------------------------------------------------------------------------------ -- -- -- 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;