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178a40928a
* module/language/tree-il.scm (<prompt>): Change to have the body and
handler be lambdas, and add an "escape-only?" field. This will make
generic prompts work better in CPS or ANF with the RTL VM, as it
doesn't make sense in that context to capture only part of a frame.
Escape-only prompts can still be fully inlined.
(parse-tree-il, unparse-tree-il): Add escape-only? to the
serialization.
(make-tree-il-folder, pre-post-order): Deal with escape-only?.
* module/language/tree-il/analyze.scm (analyze-lexicals): Handle
escape-only?, and the new expectations for the body and handler.
* module/language/tree-il/canonicalize.scm (canonicalize): Ensure that
the body of an escape-only continuation is a thunk, and that the
handler is always a lambda.
* module/language/tree-il/debug.scm (verify-tree-il): Assert that
escape-only? is a boolean.
* module/language/tree-il/cse.scm (cse):
* module/language/tree-il/effects.scm (make-effects-analyzer):
* module/language/tree-il/peval.scm (peval):
* module/language/tree-il/primitives.scm (*primitive-expand-table*):
* test-suite/tests/peval.test ("partial evaluation"):
* module/language/tree-il/compile-glil.scm (flatten-lambda-case): Adapt
to <prompt> change.
375 lines
14 KiB
Scheme
375 lines
14 KiB
Scheme
;;; Effects analysis on Tree-IL
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;; Copyright (C) 2011, 2012, 2013 Free Software Foundation, Inc.
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;;;; This library is free software; you can redistribute it and/or
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;;;; modify it under the terms of the GNU Lesser General Public
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;;;; License as published by the Free Software Foundation; either
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;;;; version 3 of the License, or (at your option) any later version.
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;;;;
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;;;; This library is distributed in the hope that it will be useful,
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;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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;;;; Lesser General Public License for more details.
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;;;;
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;;;; You should have received a copy of the GNU Lesser General Public
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;;;; License along with this library; if not, write to the Free Software
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;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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(define-module (language tree-il effects)
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#:use-module (language tree-il)
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#:use-module (language tree-il primitives)
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#:use-module (ice-9 match)
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#:export (make-effects-analyzer
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&mutable-lexical
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&toplevel
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&fluid
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&definite-bailout
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&possible-bailout
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&zero-values
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&allocation
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&mutable-data
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&type-check
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&all-effects
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effects-commute?
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exclude-effects
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effect-free?
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constant?
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depends-on-effects?
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causes-effects?))
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;;;
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;;; Hey, it's some effects analysis! If you invoke
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;;; `make-effects-analyzer', you get a procedure that computes the set
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;;; of effects that an expression depends on and causes. This
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;;; information is useful when writing algorithms that move code around,
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;;; while preserving the semantics of an input program.
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;;;
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;;; The effects set is represented by a bitfield, as a fixnum. The set
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;;; of possible effects is modelled rather coarsely. For example, a
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;;; toplevel reference to FOO is modelled as depending on the &toplevel
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;;; effect, and causing a &type-check effect. If any intervening code
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;;; sets any toplevel variable, that will block motion of FOO.
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;;;
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;;; For each effect, two bits are reserved: one to indicate that an
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;;; expression depends on the effect, and the other to indicate that an
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;;; expression causes the effect.
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;;;
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(define-syntax define-effects
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(lambda (x)
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(syntax-case x ()
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((_ all name ...)
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(with-syntax (((n ...) (iota (length #'(name ...)))))
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#'(begin
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(define-syntax name (identifier-syntax (ash 1 (* n 2))))
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...
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(define-syntax all (identifier-syntax (logior name ...)))))))))
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;; Here we define the effects, indicating the meaning of the effect.
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;;
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;; Effects that are described in a "depends on" sense can also be used
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;; in the "causes" sense.
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;;
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;; Effects that are described as causing an effect are not usually used
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;; in a "depends-on" sense. Although the "depends-on" sense is used
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;; when checking for the existence of the "causes" effect, the effects
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;; analyzer will not associate the "depends-on" sense of these effects
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;; with any expression.
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;;
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(define-effects &all-effects
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;; Indicates that an expression depends on the value of a mutable
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;; lexical variable.
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&mutable-lexical
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;; Indicates that an expression depends on the value of a toplevel
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;; variable.
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&toplevel
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;; Indicates that an expression depends on the value of a fluid
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;; variable.
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&fluid
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;; Indicates that an expression definitely causes a non-local,
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;; non-resumable exit -- a bailout. Only used in the "changes" sense.
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&definite-bailout
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;; Indicates that an expression may cause a bailout.
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&possible-bailout
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;; Indicates than an expression may return zero values -- a "causes"
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;; effect.
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&zero-values
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;; Indicates that an expression may return a fresh object -- a
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;; "causes" effect.
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&allocation
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;; Indicates that an expression depends on the value of a mutable data
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;; structure.
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&mutable-data
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;; Indicates that an expression may cause a type check. A type check,
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;; for the purposes of this analysis, is the possibility of throwing
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;; an exception the first time an expression is evaluated. If the
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;; expression did not cause an exception to be thrown, users can
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;; assume that evaluating the expression again will not cause an
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;; exception to be thrown.
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;;
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;; For example, (+ x y) might throw if X or Y are not numbers. But if
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;; it doesn't throw, it should be safe to elide a dominated, common
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;; subexpression (+ x y).
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&type-check)
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(define-syntax &no-effects (identifier-syntax 0))
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;; Definite bailout is an oddball effect. Since it indicates that an
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;; expression definitely causes bailout, it's not in the set of effects
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;; of a call to an unknown procedure. At the same time, it's also
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;; special in that a definite bailout in a subexpression doesn't always
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;; cause an outer expression to include &definite-bailout in its
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;; effects. For that reason we have to treat it specially.
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;;
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(define-syntax &all-effects-but-bailout
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(identifier-syntax
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(logand &all-effects (lognot &definite-bailout))))
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(define-inlinable (cause effect)
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(ash effect 1))
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(define-inlinable (&depends-on a)
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(logand a &all-effects))
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(define-inlinable (&causes a)
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(logand a (cause &all-effects)))
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(define (exclude-effects effects exclude)
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(logand effects (lognot (cause exclude))))
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(define (effect-free? effects)
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(zero? (&causes effects)))
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(define (constant? effects)
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(zero? effects))
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(define-inlinable (depends-on-effects? x effects)
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(not (zero? (logand (&depends-on x) effects))))
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(define-inlinable (causes-effects? x effects)
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(not (zero? (logand (&causes x) (cause effects)))))
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(define-inlinable (effects-commute? a b)
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(and (not (causes-effects? a (&depends-on b)))
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(not (causes-effects? b (&depends-on a)))))
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(define (make-effects-analyzer assigned-lexical?)
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"Returns a procedure of type EXP -> EFFECTS that analyzes the effects
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of an expression."
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(let ((cache (make-hash-table)))
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(define* (compute-effects exp #:optional (lookup (lambda (x) #f)))
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(define (compute-effects exp)
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(or (hashq-ref cache exp)
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(let ((effects (visit exp)))
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(hashq-set! cache exp effects)
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effects)))
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(define (accumulate-effects exps)
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(let lp ((exps exps) (out &no-effects))
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(if (null? exps)
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out
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(lp (cdr exps) (logior out (compute-effects (car exps)))))))
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(define (visit exp)
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(match exp
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(($ <const>)
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&no-effects)
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(($ <void>)
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&no-effects)
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(($ <lexical-ref> _ _ gensym)
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(if (assigned-lexical? gensym)
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&mutable-lexical
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&no-effects))
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(($ <lexical-set> _ name gensym exp)
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(logior (cause &mutable-lexical)
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(compute-effects exp)))
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(($ <let> _ names gensyms vals body)
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(logior (if (or-map assigned-lexical? gensyms)
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(cause &allocation)
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&no-effects)
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(accumulate-effects vals)
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(compute-effects body)))
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(($ <letrec> _ in-order? names gensyms vals body)
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(logior (if (or-map assigned-lexical? gensyms)
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(cause &allocation)
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&no-effects)
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(accumulate-effects vals)
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(compute-effects body)))
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(($ <fix> _ names gensyms vals body)
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(logior (if (or-map assigned-lexical? gensyms)
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(cause &allocation)
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&no-effects)
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(accumulate-effects vals)
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(compute-effects body)))
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(($ <let-values> _ producer consumer)
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(logior (compute-effects producer)
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(compute-effects consumer)
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(cause &type-check)))
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(($ <toplevel-ref>)
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(logior &toplevel
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(cause &type-check)))
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(($ <module-ref>)
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(logior &toplevel
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(cause &type-check)))
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(($ <module-set> _ mod name public? exp)
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(logior (cause &toplevel)
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(cause &type-check)
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(compute-effects exp)))
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(($ <toplevel-define> _ name exp)
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(logior (cause &toplevel)
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(compute-effects exp)))
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(($ <toplevel-set> _ name exp)
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(logior (cause &toplevel)
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(compute-effects exp)))
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(($ <primitive-ref>)
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&no-effects)
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(($ <conditional> _ test consequent alternate)
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(let ((tfx (compute-effects test))
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(cfx (compute-effects consequent))
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(afx (compute-effects alternate)))
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(if (causes-effects? (logior tfx (logand afx cfx))
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&definite-bailout)
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(logior tfx cfx afx)
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(exclude-effects (logior tfx cfx afx)
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&definite-bailout))))
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;; Zero values.
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(($ <primcall> _ 'values ())
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(cause &zero-values))
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;; Effect-free primitives.
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(($ <primcall> _ (or 'values 'eq? 'eqv? 'equal?) args)
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(accumulate-effects args))
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(($ <primcall> _ (or 'not 'pair? 'null? 'list? 'symbol?
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'vector? 'struct? 'string? 'number?
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'char?)
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(arg))
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(compute-effects arg))
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;; Primitives that allocate memory.
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(($ <primcall> _ 'cons (x y))
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(logior (compute-effects x) (compute-effects y)
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&allocation))
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(($ <primcall> _ (or 'list 'vector) args)
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(logior (accumulate-effects args) &allocation))
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(($ <primcall> _ 'make-prompt-tag ())
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&allocation)
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(($ <primcall> _ 'make-prompt-tag (arg))
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(logior (compute-effects arg) &allocation))
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(($ <primcall> _ 'fluid-ref (fluid))
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(logior (compute-effects fluid)
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(cause &type-check)
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&fluid))
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(($ <primcall> _ 'fluid-set! (fluid exp))
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(logior (compute-effects fluid)
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(compute-effects exp)
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(cause &type-check)
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(cause &fluid)))
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(($ <primcall> _ 'push-fluid (fluid val))
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(logior (compute-effects fluid)
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(compute-effects val)
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(cause &type-check)
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(cause &fluid)))
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(($ <primcall> _ 'pop-fluid ())
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(logior (cause &fluid)))
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;; Primitives that are normally effect-free, but which might
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;; cause type checks, allocate memory, or access mutable
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;; memory. FIXME: expand, to be more precise.
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(($ <primcall> _ (and name (? effect-free-primitive?)) args)
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(logior (accumulate-effects args)
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(cause &type-check)
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(if (constructor-primitive? name)
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(cause &allocation)
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(if (accessor-primitive? name)
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&mutable-data
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&no-effects))))
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;; Lambda applications might throw wrong-number-of-args.
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(($ <call> _ ($ <lambda> _ _ body) args)
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(logior (accumulate-effects args)
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(match body
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(($ <lambda-case> _ req #f #f #f () syms body #f)
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(logior (compute-effects body)
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(if (= (length req) (length args))
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0
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(cause &type-check))))
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(($ <lambda-case>)
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(logior (compute-effects body)
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(cause &type-check)))
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(#f
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;; Calling a case-lambda with no clauses
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;; definitely causes bailout.
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(logior (cause &definite-bailout)
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(cause &possible-bailout))))))
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;; Bailout primitives.
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(($ <primcall> _ (? bailout-primitive? name) args)
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(logior (accumulate-effects args)
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(cause &definite-bailout)
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(cause &possible-bailout)))
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;; A call to a lexically bound procedure, perhaps labels
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;; allocated.
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(($ <call> _ (and proc ($ <lexical-ref> _ _ sym)) args)
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(cond
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((lookup sym)
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=> (lambda (proc)
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(compute-effects (make-call #f proc args))))
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(else
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(logior &all-effects-but-bailout
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(cause &all-effects-but-bailout)))))
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;; A call to an unknown procedure can do anything.
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(($ <primcall> _ name args)
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(logior &all-effects-but-bailout
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(cause &all-effects-but-bailout)))
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(($ <call> _ proc args)
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(logior &all-effects-but-bailout
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(cause &all-effects-but-bailout)))
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(($ <lambda> _ meta body)
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&no-effects)
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(($ <lambda-case> _ req opt rest kw inits gensyms body alt)
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(logior (exclude-effects (accumulate-effects inits)
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&definite-bailout)
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(if (or-map assigned-lexical? gensyms)
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(cause &allocation)
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&no-effects)
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(compute-effects body)
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(if alt (compute-effects alt) &no-effects)))
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(($ <seq> _ head tail)
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(logior
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;; Returning zero values to a for-effect continuation is
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;; not observable.
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(exclude-effects (compute-effects head)
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(cause &zero-values))
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(compute-effects tail)))
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(($ <prompt> _ escape-only? tag body handler)
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(logior (compute-effects tag)
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(compute-effects body)
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(compute-effects handler)))
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(($ <abort> _ tag args tail)
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(logior &all-effects-but-bailout
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(cause &all-effects-but-bailout)))))
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(compute-effects exp))
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compute-effects))
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