| author |
Steve Losh <steve@stevelosh.com> |
| date |
Sun, 17 Jul 2016 16:59:50 +0000 |
| parents |
6c90a65137d9 |
| children |
(none) |
(in-package #:bones.wam)
;;;; .-,--. . ,. . . .
;;;; `|__/ ,-. ,-. . ,-. |- ,-. ,-. / | | | ,-. ,-. ,-. |- . ,-. ,-.
;;;; )| \ |-' | | | `-. | |-' | /~~|-. | | | | | ,-| | | | | | |
;;;; `' ` `-' `-| ' `-' `' `-' ' ,' `-' `' `' `-' `-' `-^ `' ' `-' ' '
;;;; ,|
;;;; `'
;;; 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.
;;;
;;; In the following code these variables will be called "nead variables"
;;; because:
;;;
;;; 1. They're present in the head of the clause.
;;; 2. They're present in the first term of the body (the "neck", as referred to
;;; in "neck cut" and such).
;;; 3. https://www.urbandictionary.com/define.php?term=nead&defid=1488946
;;;
;;; We now return you to your regularly scheduled Lisp code.
(defstruct allocation-state
(local-registers (make-queue) :type queue)
(stack-registers nil :type list)
(permanent-variables nil :type list)
(anonymous-variables nil :type list)
(reserved-variables nil :type list)
(reserved-arity nil :type (or null arity))
(actual-arity 0 :type arity))
(defun find-variable (state variable)
"Return the register that already contains this variable, or `nil` otherwise."
(or (when-let (r (position variable
(queue-contents
(allocation-state-local-registers state))))
(make-temporary-register r (allocation-state-actual-arity state)))
(when-let (s (position variable
(allocation-state-stack-registers state)))
(make-permanent-register s))
nil))
(defun store-variable (state variable)
"Assign `variable` to the next available local register.
It is assumed that `variable` is not already assigned to another register
(check that with `find-variable` first).
It is also assumed that this will be a non-argument register, because as
mentioned above variables cannot live directly inside argument registers.
"
(make-register
:local
(1- (enqueue variable (allocation-state-local-registers state)))))
(defun ensure-variable (state variable)
(or (find-variable state variable)
(store-variable state variable)))
(defmacro set-when-nil ((accessor instance) value-form)
(once-only (instance)
`(when (not (,accessor ,instance))
(setf (,accessor ,instance) ,value-form))))
(defun variable-anonymous-p (state variable)
"Return whether `variable` is considered anonymous in `state`."
(and (member variable (allocation-state-anonymous-variables state)) t))
(defun allocate-variable-register (state variable)
(if (variable-anonymous-p state variable)
(make-anonymous-register)
(ensure-variable state variable)))
(defun allocate-nonvariable-register (state)
"Allocate and return a register for something that's not a variable."
;; We need to allocate registers for things like structures and lists, but we
;; never need to look them up later (like we do with variables), so we'll just
;; shove a nil into the local registers array as a placeholder.
(make-temporary-register
(enqueue nil (allocation-state-local-registers state))
(allocation-state-actual-arity state)))
(defgeneric allocate-register (node allocation-state))
(defmethod allocate-register ((node top-level-node) state)
(declare (ignore node state))
nil)
(defmethod allocate-register ((node variable-node) state)
(set-when-nil (node-register node)
(allocate-variable-register state (node-variable node))))
(defmethod allocate-register ((node argument-variable-node) state)
(set-when-nil (node-secondary-register node)
(allocate-variable-register state (node-variable node))))
(defmethod allocate-register ((node structure-node) state)
(set-when-nil (node-register node)
(allocate-nonvariable-register state)))
(defmethod allocate-register ((node list-node) state)
(set-when-nil (node-register node)
(allocate-nonvariable-register state)))
(defmethod allocate-register ((node lisp-object-node) state)
(set-when-nil (node-register node)
(allocate-nonvariable-register state)))
(defun allocate-argument-registers (node)
(loop :for argument :in (top-level-node-arguments node)
:for i :from 0
:do (setf (node-register argument)
(make-register :argument i))))
(defun allocate-nonargument-registers (node clause-props &key nead)
;; JESUS TAKE THE WHEEL
(let*
((actual-arity (top-level-node-arity node))
(reserved-arity (when nead
(clause-nead-arity clause-props)))
(reserved-variables (when nead
(clause-nead-vars clause-props)))
(permanent-variables (clause-permanent-vars clause-props))
(local-registers (make-queue))
;; 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 permanent-variables)
(allocation-state
(make-allocation-state
:local-registers local-registers
:stack-registers stack-registers
:permanent-variables permanent-variables
:anonymous-variables (clause-anonymous-vars clause-props)
:reserved-variables reserved-variables
:reserved-arity reserved-arity
:actual-arity actual-arity)))
;; 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).
(loop :repeat (or reserved-arity actual-arity)
:do (enqueue nil local-registers))
;; Actually reserve the reserved (but non-permanent, see above) variables.
;; They need to live in consistent spots for the head and first body term.
(loop :for variable :in reserved-variables
:do (enqueue variable local-registers))
(recursively ((remaining (list node)))
(when remaining
(destructuring-bind (node . remaining) remaining
(allocate-register node allocation-state)
(recur (append remaining (node-children node))))))))
(defun allocate-registers (node clause-props &key nead)
(allocate-argument-registers node)
(allocate-nonargument-registers node clause-props :nead nead))