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Diffstat (limited to 'middle_end/flambda/lift_constants.mli')
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diff --git a/middle_end/flambda/lift_constants.mli b/middle_end/flambda/lift_constants.mli new file mode 100644 index 0000000000..969c365e33 --- /dev/null +++ b/middle_end/flambda/lift_constants.mli @@ -0,0 +1,65 @@ +(**************************************************************************) +(* *) +(* 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 |