66 lines
3.5 KiB
OCaml
66 lines
3.5 KiB
OCaml
(**************************************************************************)
|
|
(* *)
|
|
(* OCaml *)
|
|
(* *)
|
|
(* Pierre Chambart, OCamlPro *)
|
|
(* Mark Shinwell and Leo White, Jane Street Europe *)
|
|
(* *)
|
|
(* Copyright 2013--2016 OCamlPro SAS *)
|
|
(* Copyright 2014--2016 Jane Street Group LLC *)
|
|
(* *)
|
|
(* All rights reserved. This file is distributed under the terms of *)
|
|
(* the GNU Lesser General Public License version 2.1, with the *)
|
|
(* special exception on linking described in the file LICENSE. *)
|
|
(* *)
|
|
(**************************************************************************)
|
|
|
|
[@@@ocaml.warning "+a-4-9-30-40-41-42"]
|
|
|
|
(** The aim of this pass is to assign symbols to values known to be
|
|
constant (in other words, whose values we know at compile time), with
|
|
appropriate sharing of constants, and replace the occurrences of the
|
|
constants with their corresponding symbols.
|
|
|
|
This pass uses the results of two other passes, [Inconstant_idents] and
|
|
[Alias_analysis]. The relationship between these two deserves some
|
|
attention.
|
|
|
|
[Inconstant_idents] is a "backwards" analysis that propagates implications
|
|
about inconstantness of variables and set of closures IDs.
|
|
|
|
[Alias_analysis] is a "forwards" analysis that is analogous to the
|
|
propagation of [Simple_value_approx.t] values during [Inline_and_simplify].
|
|
It gives us information about relationships between values but not actually
|
|
about their constantness.
|
|
|
|
Combining these two into a single pass has been attempted previously,
|
|
but was not thought to be successful; this experiment could be repeated in
|
|
the future. (If "constant" is considered as "top" and "inconstant" is
|
|
considered as "bottom", then [Alias_analysis] corresponds to a least fixed
|
|
point and [Inconstant_idents] corresponds to a greatest fixed point.)
|
|
|
|
At a high level, this pass operates as follows. Symbols are assigned to
|
|
variables known to be constant and their defining expressions examined.
|
|
Based on the results of [Alias_analysis], we simplify the destructive
|
|
elements within the defining expressions (specifically, projection of
|
|
fields from blocks), to eventually yield [Flambda.constant_defining_value]s
|
|
that are entirely constructive. These will be bound to symbols in the
|
|
resulting program.
|
|
|
|
Another approach to this pass could be to only use the results of
|
|
[Inconstant_idents] and then repeatedly lift constants and run
|
|
[Inline_and_simplify] until a fixpoint. It was thought more robust to
|
|
instead use [Alias_analysis], where the fixpointing involves a less
|
|
complicated function.
|
|
|
|
We still run [Inline_and_simplify] once after this pass since the lifting
|
|
of constants may enable more functions to become closed; the simplification
|
|
pass provides an easy way of cleaning up (e.g. making sure [free_vars]
|
|
maps in sets of closures are correct).
|
|
*)
|
|
|
|
val lift_constants
|
|
: Flambda.program
|
|
-> backend:(module Backend_intf.S)
|
|
-> Flambda.program
|