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------------------------------------------------------------------------------
-- --
-- GNAT COMPILER COMPONENTS --
-- --
-- E X P _ U N S T --
-- --
-- S p e c --
-- --
-- Copyright (C) 2014-2022, Free Software Foundation, Inc. --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 3, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
-- for more details. You should have received a copy of the GNU General --
-- Public License distributed with GNAT; see file COPYING3. If not, go to --
-- http://www.gnu.org/licenses for a complete copy of the license. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
-- Expand routines for unnesting subprograms
with Table;
with Types; use Types;
package Exp_Unst is
-- -----------------
-- -- The Problem --
-- -----------------
-- Normally, nested subprograms in the source result in corresponding
-- nested subprograms in the resulting tree. We then expect the back end
-- to handle such nested subprograms, including all cases of uplevel
-- references. For example, the GCC back end can do this relatively easily
-- since GNU C (as an extension) allows nested functions with uplevel
-- references, and implements an appropriate static chain approach to
-- dealing with such uplevel references.
-- However, we also want to be able to interface with back ends that do
-- not easily handle such uplevel references. One example is the back end
-- that translates the tree into standard C source code. In the future,
-- other back ends might need the same capability (e.g. a back end that
-- generated LLVM intermediate code).
-- We could imagine simply handling such references in the appropriate
-- back end. For example the back end that generates C could recognize
-- nested subprograms and rig up some way of translating them, e.g. by
-- making a static-link source level visible.
-- Rather than take that approach, we prefer to do a semantics-preserving
-- transformation on the GNAT tree, that eliminates the problem before we
-- hand the tree over to the back end. There are two reasons for preferring
-- this approach:
-- First: the work needs only to be done once for all affected back ends
-- and we can remain within the semantics of the tree. The front end is
-- full of tree transformations, so we have all the infrastructure for
-- doing transformations of this type.
-- Second: given that the transformation will be semantics-preserving,
-- we can still use the standard GCC back end to build code from it.
-- This means we can easily run our full test suite to verify that the
-- transformations are indeed semantics preserving. It is a lot more
-- work to thoroughly test the output of specialized back ends.
-- Looking at the problem, we have three situations to deal with. Note
-- that in these examples, we use all lower case, since that is the way
-- the internal tree is cased.
-- First, cases where there are no uplevel references, for example
-- procedure case1 is
-- function max (m, n : Integer) return integer is
-- begin
-- return integer'max (m, n);
-- end max;
-- ...
-- end case1;
-- Second, cases where there are explicit uplevel references.
-- procedure case2 (b : integer) is
-- procedure Inner (bb : integer);
--
-- procedure inner2 is
-- begin
-- inner(5);
-- end;
--
-- x : integer := 77;
-- y : constant integer := 15 * 16;
-- rv : integer := 10;
--
-- procedure inner (bb : integer) is
-- begin
-- x := rv + y + bb + b;
-- end;
--
-- begin
-- inner2;
-- end case2;
-- In this second example, B, X, RV are uplevel referenced. Y is not
-- considered as an uplevel reference since it is a static constant
-- where references are replaced by the value at compile time.
-- Third, cases where there are implicit uplevel references via types
-- whose bounds depend on locally declared constants or variables:
-- function case3 (x, y : integer) return boolean is
-- subtype dynam is integer range x .. y + 3;
-- subtype static is integer range 42 .. 73;
-- xx : dynam := y;
--
-- type darr is array (dynam) of Integer;
-- type darec is record
-- A : darr;
-- B : integer;
-- end record;
-- darecv : darec;
--
-- function inner (b : integer) return boolean is
-- begin
-- return b in dynam and then darecv.b in static;
-- end inner;
--
-- begin
-- return inner (42) and then inner (xx * 3 - y * 2);
-- end case3;
--
-- In this third example, the membership test implicitly references the
-- the bounds of Dynam, which both involve uplevel references.
-- ------------------
-- -- The Solution --
-- ------------------
-- Looking at the three cases above, the first case poses no problem at
-- all. Indeed the subprogram could have been declared at the outer level
-- (perhaps changing the name). But this style is quite common as a way
-- of limiting the scope of a local procedure called only within the outer
-- procedure. We could move it to the outer level (with a name change if
-- needed), but we don't bother. We leave it nested, and the back end just
-- translates it as though it were not nested.
-- In general we leave nested procedures nested, rather than trying to move
-- them to the outer level (the back end may do that, e.g. as part of the
-- translation to C, but we don't do it in the tree itself). This saves a
-- LOT of trouble in terms of visibility and semantics.
-- But of course we have to deal with the uplevel references. The idea is
-- to rewrite these nested subprograms so that they no longer have any such
-- uplevel references, so by the time they reach the back end, they all are
-- case 1 (no uplevel references) and thus easily handled.
-- To deal with explicit uplevel references (case 2 above), we proceed with
-- the following steps:
-- All entities marked as being uplevel referenced are marked as aliased
-- since they will be accessed indirectly via an activation record as
-- described below.
-- An activation record is created containing system address values
-- for each uplevel referenced entity in a given scope. In the example
-- given before, we would have:
-- type AREC1T is record
-- b : Address;
-- x : Address;
-- rv : Address;
-- end record;
-- type AREC1PT is access all AREC1T;
-- AREC1 : aliased AREC1T;
-- AREC1P : constant AREC1PT := AREC1'Access;
-- The fields of AREC1 are set at the point the corresponding entity
-- is declared (immediately for parameters).
-- Note: the 1 in all these names is a unique index number. Different
-- scopes requiring different ARECnT declarations will have different
-- values of n to ensure uniqueness.
-- Note: normally the field names in the activation record match the
-- name of the entity. An exception is when the entity is declared in
-- a declare block, in which case we append the entity number, to avoid
-- clashes between the same name declared in different declare blocks.
-- For all subprograms nested immediately within the corresponding scope,
-- a parameter AREC1F is passed, and all calls to these routines have
-- AREC1P added as an additional formal.
-- Now within the nested procedures, any reference to an uplevel entity
-- xxx is replaced by typ'Deref(AREC1.xxx) where typ is the type of the
-- reference.
-- Note: the reason that we use Address as the component type in the
-- declaration of AREC1T is that we may create this type before we see
-- the declaration of this type.
-- The following shows example 2 above after this translation:
-- procedure case2x (b : aliased Integer) is
-- type AREC1T is record
-- b : Address;
-- x : Address;
-- rv : Address;
-- end record;
--
-- type AREC1PT is access all AREC1T;
--
-- AREC1 : aliased AREC1T;
-- AREC1P : constant AREC1PT := AREC1'Access;
--
-- AREC1.b := b'Address;
--
-- procedure inner (bb : integer; AREC1F : AREC1PT);
--
-- procedure inner2 (AREC1F : AREC1PT) is
-- begin
-- inner(5, AREC1F);
-- end;
--
-- x : aliased integer := 77;
-- AREC1.x := X'Address;
--
-- y : constant Integer := 15 * 16;
--
-- rv : aliased Integer;
-- AREC1.rv := rv'Address;
--
-- procedure inner (bb : integer; AREC1F : AREC1PT) is
-- begin
-- Integer'Deref(AREC1F.x) :=
-- Integer'Deref(AREC1F.rv) + y + b + Integer'Deref(AREC1F.b);
-- end;
--
-- begin
-- inner2 (AREC1P);
-- end case2x;
-- And now the inner procedures INNER2 and INNER have no uplevel references
-- so they have been reduced to case 1, which is the case easily handled by
-- the back end. Note that the generated code is not strictly legal Ada
-- because of the assignments to AREC1 in the declarative sequence, but the
-- GNAT tree always allows such mixing of declarations and statements, so
-- the back end must be prepared to handle this in any case.
-- Case 3 where we have uplevel references to types is a bit more complex.
-- That would especially be the case if we did a full transformation that
-- completely eliminated such uplevel references as we did for case 2. But
-- instead of trying to do that, we rewrite the subprogram so that the code
-- generator can easily detect and deal with these uplevel type references.
-- First we distinguish two cases
-- Static types are one of the two following cases:
-- Discrete types whose bounds are known at compile time. This is not
-- quite the same as what is tested by Is_OK_Static_Subtype, in that
-- it allows compile time known values that are not static expressions.
-- Composite types, whose components are (recursively) static types.
-- Dynamic types are one of the two following cases:
-- Discrete types with at least one bound not known at compile time.
-- Composite types with at least one component that is (recursively)
-- a dynamic type.
-- Uplevel references to static types are not a problem, the front end
-- or the code generator fetches the bounds as required, and since they
-- are compile time known values, this value can just be extracted and
-- no actual uplevel reference is required.
-- Uplevel references to dynamic types are a potential problem, since
-- such references may involve an implicit access to a dynamic bound,
-- and this reference is an implicit uplevel access.
-- To fully unnest such references would be messy, since we would have
-- to create local copies of the dynamic types involved, so that the
-- front end or code generator could generate an explicit uplevel
-- reference to the bound involved. Rather than do that, we set things
-- up so that this situation can be easily detected and dealt with when
-- there is an implicit reference to the bounds.
-- What we do is to always generate a local constant for any dynamic
-- bound in a dynamic subtype xx with name xx_FIRST or xx_LAST. The one
-- case where we can skip this is where the bound is already a constant.
-- E.g. in the third example above, subtype dynam is expanded as
-- dynam_LAST : constant Integer := y + 3;
-- subtype dynam is integer range x .. dynam_LAST;
-- Now if type dynam is uplevel referenced (as it is in this case), then
-- the bounds x and dynam_LAST are marked as uplevel references
-- so that appropriate entries are made in the activation record. Any
-- explicit reference to such a bound in the front end generated code
-- will be handled by the normal uplevel reference mechanism which we
-- described above for case 2. For implicit references by a back end
-- that needs to unnest things, any such implicit reference to one of
-- these bounds can be replaced by an appropriate reference to the entry
-- in the activation record for xx_FIRST or xx_LAST. Thus the back end
-- can eliminate the problematical uplevel reference without the need to
-- do the heavy tree modification to do that at the code expansion level.
-- Looking at case 3 again, here is the normal -gnatG expanded code
-- function case3 (x : integer; y : integer) return boolean is
-- dynam_LAST : constant integer := y {+} 3;
-- subtype dynam is integer range x .. dynam_LAST;
-- subtype static is integer range 42 .. 73;
--
-- [constraint_error when
-- not (y in x .. dynam_LAST)
-- "range check failed"]
--
-- xx : dynam := y;
--
-- type darr is array (x .. dynam_LAST) of integer;
-- type darec is record
-- a : darr;
-- b : integer;
-- end record;
-- [type TdarrB is array (x .. dynam_LAST range <>) of integer]
-- freeze TdarrB []
-- darecv : darec;
--
-- function inner (b : integer) return boolean is
-- begin
-- return b in x .. dynam_LAST and then darecv.b in 42 .. 73;
-- end inner;
-- begin
-- return inner (42) and then inner (xx {*} 3 {-} y {*} 2);
-- end case3;
-- Note: the actual expanded code has fully qualified names so for
-- example function inner is actually function case3__inner. For now
-- we ignore that detail to clarify the examples.
-- Here we see that some of the bounds references are expanded by the
-- front end, so that we get explicit references to y or dynam_Last. These
-- cases are handled by the normal uplevel reference mechanism described
-- above for case 2. This is the case for the constraint check for the
-- initialization of xx, and the range check in function inner.
-- But the reference darecv.b in the return statement of function
-- inner has an implicit reference to the bounds of dynam, since to
-- compute the location of b in the record, we need the length of a.
-- Here is the full translation of the third example:
-- function case3x (x, y : integer) return boolean is
-- type AREC1T is record
-- x : Address;
-- dynam_LAST : Address;
-- end record;
--
-- type AREC1PT is access all AREC1T;
--
-- AREC1 : aliased AREC1T;
-- AREC1P : constant AREC1PT := AREC1'Access;
--
-- AREC1.x := x'Address;
--
-- dynam_LAST : constant integer := y {+} 3;
-- AREC1.dynam_LAST := dynam_LAST'Address;
-- subtype dynam is integer range x .. dynam_LAST;
-- xx : dynam := y;
--
-- [constraint_error when
-- not (y in x .. dynam_LAST)
-- "range check failed"]
--
-- subtype static is integer range 42 .. 73;
--
-- type darr is array (x .. dynam_LAST) of Integer;
-- type darec is record
-- A : darr;
-- B : integer;
-- end record;
-- darecv : darec;
--
-- function inner (b : integer; AREC1F : AREC1PT) return boolean is
-- begin
-- return b in x .. Integer'Deref(AREC1F.dynam_LAST)
-- and then darecv.b in 42 .. 73;
-- end inner;
--
-- begin
-- return inner (42, AREC1P) and then inner (xx * 3, AREC1P);
-- end case3x;
-- And now the back end when it processes darecv.b will access the bounds
-- of darecv.a by referencing the d and dynam_LAST fields of AREC1P.
-----------------------------
-- Multiple Nesting Levels --
-----------------------------
-- In our examples so far, we have only nested to a single level, but the
-- scheme generalizes to multiple levels of nesting and in this section we
-- discuss how this generalization works.
-- Consider this example with two nesting levels
-- To deal with elimination of uplevel references, we follow the same basic
-- approach described above for case 2, except that we need an activation
-- record at each nested level. Basically the rule is that any procedure
-- that has nested procedures needs an activation record. When we do this,
-- the inner activation records have a pointer (uplink) to the immediately
-- enclosing activation record, the normal arrangement of static links. The
-- following shows the full translation of this fourth case.
-- function case4x (x : integer) return integer is
-- type AREC1T is record
-- v1 : Address;
-- end record;
--
-- type AREC1PT is access all AREC1T;
--
-- AREC1 : aliased AREC1T;
-- AREC1P : constant AREC1PT := AREC1'Access;
--
-- v1 : integer := x;
-- AREC1.v1 := v1'Address;
--
-- function inner1 (y : integer; AREC1F : AREC1PT) return integer is
-- type AREC2T is record
-- AREC1U : AREC1PT;
-- v2 : Address;
-- end record;
--
-- type AREC2PT is access all AREC2T;
--
-- AREC2 : aliased AREC2T;
-- AREC2P : constant AREC2PT := AREC2'Access;
--
-- AREC2.AREC1U := AREC1F;
--
-- v2 : integer := Integer'Deref (AREC1F.v1) {+} 1;
-- AREC2.v2 := v2'Address;
--
-- function inner2
-- (z : integer; AREC2F : AREC2PT) return integer
-- is
-- begin
-- return integer(z {+}
-- Integer'Deref (AREC2F.AREC1U.v1) {+}
-- Integer'Deref (AREC2F.v2).all);
-- end inner2;
-- begin
-- return integer(y {+}
-- inner2 (Integer'Deref (AREC1F.v1), AREC2P));
-- end inner1;
-- begin
-- return inner1 (x, AREC1P);
-- end case4x;
-- As can be seen in this example, the index numbers following AREC in the
-- generated names avoid confusion between AREC names at different levels.
-------------------------
-- Name Disambiguation --
-------------------------
-- As described above, the translation scheme would raise issues when the
-- code generator did the actual unnesting if identically named nested
-- subprograms exist. Similarly overloading would cause a naming issue.
-- In fact, the expanded code includes qualified names which eliminate this
-- problem. We omitted the qualification from the expanded examples above
-- for simplicity. But to see this in action, consider this example:
-- function Mnames return Boolean is
-- procedure Inner is
-- procedure Inner is
-- begin
-- null;
-- end;
-- begin
-- Inner;
-- end;
-- function F (A : Boolean) return Boolean is
-- begin
-- return not A;
-- end;
-- function F (A : Integer) return Boolean is
-- begin
-- return A > 42;
-- end;
-- begin
-- Inner;
-- return F (42) or F (True);
-- end;
-- The expanded code actually looks like:
-- function mnames return boolean is
-- procedure mnames__inner is
-- procedure mnames__inner__inner is
-- begin
-- null;
-- return;
-- end mnames__inner__inner;
-- begin
-- mnames__inner__inner;
-- return;
-- end mnames__inner;
-- function mnames__f (a : boolean) return boolean is
-- begin
-- return not a;
-- end mnames__f;
-- function mnames__f__2 (a : integer) return boolean is
-- begin
-- return a > 42;
-- end mnames__f__2;
-- begin
-- mnames__inner;
-- return mnames__f__2 (42) or mnames__f (true);
-- end mnames;
-- As can be seen from studying this example, the qualification deals both
-- with the issue of clashing names (mnames__inner, mnames__inner__inner),
-- and with overloading (mnames__f, mnames__f__2).
-- In addition, the declarations of ARECnT and ARECnPT get moved to the
-- outer level when we actually generate C code, so we suffix these names
-- with the corresponding entity name to make sure they are unique.
---------------------------
-- Terminology for Calls --
---------------------------
-- The level of a subprogram in the nest being analyzed is defined to be
-- the level of nesting, so the outer level subprogram (the one passed to
-- Unnest_Subprogram) is 1, subprograms immediately nested within this
-- outer level subprogram have a level of 2, etc.
-- Calls within the nest being analyzed are of three types:
-- Downward call: this is a call from a subprogram to a subprogram that
-- is immediately nested with in the caller, and thus has a level that
-- is one greater than the caller. It is a fundamental property of the
-- nesting structure and visibility that it is not possible to make a
-- call from level N to level M, where M is greater than N + 1.
-- Parallel call: this is a call from a nested subprogram to another
-- nested subprogram that is at the same level.
-- Upward call: this is a call from a subprogram to a subprogram that
-- encloses the caller. The level of the callee is less than the level
-- of the caller, and there is no limit on the difference, e.g. for an
-- uplevel call, a subprogram at level 5 can call one at level 2 or even
-- the outer level subprogram at level 1.
-------------------------------------
-- Handling of unconstrained types --
-------------------------------------
-- Objects whose nominal subtype is an unconstrained array type present
-- additional complications for translation into LLVM. The address
-- attribute of such objects points to the first component of the
-- array, and the bounds are found elsewhere, typically ahead of the
-- components. In many cases the bounds of an object are stored ahead
-- of the components and can be retrieved from it. However, if the
-- object is an expression (e.g. a slice) the bounds are not adjacent
-- and thus must be conveyed explicitly by means of a so-called
-- fat pointer. This leads to the following enhancements to the
-- handling of uplevel references described so far. This applies only
-- to uplevel references to unconstrained formals of enclosing
-- subprograms:
--
-- a) Uplevel references are detected as before during the tree traversal
-- in Visit_Node. For reference to uplevel formals, we include those with
-- an unconstrained array type (e.g. String) even if such a type has
-- static bounds.
--
-- b) references to unconstrained formals are recognized in the Subp
-- table by means of the predicate Needs_Fat_Pointer.
--
-- c) When constructing the required activation record we also construct
-- a named access type whose designated type is the unconstrained array
-- type. The activation record of a subprogram that contains such an
-- uplevel reference includes a component of this access type. The
-- declaration for that access type is introduced and analyzed before
-- that of the activation record, so it appears in the subprogram that
-- has that formal.
--
-- d) The uplevel reference is rewritten as an explicit dereference (.all)
-- of the corresponding pointer component.
--
-----------
-- Subps --
-----------
-- Table to record subprograms within the nest being currently analyzed.
-- Entries in this table are made for each subprogram expanded, and do not
-- get cleared as we complete the expansion, since we want the table info
-- around in Cprint for the actual unnesting operation. Subps_First in this
-- unit records the starting entry in the table for the entries for Subp
-- and this is also recorded in the Subps_Index field of the outer level
-- subprogram in the nest. The last subps index for the nest can be found
-- in the Subp_Entry Last field of this first entry.
subtype SI_Type is Nat;
-- Index type for the table
Subps_First : SI_Type;
-- Record starting index for entries in the current nest (this is the table
-- index of the entry for Subp itself, and is recorded in the Subps_Index
-- field of the entity for this subprogram).
type Subp_Entry is record
Ent : Entity_Id;
-- Entity of the subprogram
Bod : Node_Id;
-- Subprogram_Body node for this subprogram
Lev : Nat;
-- Subprogram level (1 = outer subprogram (Subp argument), 2 = nested
-- immediately within this outer subprogram etc.)
Reachable : Boolean;
-- This flag is set True if there is a call path from the outer level
-- subprogram to this subprogram. If Reachable is False, it means that
-- the subprogram is declared but not actually referenced. We remove
-- such subprograms from the tree, which simplifies our task, because
-- we don't have to worry about e.g. uplevel references from such an
-- unreferenced subprogram, which might require (useless) activation
-- records to be created. This is computed by setting the outer level
-- subprogram (Subp itself) as reachable, and then doing a transitive
-- closure following all calls.
Uplevel_Ref : Nat;
-- The outermost level which defines entities which this subprogram
-- references either directly or indirectly via a call. This cannot
-- be greater than Lev. If it is equal to Lev, then it means that the
-- subprogram does not make any uplevel references and that thus it
-- does not need an activation record pointer passed. If it is less than
-- Lev, then an activation record pointer is needed, since there is at
-- least one uplevel reference. This is computed by initially setting
-- Uplevel_Ref to Lev for all subprograms. Then on the initial tree
-- traversal, decreasing Uplevel_Ref for an explicit uplevel reference,
-- and finally by doing a transitive closure that follows calls (if A
-- calls B and B has an uplevel reference to level X, then A references
-- level X indirectly).
Declares_AREC : Boolean;
-- This is set True for a subprogram which include the declarations
-- for a local activation record to be passed on downward calls. It
-- is set True for the target level of an uplevel reference, and for
-- all intervening nested subprograms. For example, if a subprogram X
-- at level 5 makes an uplevel reference to an entity declared in a
-- level 2 subprogram, then the subprograms at levels 4,3,2 enclosing
-- the level 5 subprogram will have this flag set True.
Uents : Elist_Id;
-- This is a list of entities declared in this subprogram which are
-- uplevel referenced. It contains both objects (which will be put in
-- the corresponding AREC activation record), and types. The types are
-- not put in the AREC activation record, but referenced bounds (i.e.
-- generated _FIRST and _LAST entities, and formal parameters) will be
-- in the list in their own right.
Last : SI_Type;
-- This field is set only in the entry for the outer level subprogram
-- in a nest, and records the last index in the Subp table for all the
-- entries for subprograms in this nest.
ARECnF : Entity_Id;
-- This entity is defined for all subprograms which need an extra formal
-- that contains a pointer to the activation record needed for uplevel
-- references. ARECnF must be defined for any subprogram which has a
-- direct or indirect uplevel reference (i.e. Reference_Level < Lev).
ARECn : Entity_Id;
ARECnT : Entity_Id;
ARECnPT : Entity_Id;
ARECnP : Entity_Id;
-- These AREC entities are defined only for subprograms for which we
-- generate an activation record declaration, i.e. for subprograms for
-- which the Declares_AREC flag is set True.
ARECnU : Entity_Id;
-- This AREC entity is the uplink component. It is other than Empty only
-- for nested subprograms that declare an activation record as indicated
-- by Declares_AREC being True, and which have uplevel references (Lev
-- greater than Uplevel_Ref). It is the additional component in the
-- activation record that references the ARECnF pointer (which points
-- the activation record one level higher, thus forming the chain).
end record;
package Subps is new Table.Table (
Table_Component_Type => Subp_Entry,
Table_Index_Type => SI_Type,
Table_Low_Bound => 1,
Table_Initial => 1000,
Table_Increment => 200,
Table_Name => "Unnest_Subps");
-- Records the subprograms in the nest whose outer subprogram is Subp
-----------------
-- Subprograms --
-----------------
function Get_Level (Subp : Entity_Id; Sub : Entity_Id) return Nat;
-- Sub is either Subp itself, or a subprogram nested within Subp. This
-- function returns the level of nesting (Subp = 1, subprograms that
-- are immediately nested within Subp = 2, etc.).
function In_Synchronized_Unit (Subp : Entity_Id) return Boolean;
-- Predicate to identify subprograms declared in task and protected types.
-- These subprograms are called from outside the compilation and therefore
-- must be considered reachable (and cannot be eliminated) because we must
-- generate code for them.
function Subp_Index (Sub : Entity_Id) return SI_Type;
-- Given the entity for a subprogram, return corresponding Subp's index
procedure Unnest_Subprograms (N : Node_Id);
-- Called to unnest subprograms. If we are in unnest subprogram mode, this
-- is the call that traverses the tree N and locates all the library-level
-- subprograms with nested subprograms to process them.
end Exp_Unst;