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Update effects-analysis docstring.

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* module/language/cps/effects-analysis.scm: Update docs.
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Andy Wingo 2014-05-16 16:57:58 +02:00
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commit 146c8e72a9
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module/language/cps/effects-analysis.scm
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@ -18,24 +18,24 @@
;;; Commentary:
;;;
;;; A helper module to compute the set of effects that an expression
;;; depends on and causes. This information is useful when writing
;;; algorithms that move code around, while preserving the semantics of
;;; an input program.
;;; A helper module to compute the set of effects caused by an
;;; expression. This information is useful when writing algorithms that
;;; move code around, while preserving the semantics of an input
;;; program.
;;;
;;; The effects set is represented by a bitfield, as a fixnum. The set
;;; of possible effects is modelled rather coarsely. For example, a
;;; "car" call modelled as depending on the &car effect, and causing a
;;; &type-check effect. If any intervening code sets the car of any
;;; pair, that will block motion of the "car" call.
;;; The effects set is represented as an integer with three parts. The
;;; low 4 bits indicate effects caused by an expression, as a bitfield.
;;; The next 4 bits indicate the kind of memory accessed by the
;;; expression, if it accesses mutable memory. Finally the rest of the
;;; bits indicate the field in the object being accessed, if known, or
;;; -1 for unknown.
;;;
;;; For each effect, two bits are reserved: one to indicate that an
;;; expression depends on the effect, and the other to indicate that an
;;; expression causes the effect.
;;;
;;; Since we have more bits in a fixnum on 64-bit systems, we can be
;;; more precise without losing efficiency. On a 32-bit system, some of
;;; the more precise effects map to fewer bits.
;;; In this way we embed a coarse type-based alias analysis in the
;;; effects analysis. For example, a "car" call is modelled as causing
;;; a read to field 0 on a &pair, and causing a &type-check effect. If
;;; any intervening code sets the car of any pair, that will block
;;; motion of the "car" call, because any write to field 0 of a pair is
;;; seen by effects analysis as being a write to field 0 of all pairs.
;;;
;;; Code: