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;;; transformation of top-level bindings into letrec*
;; Copyright (C) 2019-2021,2023 Free Software Foundation, Inc.
;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
;;;; License as published by the Free Software Foundation; either
;;;; version 3 of the License, or (at your option) any later version.
;;;;
;;;; This library is distributed in the hope that it will be useful,
;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;;;; Lesser General Public License for more details.
;;;;
;;;; You should have received a copy of the GNU Lesser General Public
;;;; License along with this library; if not, write to the Free Software
;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
(define-module (language tree-il letrectify)
#:use-module ((srfi srfi-1) #:select (fold-right))
#:use-module (srfi srfi-11)
#:use-module (ice-9 match)
#:use-module (language tree-il)
#:use-module (language tree-il effects)
#:export (letrectify))
;; Take a sequence of top-level definitions and turn the defintions into
;; letrec*. From this:
;;
;; (begin
;; (define a 10)
;; (define b (lambda () a))
;; (foo a)
;; (define c (lambda () (set! c b) (c))))
;;
;; To this:
;;
;; (letrec* ((a-var (module-make-local-var! (current-module) 'a))
;; (a 10)
;; (_ (begin (variable-set! a-var a)))
;; (b-var (module-make-local-var! (current-module) 'b))
;; (b (lambda () a))
;; (_ (begin (variable-set! b-var b)))
;; (_ (begin (foo a) #t))
;; (c-var (module-make-local-var! (current-module) 'c)))
;; (c (lambda () (variable-set! c-var b) ((variable-ref c-var))))
;; (_ (begin (variable-set! c-var c))))
;; (void))
;;
;; Inside the compilation unit, references to "declarative" top-level
;; definitions are accessed directly as lexicals. A declarative
;; definition is a variable for which the expander knows the module,
;; which is defined in the compilation unit exactly one time, and which
;; is not assigned in the compilation unit.
;;
;; The assumption is that it's safe for the compiler to reason about the
;; *values* of declarative bindings, because they are immutable in
;; practice. Of course someone can come later from another compilation
;; unit or another module and use the private module API to mutate
;; definitions from this compilation unit; in that case, updates from
;; that third party may not be visible to users of declarative
;; definitions. That kind of use is not common, though. The letrectify
;; transformation is so important for performance that most users are
;; willing to accept the restrictions of this transformation.
;;
;; Incidentally, the later fix-letrec and peval passes should optimize
;; the above example to:
;;
;; (begin
;; (variable-set! (module-make-local-var! (current-module) 'a) 10)
;; (variable-set! (module-make-local-var! (current-module) 'b)
;; (lambda () 10))
;; (foo 10)
;; (let ((c-var (module-make-local-var! (current-module) 'c)))
;; (variable-set! c-var
;; (lambda ()
;; (variable-set! c-var (lambda () 10))
;; ((variable-ref c-var))))
;; (void)))
;;
;; As you can see, letrectification allowed for inlining of the uses of
;; both A and B.
;;
(define for-each-fold (make-tree-il-folder))
(define (tree-il-for-each f x)
(for-each-fold x (lambda (x) (f x) (values)) (lambda (x) (values))))
(define (compute-declarative-toplevels x)
(define dynamic (make-hash-table))
(define defined (make-hash-table))
(define assigned (make-hash-table))
(tree-il-for-each
(lambda (x)
(match x
(($ <toplevel-set> src mod name)
(if mod
(hash-set! assigned (cons mod name) #t)
(hashq-set! dynamic name #t)))
(($ <toplevel-define> src mod name expr)
(if mod
(hash-set! (if (hash-ref defined (cons mod name))
assigned
defined)
(cons mod name) expr)
(hashq-set! dynamic name #t)))
(_ (values))))
x)
(let ((declarative (make-hash-table)))
(define (declarative-module? mod)
(let ((m (resolve-module mod #f #:ensure #f)))
(and m (module-declarative? m))))
(hash-for-each (lambda (k expr)
(match k
((mod . name)
(unless (or (hash-ref assigned k)
(hashq-ref dynamic name)
(not (declarative-module? mod)))
(hash-set! declarative k expr)))))
defined)
declarative))
(define (compute-private-toplevels declarative)
;; Set of variables exported by the modules of declarative bindings in
;; this compilation unit.
(define exports (make-hash-table))
;; If a module exports a macro, that macro could implicitly export any
;; top-level binding in a module; we have to avoid sealing private
;; bindings in that case.
(define exports-macro? (make-hash-table))
(hash-for-each
(lambda (k _)
(match k
((mod . name)
(unless (hash-get-handle exports-macro? mod)
(hash-set! exports-macro? mod #f)
(let ((i (module-public-interface (resolve-module mod))))
(when i
(module-for-each
(lambda (k v)
(hashq-set! exports v k)
(when (and (variable-bound? v) (macro? (variable-ref v)))
(hash-set! exports-macro? mod #t)))
i)))))))
declarative)
(let ((private (make-hash-table)))
(hash-for-each
(lambda (k _)
(match k
((mod . name)
(unless (or (hash-ref exports-macro? mod)
(hashq-ref exports
(module-local-variable (resolve-module mod) name)))
(hash-set! private k #t)))))
declarative)
private))
(define* (letrectify expr #:key (seal-private-bindings? #f))
(define declarative (compute-declarative-toplevels expr))
(define private
(if seal-private-bindings?
(compute-private-toplevels declarative)
(make-hash-table)))
(define declarative-box+value
(let ((tab (make-hash-table)))
(hash-for-each (lambda (key val)
(let ((box (and (not (hash-ref private key))
(gensym)))
(val (gensym)))
(hash-set! tab key (cons box val))))
declarative)
(lambda (mod name)
(hash-ref tab (cons mod name)))))
(define compute-effects
;; Assume all lexicals are assigned, for the purposes of this
;; transformation. (It doesn't matter.)
(let ((assigned? (lambda (sym) #t)))
(make-effects-analyzer assigned?)))
(define (can-elide-statement? stmt)
(let ((effects (compute-effects stmt)))
(effect-free?
(exclude-effects effects (logior &allocation &zero-values)))))
(define (add-binding name var val tail)
(match tail
(($ <letrec> src #t names vars vals tail)
(make-letrec src #t
(cons name names) (cons var vars) (cons val vals)
tail))
(_
(make-letrec (tree-il-srcv tail) #t
(list name) (list var) (list val)
tail))))
(define (add-statement src stmt tail)
(if (can-elide-statement? stmt)
tail
(add-binding '_ (gensym "_") (make-seq src stmt (make-void src))
tail)))
(define (visit-expr expr)
(post-order
(lambda (expr)
(match expr
(($ <toplevel-ref> src mod name)
(match (declarative-box+value mod name)
(#f expr)
((box . value)
(make-lexical-ref src name value))))
(_ expr)))
expr))
(define (visit-top-level expr mod-vars)
(match expr
(($ <toplevel-define> src mod name exp)
(match (declarative-box+value mod name)
(#f (values (visit-expr expr) mod-vars))
((#f . value)
(values (add-binding name value (visit-expr exp) (make-void src))
mod-vars))
((box . value)
(match (assoc-ref mod-vars mod)
(#f
(let* ((mod-var (gensym "mod"))
(mod-vars (acons mod mod-var mod-vars)))
(call-with-values (lambda () (visit-top-level expr mod-vars))
(lambda (tail mod-vars)
(values
(add-binding 'mod
mod-var
(make-primcall src 'current-module '())
tail)
mod-vars)))))
(mod-var
(let* ((loc
(make-primcall src 'module-ensure-local-variable!
(list (make-lexical-ref src 'mod mod-var)
(make-const src name))))
(exp (visit-expr exp))
(ref (make-lexical-ref src name value))
(init
(make-primcall src '%variable-set!
(list (make-lexical-ref src name box)
ref))))
(values
(add-binding
name box loc
(add-binding
name value exp
(add-statement src init (make-void src))))
mod-vars)))))))
(($ <let> src names vars vals body)
(let lp ((names names) (vars vars) (vals vals) (mod-vars mod-vars))
(match (vector names vars vals)
(#(() () ())
(values (visit-expr body) mod-vars))
(#((name . names) (var . vars) (val . vals))
(let* ((val (visit-expr val))
(mod-vars
(match val
(($ <call> _
($ <module-ref> _ '(guile) 'define-module* #f)
(($ <const> _ mod) . args))
(acons mod var mod-vars))
(_ mod-vars))))
(let-values (((exp mod-vars) (lp names vars vals mod-vars)))
(values (add-binding name var val exp)
mod-vars)))))))
(($ <seq> src head tail)
(let*-values (((head mod-vars) (visit-top-level head mod-vars))
((tail mod-vars) (visit-top-level tail mod-vars)))
(values (match head
(($ <letrec> src2 #t names vars vals head)
(fold-right add-binding (add-statement src head tail)
names vars vals))
(else
(add-statement src head tail)))
mod-vars)))
;; What would the advantages/disadvantages be if we flattened all
;; bindings here, even those from nested let/letrec?
(_ (values (visit-expr expr) mod-vars))))
(values (visit-top-level expr '())))
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