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(* *)
(* 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 analagous 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
|