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-------------------------------------------------------------------------------
--- --
--- GNAT COMPILER COMPONENTS --
--- --
--- S E M --
--- --
--- S p e c --
--- --
--- Copyright (C) 1992-2005, 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 2, 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 COPYING. If not, write --
--- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, --
--- Boston, MA 02110-1301, USA. --
--- --
--- GNAT was originally developed by the GNAT team at New York University. --
--- Extensive contributions were provided by Ada Core Technologies Inc. --
--- --
-------------------------------------------------------------------------------
-
---------------------------------------
--- Semantic Analysis: General Model --
---------------------------------------
-
--- Semantic processing involves 3 phases which are highly interwined
--- (ie mutually recursive):
-
--- Analysis implements the bulk of semantic analysis such as
--- name analysis and type resolution for declarations,
--- instructions and expressions. The main routine
--- driving this process is procedure Analyze given below.
--- This analysis phase is really a bottom up pass that is
--- achieved during the recursive traversal performed by the
--- Analyze_... procedures implemented in the sem_* packages.
--- For expressions this phase determines unambiguous types
--- and collects sets of possible types where the
--- interpretation is potentially ambiguous.
-
--- Resolution is carried out only for expressions to finish type
--- resolution that was initiated but not necessarily
--- completed during analysis (because of overloading
--- ambiguities). Specifically, after completing the bottom
--- up pass carried out during analysis for expressions, the
--- Resolve routine (see the spec of sem_res for more info)
--- is called to perform a top down resolution with
--- recursive calls to itself to resolve operands.
-
--- Expansion if we are not generating code this phase is a no-op.
--- otherwise this phase expands, ie transforms, original
--- declaration, expressions or instructions into simpler
--- structures that can be handled by the back-end. This
--- phase is also in charge of generating code which is
--- implicit in the original source (for instance for
--- default initializations, controlled types, etc.)
--- There are two separate instances where expansion is
--- invoked. For declarations and instructions, expansion is
--- invoked just after analysis since no resolution needs
--- to be performed. For expressions, expansion is done just
--- after resolution. In both cases expansion is done from the
--- bottom up just before the end of Analyze for instructions
--- and declarations or the call to Resolve for expressions.
--- The main routine driving expansion is Expand.
--- See the spec of Expander for more details.
-
--- To summarize, in normal code generation mode we recursively traverse the
--- abstract syntax tree top-down performing semantic analysis bottom
--- up. For instructions and declarations, before the call to the Analyze
--- routine completes we perform expansion since at that point we have all
--- semantic information needed. For expression nodes, after the call to
--- Analysis terminates we invoke the Resolve routine to transmit top-down
--- the type that was gathered by Analyze which will resolve possible
--- ambiguities in the expression. Just before the call to Resolve
--- terminates, the expression can be expanded since all the semantic
--- information is available at that point.
-
--- If we are not generating code then the expansion phase is a no-op
-
--- When generating code there are a number of exceptions to the basic
--- Analysis-Resolution-Expansion model for expressions. The most prominent
--- examples are the handling of default expressions and aggregates.
-
-----------------------------------------------------
--- Handling of Default and Per-Object Expressions --
-----------------------------------------------------
-
--- The default expressions in component declarations and in procedure
--- specifications (but not the ones in object declarations) are quite
--- tricky to handle. The problem is that some processing is required
--- at the point where the expression appears:
-
--- visibility analysis (including user defined operators)
--- freezing of static expressions
-
--- but other processing must be deferred until the enclosing entity
--- (record or procedure specification) is frozen:
-
--- freezing of any other types in the expression
--- expansion
-
--- A similar situation occurs with the argument of priority and interrupt
--- priority pragmas that appear in task and protected definition specs and
--- other cases of per-object expressions (see RM 3.8(18)).
-
--- Expansion has to be deferred since you can't generate code for
--- expressions that refernce types that have not been frozen yet. As an
--- example, consider the following:
-
--- type x is delta 0.5 range -10.0 .. +10.0;
--- ...
--- type q is record
--- xx : x := y * z;
--- end record;
-
--- for x'small use 0.25
-
--- The expander is in charge of dealing with fixed-point, and of course
--- the small declaration, which is not too late, since the declaration of
--- type q does *not* freeze type x, definitely affects the expanded code.
-
--- Another reason that we cannot expand early is that expansion can generate
--- range checks. These range checks need to be inserted not at the point of
--- definition but at the point of use. The whole point here is that the value
--- of the expression cannot be obtained at the point of declaration, only at
--- the point of use.
-
--- Generally our model is to combine analysis resolution and expansion, but
--- this is the one case where this model falls down. Here is how we patch
--- it up without causing too much distortion to our basic model.
-
--- A switch (sede below) is set to indicate that we are in the initial
--- occurence of a default expression. The analyzer is then called on this
--- expression with the switch set true. Analysis and resolution proceed
--- almost as usual, except that Freeze_Expression will not freeze
--- non-static expressions if this switch is set, and the call to Expand at
--- the end of resolution is skipped. This also skips the code that normally
--- sets the Analyzed flag to True). The result is that when we are done the
--- tree is still marked as unanalyzed, but all types for static expressions
--- are frozen as required, and all entities of variables have been
--- recorded. We then turn off the switch, and later on reanalyze the
--- expression with the switch off. The effect is that this second analysis
--- freezes the rest of the types as required, and generates code but
--- visibility analysis is not repeated since all the entities are marked.
-
--- The second analysis (the one that generates code) is in the context
--- where the code is required. For a record field default, this is in
--- the initialization procedure for the record and for a subprogram
--- default parameter, it is at the point the subprogram is frozen.
--- For a priority or storage size pragma it is in the context of the
--- Init_Proc for the task or protected object.
-
-------------------
--- Pre-Analysis --
-------------------
-
--- For certain kind of expressions, such as aggregates, we need to defer
--- expansion of the aggregate and its inner expressions after the whole
--- set of expressions appearing inside the aggregate have been analyzed.
--- Consider, for instance the following example:
---
--- (1 .. 100 => new Thing (Function_Call))
---
--- The normal Analysis-Resolution-Expansion mechanism where expansion
--- of the children is performed before expansion of the parent does not
--- work if the code generated for the children by the expander needs
--- to be evaluated repeatdly (for instance in the above aggregate
--- "new Thing (Function_Call)" needs to be called 100 times.)
--- The reason why this mecanism does not work is that, the expanded code
--- for the children is typically inserted above the parent and thus
--- when the father gets expanded no re-evaluation takes place. For instance
--- in the case of aggregates if "new Thing (Function_Call)" is expanded
--- before of the aggregate the expanded code will be placed outside
--- of the aggregate and when expanding the aggregate the loop from 1 to 100
--- will not surround the expanded code for "new Thing (Function_Call)".
---
--- To remedy this situation we introduce a new flag which signals whether
--- we want a full analysis (ie expansion is enabled) or a pre-analysis
--- which performs Analysis and Resolution but no expansion.
---
--- After the complete pre-analysis of an expression has been carried out
--- we can transform the expression and then carry out the full
--- Analyze-Resolve-Expand cycle on the transformed expression top-down
--- so that the expansion of inner expressions happens inside the newly
--- generated node for the parent expression.
---
--- Note that the difference between processing of default expressions and
--- pre-analysis of other expressions is that we do carry out freezing in
--- the latter but not in the former (except for static scalar expressions).
--- The routine that performs pre-analysis is called Pre_Analyze_And_Resolve
--- and is in Sem_Res.
-
-with Alloc;
-with Einfo; use Einfo;
-with Opt; use Opt;
-with Table;
-with Types; use Types;
-
-package Sem is
-
- New_Nodes_OK : Int := 1;
- -- Temporary flag for use in checking out HLO. Set non-zero if it is
- -- OK to generate new nodes.
-
- -----------------------------
- -- Semantic Analysis Flags --
- -----------------------------
-
- Explicit_Overriding : Boolean := False;
- -- Switch to indicate whether checking mechanism described in AI-218
- -- is enforced: subprograms that override inherited operations must be
- -- be marked explicitly, to prevent accidental or omitted overriding.
-
- Full_Analysis : Boolean := True;
- -- Switch to indicate whether we are doing a full analysis or a
- -- pre-analysis. In normal analysis mode (Analysis-Expansion for
- -- instructions or declarations) or (Analysis-Resolution-Expansion for
- -- expressions) this flag is set. Note that if we are not generating
- -- code the expansion phase merely sets the Analyzed flag to True in
- -- this case. If we are in Pre-Analysis mode (see above) this flag is
- -- set to False then the expansion phase is skipped.
- -- When this flag is False the flag Expander_Active is also False
- -- (the Expander_Activer flag defined in the spec of package Expander
- -- tells you whether expansion is currently enabled).
- -- You should really regard this as a read only flag.
-
- In_Default_Expression : Boolean := False;
- -- Switch to indicate that we are in a default expression, as described
- -- above. Note that this must be recursively saved on a Semantics call
- -- since it is possible for the analysis of an expression to result in
- -- a recursive call (e.g. to get the entity for System.Address as part
- -- of the processing of an Address attribute reference).
- -- When this switch is True then Full_Analysis above must be False.
- -- You should really regard this as a read only flag.
-
- In_Deleted_Code : Boolean := False;
- -- If the condition in an if-statement is statically known, the branch
- -- that is not taken is analyzed with expansion disabled, and the tree
- -- is deleted after analysis. Itypes generated in deleted code must be
- -- frozen from start, because the tree on which they depend will not
- -- be available at the freeze point.
-
- In_Inlined_Body : Boolean := False;
- -- Switch to indicate that we are analyzing and resolving an inlined
- -- body. Type checking is disabled in this context, because types are
- -- known to be compatible. This avoids problems with private types whose
- -- full view is derived from private types.
-
- Inside_A_Generic : Boolean := False;
- -- This flag is set if we are processing a generic specification,
- -- generic definition, or generic body. When this flag is True the
- -- Expander_Active flag is False to disable any code expansion (see
- -- package Expander). Only the generic processing can modify the
- -- status of this flag, any other client should regard it as read-only.
-
- Unloaded_Subunits : Boolean := False;
- -- This flag is set True if we have subunits that are not loaded. This
- -- occurs when the main unit is a subunit, and contains lower level
- -- subunits that are not loaded. We use this flag to suppress warnings
- -- about unused variables, since these warnings are unreliable in this
- -- case. We could perhaps do a more accurate job and retain some of the
- -- warnings, but it is quite a tricky job. See test 4323-002.
-
- -----------------
- -- Scope Stack --
- -----------------
-
- -- The scope stack holds all entries of the scope table. As in the parser,
- -- we use Last as the stack pointer, so that we can always find the scope
- -- that is currently open in Scope_Stack.Table (Scope_Stack.Last). The
- -- oldest entry, at Scope_Stack (0) is Standard. The entries in the table
- -- include the entity for the referenced scope, together with information
- -- used to restore the proper setting of check suppressions on scope exit.
-
- type Scope_Stack_Entry is record
- Entity : Entity_Id;
- -- Entity representing the scope
-
- Last_Subprogram_Name : String_Ptr;
- -- Pointer to name of last subprogram body in this scope. Used for
- -- testing proper alpha ordering of subprogram bodies in scope.
-
- Save_Scope_Suppress : Suppress_Array;
- -- Save contents of Scope_Suppress on entry
-
- Save_Local_Entity_Suppress : Int;
- -- Save contents of Local_Entity_Suppress.Last on entry
-
- Is_Transient : Boolean;
- -- Marks Transient Scopes (See Exp_Ch7 body for details)
-
- Previous_Visibility : Boolean;
- -- Used when installing the parent (s) of the current compilation
- -- unit. The parent may already be visible because of an ongoing
- -- compilation, and the proper visibility must be restored on exit.
-
- Node_To_Be_Wrapped : Node_Id;
- -- Only used in transient scopes. Records the node which will
- -- be wrapped by the transient block.
-
- Actions_To_Be_Wrapped_Before : List_Id;
- Actions_To_Be_Wrapped_After : List_Id;
- -- Actions that have to be inserted at the start or at the end of a
- -- transient block. Used to temporarily hold these actions until the
- -- block is created, at which time the actions are moved to the
- -- block.
-
- Pending_Freeze_Actions : List_Id;
- -- Used to collect freeze entity nodes and associated actions that
- -- are generated in a inner context but need to be analyzed outside,
- -- such as records and initialization procedures. On exit from the
- -- scope, this list of actions is inserted before the scope construct
- -- and analyzed to generate the corresponding freeze processing and
- -- elaboration of other associated actions.
-
- First_Use_Clause : Node_Id;
- -- Head of list of Use_Clauses in current scope. The list is built
- -- when the declarations in the scope are processed. The list is
- -- traversed on scope exit to undo the effect of the use clauses.
-
- Component_Alignment_Default : Component_Alignment_Kind;
- -- Component alignment to be applied to any record or array types
- -- that are declared for which a specific component alignment pragma
- -- does not set the alignment.
-
- Is_Active_Stack_Base : Boolean;
- -- Set to true only when entering the scope for Standard_Standard from
- -- from within procedure Semantics. Indicates the base of the current
- -- active set of scopes. Needed by In_Open_Scopes to handle cases
- -- where Standard_Standard can be pushed in the middle of the active
- -- set of scopes (occurs for instantiations of generic child units).
- end record;
-
- package Scope_Stack is new Table.Table (
- Table_Component_Type => Scope_Stack_Entry,
- Table_Index_Type => Int,
- Table_Low_Bound => 0,
- Table_Initial => Alloc.Scope_Stack_Initial,
- Table_Increment => Alloc.Scope_Stack_Increment,
- Table_Name => "Sem.Scope_Stack");
-
- -----------------------------------
- -- Handling of Check Suppression --
- -----------------------------------
-
- -- There are two kinds of suppress checks: scope based suppress checks,
- -- and entity based suppress checks.
-
- -- Scope based suppress chems (from initial command line arguments,
- -- or from Suppress pragmas not including an entity name) are recorded
- -- in the Sem.Supress variable, and all that is necessary is to save the
- -- state of this variable on scope entry, and restore it on scope exit.
-
- -- Entity based suppress checks, from Suppress pragmas giving an Entity_Id,
- -- are handled as follows. If a suppress or unsuppress pragma is
- -- encountered for a given entity, then the flag Checks_May_Be_Suppressed
- -- is set in the entity and an entry is made in either the
- -- Local_Entity_Suppress table (case of pragma that appears in other than
- -- a package spec), or in the Global_Entity_Suppress table (case of pragma
- -- that appears in a package spec, which is by the rule of RM 11.5(7)
- -- applicable throughout the life of the entity).
-
- -- If the Checks_May_Be_Suppressed flag is set in an entity then the
- -- procedure is to search first the local and then the global suppress
- -- tables (the local one being searched in reverse order, i.e. last in
- -- searched first). The only other point is that we have to make sure
- -- that we have proper nested interaction between such specific pragmas
- -- and locally applied general pragmas applying to all entities. This
- -- is achieved by including in the Local_Entity_Suppress table dummy
- -- entries with an empty Entity field that are applicable to all entities.
-
- Scope_Suppress : Suppress_Array := Suppress_Options;
- -- This array contains the current scope based settings of the suppress
- -- switches. It is initialized from the options as shown, and then modified
- -- by pragma Suppress. On entry to each scope, the current setting is saved
- -- the scope stack, and then restored on exit from the scope. This record
- -- may be rapidly checked to determine the current status of a check if
- -- no specific entity is involved or if the specific entity involved is
- -- one for which no specific Suppress/Unsuppress pragma has been set (as
- -- indicated by the Checks_May_Be_Suppressed flag being set).
-
- -- This scheme is a little complex, but serves the purpose of enabling
- -- a very rapid check in the common case where no entity specific pragma
- -- applies, and gives the right result when such pragmas are used even
- -- in complex cases of nested Suppress and Unsuppress pragmas.
-
- type Entity_Check_Suppress_Record is record
- Entity : Entity_Id;
- -- Entity to which the check applies, or Empty for a local check
- -- that has no entity name (and thus applies to all entities).
-
- Check : Check_Id;
- -- Check which is set (note this cannot be All_Checks, if the All_Checks
- -- case, a sequence of eentries appears for the individual checks.
-
- Suppress : Boolean;
- -- Set True for Suppress, and False for Unsuppress
- end record;
-
- -- The Local_Entity_Suppress table is a stack, to which new entries are
- -- added for Suppress and Unsuppress pragmas appearing in other than
- -- package specs. Such pragmas are effective only to the end of the scope
- -- in which they appear. This is achieved by marking the stack on entry
- -- to a scope and then cutting back the stack to that marked point on
- -- scope exit.
-
- package Local_Entity_Suppress is new Table.Table (
- Table_Component_Type => Entity_Check_Suppress_Record,
- Table_Index_Type => Int,
- Table_Low_Bound => 0,
- Table_Initial => Alloc.Entity_Suppress_Initial,
- Table_Increment => Alloc.Entity_Suppress_Increment,
- Table_Name => "Local_Entity_Suppress");
-
- -- The Global_Entity_Suppress table is used for entities which have
- -- a Suppress or Unsuppress pragma naming a specific entity in a
- -- package spec. Such pragmas always refer to entities in the package
- -- spec and are effective throughout the lifetime of the named entity.
-
- package Global_Entity_Suppress is new Table.Table (
- Table_Component_Type => Entity_Check_Suppress_Record,
- Table_Index_Type => Int,
- Table_Low_Bound => 0,
- Table_Initial => Alloc.Entity_Suppress_Initial,
- Table_Increment => Alloc.Entity_Suppress_Increment,
- Table_Name => "Global_Entity_Suppress");
-
- -----------------
- -- Subprograms --
- -----------------
-
- procedure Initialize;
- -- Initialize internal tables
-
- procedure Lock;
- -- Lock internal tables before calling back end
-
- procedure Semantics (Comp_Unit : Node_Id);
- -- This procedure is called to perform semantic analysis on the specified
- -- node which is the N_Compilation_Unit node for the unit.
-
- procedure Analyze (N : Node_Id);
- procedure Analyze (N : Node_Id; Suppress : Check_Id);
- -- This is the recursive procedure which is applied to individual nodes
- -- of the tree, starting at the top level node (compilation unit node)
- -- and then moving down the tree in a top down traversal. It calls
- -- individual routines with names Analyze_xxx to analyze node xxx. Each
- -- of these routines is responsible for calling Analyze on the components
- -- of the subtree.
- --
- -- Note: In the case of expression components (nodes whose Nkind is in
- -- N_Subexpr), the call to Analyze does not complete the semantic analysis
- -- of the node, since the type resolution cannot be completed until the
- -- complete context is analyzed. The completion of the type analysis occurs
- -- in the corresponding Resolve routine (see Sem_Res).
- --
- -- Note: for integer and real literals, the analyzer sets the flag to
- -- indicate that the result is a static expression. If the expander
- -- generates a literal that does NOT correspond to a static expression,
- -- e.g. by folding an expression whose value is known at compile-time,
- -- but is not technically static, then the caller should reset the
- -- Is_Static_Expression flag after analyzing but before resolving.
- --
- -- If the Suppress argument is present, then the analysis is done
- -- with the specified check suppressed (can be All_Checks to suppress
- -- all checks).
-
- procedure Analyze_List (L : List_Id);
- procedure Analyze_List (L : List_Id; Suppress : Check_Id);
- -- Analyzes each element of a list. If the Suppress argument is present,
- -- then the analysis is done with the specified check suppressed (can
- -- be All_Checks to suppress all checks).
-
- procedure Copy_Suppress_Status
- (C : Check_Id;
- From : Entity_Id;
- To : Entity_Id);
- -- If From is an entity for which check C is explicitly suppressed
- -- then also explicitly suppress the corresponding check in To.
-
- procedure Insert_List_After_And_Analyze
- (N : Node_Id; L : List_Id);
- procedure Insert_List_After_And_Analyze
- (N : Node_Id; L : List_Id; Suppress : Check_Id);
- -- Inserts list L after node N using Nlists.Insert_List_After, and then,
- -- after this insertion is complete, analyzes all the nodes in the list,
- -- including any additional nodes generated by this analysis. If the list
- -- is empty or be No_List, the call has no effect. If the Suppress
- -- argument is present, then the analysis is done with the specified
- -- check suppressed (can be All_Checks to suppress all checks).
-
- procedure Insert_List_Before_And_Analyze
- (N : Node_Id; L : List_Id);
- procedure Insert_List_Before_And_Analyze
- (N : Node_Id; L : List_Id; Suppress : Check_Id);
- -- Inserts list L before node N using Nlists.Insert_List_Before, and then,
- -- after this insertion is complete, analyzes all the nodes in the list,
- -- including any additional nodes generated by this analysis. If the list
- -- is empty or be No_List, the call has no effect. If the Suppress
- -- argument is present, then the analysis is done with the specified
- -- check suppressed (can be All_Checks to suppress all checks).
-
- procedure Insert_After_And_Analyze
- (N : Node_Id; M : Node_Id);
- procedure Insert_After_And_Analyze
- (N : Node_Id; M : Node_Id; Suppress : Check_Id);
- -- Inserts node M after node N and then after the insertion is complete,
- -- analyzes the inserted node and all nodes that are generated by
- -- this analysis. If the node is empty, the call has no effect. If the
- -- Suppress argument is present, then the analysis is done with the
- -- specified check suppressed (can be All_Checks to suppress all checks).
-
- procedure Insert_Before_And_Analyze
- (N : Node_Id; M : Node_Id);
- procedure Insert_Before_And_Analyze
- (N : Node_Id; M : Node_Id; Suppress : Check_Id);
- -- Inserts node M before node N and then after the insertion is complete,
- -- analyzes the inserted node and all nodes that could be generated by
- -- this analysis. If the node is empty, the call has no effect. If the
- -- Suppress argument is present, then the analysis is done with the
- -- specified check suppressed (can be All_Checks to suppress all checks).
-
- function External_Ref_In_Generic (E : Entity_Id) return Boolean;
- -- Return True if we are in the context of a generic and E is
- -- external (more global) to it.
-
- procedure Enter_Generic_Scope (S : Entity_Id);
- -- Shall be called each time a Generic subprogram or package scope is
- -- entered. S is the entity of the scope.
- -- ??? At the moment, only called for package specs because this mechanism
- -- is only used for avoiding freezing of external references in generics
- -- and this can only be an issue if the outer generic scope is a package
- -- spec (otherwise all external entities are already frozen)
-
- procedure Exit_Generic_Scope (S : Entity_Id);
- -- Shall be called each time a Generic subprogram or package scope is
- -- exited. S is the entity of the scope.
- -- ??? At the moment, only called for package specs exit.
-
- function Explicit_Suppress (E : Entity_Id; C : Check_Id) return Boolean;
- -- This function returns True if an explicit pragma Suppress for check C
- -- is present in the package defining E.
-
- function Is_Check_Suppressed (E : Entity_Id; C : Check_Id) return Boolean;
- -- This function is called if Checks_May_Be_Suppressed (E) is True to
- -- determine whether check C is suppressed either on the entity E or
- -- as the result of a scope suppress pragma. If Checks_May_Be_Suppressed
- -- is False, then the status of the check can be determined simply by
- -- examining Scope_Checks (C), so this routine is not called in that case.
-
-end Sem;