongst the given registers.
Each entry will be a cons of `(a . b)` if register `a` depends on `b`.
"
(mapcan
(lambda (assignment)
(cond
; Variable assignments (X1 <- Foo) don't depend on anything else.
((variable-assignment-p assignment)
())
; Register assignments (A0 <- X5) have one obvious dependency.
((register-assignment-p assignment)
(destructuring-bind (argument . contents) assignment
(list `(,contents . ,argument))))
; Structure assignments depend on all the functor's arguments.
((structure-assignment-p assignment)
(destructuring-bind (target . (functor . reqs))
assignment
(declare (ignore functor))
(loop :for req :in reqs
:collect (cons req target))))
(t (error "Cannot find dependencies for assignment ~S." assignment))))
assignments))
(defun flatten (assignments)
"Flatten the set of register assignments into a minimal set.
We remove the plain old variable assignments (in non-argument registers)
because they're not actually needed in the end.
"
(-<> assignments
(topological-sort <> (find-dependencies assignments)
:key #'car
:key-test #'register=
:test #'eql)
(remove-if #'variable-assignment-p <>)))
(defun flatten-query (assignments)
(flatten assignments))
(defun flatten-program (assignments)
(reverse (flatten assignments)))
;;;; Tokenization
;;; Tokenizing takes a flattened set of assignments and turns it into a stream
;;; of structure assignments and bare registers.
;;;
;;; It turns:
;;;
;;; X2 <- q(X1, X3)
;;; X0 <- p(X1, X2)
;;; A3 <- X4
;;;
;;; into something like:
;;;
;;; (X2 = q/2), X1, X3, (X0 = p/2), X1, X2, (A3 = X4)
(defun tokenize-assignments (assignments)
"Tokenize a flattened set of register assignments into a stream."
(mapcan
(lambda (ass)
;; Take a single assignment like:
;; X1 = f(X4, Y1) (X1 . (f X4 Y1))
;; A0 = X5 (A0 . X5)
;;
;; And turn it into a stream of tokens:
;; (X1 = f/2), X4, Y1 ((:structure X1 f 2) X4 Y1
;; (A0 = X5) (:argument A0 X5))
(if (register-assignment-p ass)
;; It might be a register assignment for an argument register.
(destructuring-bind (argument-register . target-register) ass
(list (list :argument argument-register target-register)))
;; Otherwise it's a structure assignment. We know the others have
;; gotten flattened away by now.
(destructuring-bind (register . (functor . arguments)) ass
(cons (list :structure register functor (length arguments))
arguments))))
assignments))
(defun tokenize-term
(term permanent-variables reserved-variables reserved-arity flattener)
(multiple-value-bind (assignments functor arity)
(parse-term term permanent-variables reserved-variables reserved-arity)
(values (->> assignments
(funcall flattener)
tokenize-assignments)
functor
arity)))
(defun tokenize-program-term
(term permanent-variables reserved-variables reserved-arity)
"Tokenize `term` as a program term, returning its tokens, functor, and arity."
(tokenize-term term
permanent-variables
reserved-variables
reserved-arity
#'flatten-program))
(defun tokenize-query-term
(term permanent-variables &optional reserved-variables reserved-arity)
"Tokenize `term` as a query term, returning its stream of tokens."
(multiple-value-bind (tokens functor arity)
(tokenize-term term
permanent-variables
reserved-variables
reserved-arity
#'flatten-query)
;; We need to shove a CALL token onto the end.
(append tokens `((:call ,functor ,arity)))))
;;;; Bytecode
;;; Once we have a tokenized stream we can generate the machine instructions
;;; from it.
;;;
;;; We turn:
;;;
;;; (X2 = q/2), X1, X3, (X0 = p/2), X1, X2
;;;
;;; into something like:
;;;
;;; (#'%put-structure 2 q 2)
;;; (#'%set-variable 1)
;;; (#'%set-variable 3)
;;; (#'%put-structure 0 p 2)
;;; (#'%set-value 1)
;;; (#'%set-value 2)
(defun find-opcode (opcode newp mode &optional register)
(flet ((find-variant (register)
(when register
(if (register-temporary-p register)
:local
:stack))))
(eswitch ((list opcode newp mode (find-variant register)) :test #'equal)
('(:argument t :program :local) +opcode-get-variable-local+)
('(:argument t :program :stack) +opcode-get-variable-stack+)
('(:argument t :query :local) +opcode-put-variable-local+)
('(:argument t :query :stack) +opcode-put-variable-stack+)
('(:argument nil :program :local) +opcode-get-value-local+)
('(:argument nil :program :stack) +opcode-get-value-stack+)
('(:argument nil :query :local) +opcode-put-value-local+)
('(:argument nil :query :stack) +opcode-put-value-stack+)
;; Structures can only live locally, they never go on the stack
('(:structure nil :program :local) +opcode-get-structure-local+)
('(:structure nil :query :local) +opcode-put-structure-local+)
('(:register t :program :local) +opcode-unify-variable-local+)
('(:register t :program :stack) +opcode-unify-variable-stack+)
('(:register t :query :local) +opcode-set-variable-local+)
('(:register t :query :stack) +opcode-set-variable-stack+)
('(:register nil :program :local) +opcode-unify-value-local+)
('(:register nil :program :stack) +opcode-unify-value-stack+)
('(:register nil :query :local) +opcode-set-value-local+)
('(:register nil :query :stack) +opcode-set-value-stack+))))
;;;; UI
(defun find-shared-variables (terms)
"Return a list of all variables shared by two or more terms."
(let* ((variables (remove-duplicates (tree-collect #'variable-p terms))))
(labels
((count-uses (variable)
(count-if (curry #'tree-member-p variable)
terms))
(shared-p (variable)
(> (count-uses variable) 1)))
(remove-if-not #'shared-p variables))))
(defun find-permanent-variables (clause)
"Return a list of all the 'permanent' variables in `clause`.
Permanent variables are those that appear in more than one goal of the clause,
where the head of the clause is considered to be a part of the first goal.
"
(if (<= (length clause) 2)
(list) ; facts and chain rules have no permanent variables at all
(destructuring-bind (head body-first . body-rest) clause
;; the head is treated as part of the first goal for the purposes of
;; finding permanent variables
(find-shared-variables (cons (cons head body-first) body-rest)))))
(defun find-head-variables (clause)
(if (<= (length clause) 1)
(list)
(destructuring-bind (head body-first . body-rest) clause
(declare (ignore body-rest))
(find-shared-variables (list head body-first)))))
(defun mark-label (wam functor arity store)
"Set the code label `(functor . arity)` to point at the next space in `store`."
;; todo make this less ugly
(setf (wam-code-label wam (wam-ensure-functor-index wam (cons functor arity)))
(fill-pointer store)))
(defun make-query-code-store ()
(make-array 64
:fill-pointer 0
:adjustable t
:element-type 'code-word))
(defun compile-clause (wam store head body)
"Compile the clause into the given store array.
`head` should be the head of the clause for program clauses, or may be `nil`
for query clauses.
"
(let* ((permanent-variables
(find-permanent-variables (cons head body)))
(head-variables
(set-difference (find-head-variables (cons head body))
permanent-variables))
(head-arity
(max (1- (length head))
(1- (length (car body)))))
(head-tokens
(when head
(multiple-value-bind (tokens functor arity)
(tokenize-program-term head
permanent-variables
head-variables
head-arity)
(mark-label wam functor arity store) ; TODO: this is ugly
tokens)))
(body-tokens
(when body
(append
(tokenize-query-term (first body)
permanent-variables
head-variables
head-arity)
(loop :for term :in (rest body) :append
(tokenize-query-term term
permanent-variables))))))
(flet ((compile% () (compile-tokens wam head-tokens body-tokens store)))
;; We need to compile facts and rules differently. Facts end with
;; a PROCEED and rules are wrapped in ALOC/DEAL.
(cond
((and head body) ; a full-ass rule
(code-push-instruction! store +opcode-allocate+ (length permanent-variables))
(compile%)
(code-push-instruction! store +opcode-deallocate+))
((and head (null body)) ; a bare fact
(compile%)
(code-push-instruction! store +opcode-proceed+))
(t ; just a query
(compile%)))))
(values))
(defun compile-query (wam query)
"Compile `query` into a fresh array of bytecode.
`query` should be a list of goal terms.
"
(let ((store (make-query-code-store)))
(compile-clause wam store nil query)
(code-push-instruction! store +opcode-done+)
store))
(defun compile-program (wam rule)
"Compile `rule` into the WAM's code store.
`rule` should be a clause consisting of a head term and zero or more body
terms. A rule with no body is also called a \"fact\".
"
(compile-clause wam (wam-code wam) (first rule) (rest rule))
(values))