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------------------------------------------------------------------------------

--                                                                          --

--                         GNAT COMPILER COMPONENTS                         --

--                                                                          --

--                             S E M _ E V A L                              --

--                                                                          --

--                                 S p e c                                  --

--                                                                          --

--          Copyright (C) 1992-2010, 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.      --

--                                                                          --

------------------------------------------------------------------------------

--  This package contains various subprograms involved in compile time

--  evaluation of expressions and checks for staticness of expressions and

--  types. It also contains the circuitry for checking for violations of pure

--  and preelaborated conditions (this naturally goes here, since these rules

--  involve consideration of staticness).

--  Note: the static evaluation for attributes is found in Sem_Attr even though

--  logically it belongs here. We have done this so that it is easier to add

--  new attributes to GNAT.

with Types;  use Types;

with Uintp;  use Uintp;

with Urealp; use Urealp;

package Sem_Eval is

   ------------------------------------

   -- Handling of Static Expressions --

   ------------------------------------

   --  This package contains a set of routines that process individual

   --  subexpression nodes with the objective of folding (precomputing) the

   --  value of static expressions that are known at compile time and properly

   --  computing the setting of two flags that appear in every subexpression

   --  node:

   --    Is_Static_Expression

   --      This flag is set on any expression that is static according to the

   --      rules in (RM 4.9(3-32)).

   --    Raises_Constraint_Error

   --      This flag indicates that it is known at compile time that the

   --      evaluation of an expression raises constraint error. If the

   --      expression is static, and this flag is off, then it is also known at

   --      compile time that the expression does not raise constraint error

   --      (i.e. the flag is accurate for static expressions, and conservative

   --      for non-static expressions.

   --  If a static expression does not raise constraint error, then the

   --  Raises_Constraint_Error flag is off, and the expression must be computed

   --  at compile time, which means that it has the form of either a literal,

   --  or a constant that is itself (recursively) either a literal or a

   --  constant.

   --  The above rules must be followed exactly in order for legality checks to

   --  be accurate. For subexpressions that are not static according to the RM

   --  definition, they are sometimes folded anyway, but of course in this case

   --  Is_Static_Expression is not set.

   -------------------------------

   -- Compile-Time Known Values --

   -------------------------------

   --  For most legality checking purposes the flag Is_Static_Expression

   --  defined in Sinfo should be used. This package also provides a routine

   --  called Is_OK_Static_Expression which in addition of checking that an

   --  expression is static in the RM 4.9 sense, it checks that the expression

   --  does not raise constraint error. In fact for certain legality checks not

   --  only do we need to ascertain that the expression is static, but we must

   --  also ensure that it does not raise constraint error.

   --

   --  Neither of Is_Static_Expression and Is_OK_Static_Expression should be

   --  used for compile time evaluation purposes. In fact certain expression

   --  whose value is known at compile time are not static in the RM 4.9 sense.

   --  A typical example is:

   --

   --     C : constant Integer := Record_Type'Size;

   --

   --  The expression 'C' is not static in the technical RM sense, but for many

   --  simple record types, the size is in fact known at compile time. When we

   --  are trying to perform compile time constant folding (for instance for

   --  expressions like C + 1, Is_Static_Expression or Is_OK_Static_Expression

   --  are not the right functions to test if folding is possible. Instead, we

   --  use Compile_Time_Known_Value. All static expressions that do not raise

   --  constraint error (i.e. those for which Is_OK_Static_Expression is true)

   --  are known at compile time, but as shown by the above example, there are

   --  cases of non-static expressions which are known at compile time.

   -----------------

   -- Subprograms --

   -----------------

   procedure Check_Non_Static_Context (N : Node_Id);

   --  Deals with the special check required for a static expression that

   --  appears in a non-static context, i.e. is not part of a larger static

   --  expression (see RM 4.9(35)), i.e. the value of the expression must be

   --  within the base range of the base type of its expected type. A check is

   --  also made for expressions that are inside the base range, but outside

   --  the range of the expected subtype (this is a warning message rather than

   --  an illegality).

   --

   --  Note: most cases of non-static context checks are handled within

   --  Sem_Eval itself, including all cases of expressions at the outer level

   --  (i.e. those that are not a subexpression). Currently the only outside

   --  customer for this procedure is Sem_Attr (because Eval_Attribute is

   --  there). There is also one special case arising from ranges (see body of

   --  Resolve_Range).

   procedure Check_String_Literal_Length (N : Node_Id; Ttype : Entity_Id);

   --  N is either a string literal, or a constraint error node. In the latter

   --  case, the situation is already dealt with, and the call has no effect.

   --  In the former case, if the target type, Ttyp is constrained, then a

   --  check is made to see if the string literal is of appropriate length.

   type Compare_Result is (LT, LE, EQ, GT, GE, NE, Unknown);

   subtype Compare_GE is Compare_Result range EQ .. GE;

   subtype Compare_LE is Compare_Result range LT .. EQ;

   --  Result subtypes for Compile_Time_Compare subprograms

   function Compile_Time_Compare

     (L, R         : Node_Id;

      Assume_Valid : Boolean) return Compare_Result;

   pragma Inline (Compile_Time_Compare);

   --  Given two expression nodes, finds out whether it can be determined at

   --  compile time how the runtime values will compare. An Unknown result

   --  means that the result of a comparison cannot be determined at compile

   --  time, otherwise the returned result indicates the known result of the

   --  comparison, given as tightly as possible (i.e. EQ or LT is preferred

   --  returned value to LE). If Assume_Valid is true, the result reflects

   --  the result of assuming that entities involved in the comparison have

   --  valid representations. If Assume_Valid is false, then the base type of

   --  any involved entity is used so that no assumption of validity is made.

   function Compile_Time_Compare

     (L, R         : Node_Id;

      Diff         : access Uint;

      Assume_Valid : Boolean;

      Rec          : Boolean := False) return Compare_Result;

   --  This version of Compile_Time_Compare returns extra information if the

   --  result is GT or LT. In these cases, if the magnitude of the difference

   --  can be determined at compile time, this (positive) magnitude is returned

   --  in Diff.all. If the magnitude of the difference cannot be determined

   --  then Diff.all contains No_Uint on return. Rec is a parameter that is set

   --  True for a recursive call from within Compile_Time_Compare to avoid some

   --  infinite recursion cases. It should never be set by a client.

   procedure Flag_Non_Static_Expr (Msg : String; Expr : Node_Id);

   --  This procedure is called after it has been determined that Expr is not

   --  static when it is required to be. Msg is the text of a message that

   --  explains the error. This procedure checks if an error is already posted

   --  on Expr, if so, it does nothing unless All_Errors_Mode is set in which

   --  case this flag is ignored. Otherwise the given message is posted using

   --  Error_Msg_F, and then Why_Not_Static is called on Expr to generate

   --  additional messages. The string given as Msg should end with ! to make

   --  it an unconditional message, to ensure that if it is posted, the entire

   --  set of messages is all posted.

   function Is_OK_Static_Expression (N : Node_Id) return Boolean;

   --  An OK static expression is one that is static in the RM definition sense

   --  and which does not raise constraint error. For most legality checking

   --  purposes you should use Is_Static_Expression. For those legality checks

   --  where the expression N should not raise constraint error use this

   --  routine. This routine is *not* to be used in contexts where the test is

   --  for compile time evaluation purposes. Use Compile_Time_Known_Value

   --  instead (see section on "Compile-Time Known Values" above).

   function Is_Static_Range (N : Node_Id) return Boolean;

   --  Determine if range is static, as defined in RM 4.9(26). The only allowed

   --  argument is an N_Range node (but note that the semantic analysis of

   --  equivalent range attribute references already turned them into the

   --  equivalent range).

   function Is_OK_Static_Range (N : Node_Id) return Boolean;

   --  Like Is_Static_Range, but also makes sure that the bounds of the range

   --  are compile-time evaluable (i.e. do not raise constraint error). A

   --  result of true means that the bounds are compile time evaluable. A

   --  result of false means they are not (either because the range is not

   --  static, or because one or the other bound raises CE).

   function Is_Static_Subtype (Typ : Entity_Id) return Boolean;

   --  Determines whether a subtype fits the definition of an Ada static

   --  subtype as given in (RM 4.9(26)).

   function Is_OK_Static_Subtype (Typ : Entity_Id) return Boolean;

   --  Like Is_Static_Subtype but also makes sure that the bounds of the

   --  subtype are compile-time evaluable (i.e. do not raise constraint error).

   --  A result of true means that the bounds are compile time evaluable. A

   --  result of false means they are not (either because the range is not

   --  static, or because one or the other bound raises CE).

   function Subtypes_Statically_Compatible

     (T1 : Entity_Id;

      T2 : Entity_Id) return Boolean;

   --  Returns true if the subtypes are unconstrained or the constraint on

   --  on T1 is statically compatible with T2 (as defined by 4.9.1(4)).

   --  Otherwise returns false.

   function Subtypes_Statically_Match (T1, T2 : Entity_Id) return Boolean;

   --  Determine whether two types T1, T2, which have the same base type,

   --  are statically matching subtypes (RM 4.9.1(1-2)).

   function Compile_Time_Known_Value (Op : Node_Id) return Boolean;

   --  Returns true if Op is an expression not raising constraint error whose

   --  value is known at compile time. This is true if Op is a static

   --  expression, but can also be true for expressions which are technically

   --  non-static but which are in fact known at compile time, such as the

   --  static lower bound of a non-static range or the value of a constant

   --  object whose initial value is static. Note that this routine is defended

   --  against unanalyzed expressions. Such expressions will not cause a

   --  blowup, they may cause pessimistic (i.e. False) results to be returned.

   function Compile_Time_Known_Value_Or_Aggr (Op : Node_Id) return Boolean;

   --  Similar to Compile_Time_Known_Value, but also returns True if the value

   --  is a compile time known aggregate, i.e. an aggregate all of whose

   --  constituent expressions are either compile time known values or compile

   --  time known aggregates.

   function Compile_Time_Known_Bounds (T : Entity_Id) return Boolean;

   --  If T is an array whose index bounds are all known at compile time, then

   --  True is returned, if T is not an array, or one or more of its index

   --  bounds is not known at compile time, then False is returned.

   function Expr_Value (N : Node_Id) return Uint;

   --  Returns the folded value of the expression N. This function is called in

   --  instances where it has already been determined that the expression is

   --  static or its value is compile time known (Compile_Time_Known_Value (N)

   --  returns True). This version is used for integer values, and enumeration

   --  or character literals. In the latter two cases, the value returned is

   --  the Pos value in the relevant enumeration type. It can also be used for

   --  fixed-point values, in which case it returns the corresponding integer

   --  value. It cannot be used for floating-point values.

   function Expr_Value_E (N : Node_Id) return Entity_Id;

   --  Returns the folded value of the expression. This function is called in

   --  instances where it has already been determined that the expression is

   --  static or its value known at compile time. This version is used for

   --  enumeration types and returns the corresponding enumeration literal.

   function Expr_Value_R (N : Node_Id) return Ureal;

   --  Returns the folded value of the expression. This function is called in

   --  instances where it has already been determined that the expression is

   --  static or its value known at compile time. This version is used for real

   --  values (including both the floating-point and fixed-point cases). In the

   --  case of a fixed-point type, the real value is returned (cf above version

   --  returning Uint).

   function Expr_Value_S (N : Node_Id) return Node_Id;

   --  Returns the folded value of the expression. This function is called

   --  in instances where it has already been determined that the expression

   --  is static or its value is known at compile time. This version is used

   --  for string types and returns the corresponding N_String_Literal node.

   function Expr_Rep_Value (N : Node_Id) return Uint;

   --  This is identical to Expr_Value, except in the case of enumeration

   --  literals of types for which an enumeration representation clause has

   --  been given, in which case it returns the representation value rather

   --  than the pos value. This is the value that is needed for generating code

   --  sequences, while the Expr_Value value is appropriate for compile time

   --  constraint errors or getting the logical value. Note that this function

   --  does NOT concern itself with biased values, if the caller needs a

   --  properly biased value, the subtraction of the bias must be handled

   --  explicitly.

   procedure Eval_Actual                 (N : Node_Id);

   procedure Eval_Allocator              (N : Node_Id);

   procedure Eval_Arithmetic_Op          (N : Node_Id);

   procedure Eval_Call                   (N : Node_Id);

   procedure Eval_Case_Expression        (N : Node_Id);

   procedure Eval_Character_Literal      (N : Node_Id);

   procedure Eval_Concatenation          (N : Node_Id);

   procedure Eval_Conditional_Expression (N : Node_Id);

   procedure Eval_Entity_Name            (N : Node_Id);

   procedure Eval_Indexed_Component      (N : Node_Id);

   procedure Eval_Integer_Literal        (N : Node_Id);

   procedure Eval_Logical_Op             (N : Node_Id);

   procedure Eval_Membership_Op          (N : Node_Id);

   procedure Eval_Named_Integer          (N : Node_Id);

   procedure Eval_Named_Real             (N : Node_Id);

   procedure Eval_Op_Expon               (N : Node_Id);

   procedure Eval_Op_Not                 (N : Node_Id);

   procedure Eval_Real_Literal           (N : Node_Id);

   procedure Eval_Relational_Op          (N : Node_Id);

   procedure Eval_Shift                  (N : Node_Id);

   procedure Eval_Short_Circuit          (N : Node_Id);

   procedure Eval_Slice                  (N : Node_Id);

   procedure Eval_String_Literal         (N : Node_Id);

   procedure Eval_Qualified_Expression   (N : Node_Id);

   procedure Eval_Type_Conversion        (N : Node_Id);

   procedure Eval_Unary_Op               (N : Node_Id);

   procedure Eval_Unchecked_Conversion   (N : Node_Id);

   procedure Fold_Str (N : Node_Id; Val : String_Id; Static : Boolean);

   --  Rewrite N with a new N_String_Literal node as the result of the compile

   --  time evaluation of the node N. Val is the resulting string value from

   --  the folding operation. The Is_Static_Expression flag is set in the

   --  result node. The result is fully analyzed and resolved. Static indicates

   --  whether the result should be considered static or not (True = consider

   --  static). The point here is that normally all string literals are static,

   --  but if this was the result of some sequence of evaluation where values

   --  were known at compile time but not static, then the result is not

   --  static.

   procedure Fold_Uint (N : Node_Id; Val : Uint; Static : Boolean);

   --  Rewrite N with a (N_Integer_Literal, N_Identifier, N_Character_Literal)

   --  node as the result of the compile time evaluation of the node N. Val is

   --  the result in the integer case and is the position of the literal in the

   --  literals list for the enumeration case. Is_Static_Expression is set True

   --  in the result node. The result is fully analyzed/resolved. Static

   --  indicates whether the result should be considered static or not (True =

   --  consider static). The point here is that normally all integer literals

   --  are static, but if this was the result of some sequence of evaluation

   --  where values were known at compile time but not static, then the result

   --  is not static.

   procedure Fold_Ureal (N : Node_Id; Val : Ureal; Static : Boolean);

   --  Rewrite N with a new N_Real_Literal node as the result of the compile

   --  time evaluation of the node N. Val is the resulting real value from the

   --  folding operation. The Is_Static_Expression flag is set in the result

   --  node. The result is fully analyzed and result. Static indicates whether

   --  the result should be considered static or not (True = consider static).

   --  The point here is that normally all string literals are static, but if

   --  this was the result of some sequence of evaluation where values were

   --  known at compile time but not static, then the result is not static.

   function Is_In_Range

     (N            : Node_Id;

      Typ          : Entity_Id;

      Assume_Valid : Boolean := False;

      Fixed_Int    : Boolean := False;

      Int_Real     : Boolean := False) return Boolean;

   --  Returns True if it can be guaranteed at compile time that expression is

   --  known to be in range of the subtype Typ. A result of False does not mean

   --  that the expression is out of range, merely that it cannot be determined

   --  at compile time that it is in range. If Typ is a floating point type or

   --  Int_Real is set, any integer value is treated as though it was a real

   --  value (i.e. the underlying real value is used). In this case we use the

   --  corresponding real value, both for the bounds of Typ, and for the value

   --  of the expression N. If Typ is a fixed type or a discrete type and

   --  Int_Real is False but flag Fixed_Int is True then any fixed-point value

   --  is treated as though it was discrete value (i.e. the underlying integer

   --  value is used). In this case we use the corresponding integer value,

   --  both for the bounds of Typ, and for the value of the expression N. If

   --  Typ is a discrete type and Fixed_Int as well as Int_Real are false,

   --  integer values are used throughout.

   --

   --  If Assume_Valid is set True, then N is always assumed to contain a valid

   --  value. If Assume_Valid is set False, then N may be invalid (unless there

   --  is some independent way of knowing that it is valid, i.e. either it is

   --  an entity with Is_Known_Valid set, or Assume_No_Invalid_Values is True.

   function Is_Out_Of_Range

     (N            : Node_Id;

      Typ          : Entity_Id;

      Assume_Valid : Boolean := False;

      Fixed_Int    : Boolean := False;

      Int_Real     : Boolean := False) return Boolean;

   --  Returns True if it can be guaranteed at compile time that expression is

   --  known to be out of range of the subtype Typ. True is returned if Typ is

   --  a scalar type, and the value of N can be determined to be outside the

   --  range of Typ. A result of False does not mean that the expression is in

   --  range, but rather merely that it cannot be determined at compile time

   --  that it is out of range. The parameters Assume_Valid, Fixed_Int, and

   --  Int_Real are as described for Is_In_Range above.

   function In_Subrange_Of

     (T1        : Entity_Id;

      T2        : Entity_Id;

      Fixed_Int : Boolean := False) return Boolean;

   --  Returns True if it can be guaranteed at compile time that the range of

   --  values for scalar type T1 are always in the range of scalar type T2. A

   --  result of False does not mean that T1 is not in T2's subrange, only that

   --  it cannot be determined at compile time. Flag Fixed_Int is used as in

   --  routine Is_In_Range above.

   function Is_Null_Range (Lo : Node_Id; Hi : Node_Id) return Boolean;

   --  Returns True if it can guarantee that Lo .. Hi is a null range. If it

   --  cannot (because the value of Lo or Hi is not known at compile time) then

   --  it returns False.

   function Not_Null_Range (Lo : Node_Id; Hi : Node_Id) return Boolean;

   --  Returns True if it can guarantee that Lo .. Hi is not a null range. If

   --  it cannot (because the value of Lo or Hi is not known at compile time)

   --  then it returns False.

   procedure Why_Not_Static (Expr : Node_Id);

   --  This procedure may be called after generating an error message that

   --  complains that something is non-static. If it finds good reasons, it

   --  generates one or more error messages pointing the appropriate offending

   --  component of the expression. If no good reasons can be figured out, then

   --  no messages are generated. The expectation here is that the caller has

   --  already issued a message complaining that the expression is non-static.

   --  Note that this message should be placed using Error_Msg_F or

   --  Error_Msg_FE, so that it will sort before any messages placed by this

   --  call. Note that it is fine to call Why_Not_Static with something that is

   --  not an expression, and usually this has no effect, but in some cases

   --  (N_Parameter_Association or N_Range), it makes sense for the internal

   --  recursive calls.

   procedure Initialize;

   --  Initializes the internal data structures. Must be called before each

   --  separate main program unit (e.g. in a GNSA/ASIS context).

private

   --  The Eval routines are all marked inline, since they are called once

   pragma Inline (Eval_Actual);

   pragma Inline (Eval_Allocator);

   pragma Inline (Eval_Character_Literal);

   pragma Inline (Eval_Conditional_Expression);

   pragma Inline (Eval_Indexed_Component);

   pragma Inline (Eval_Named_Integer);

   pragma Inline (Eval_Named_Real);

   pragma Inline (Eval_Real_Literal);

   pragma Inline (Eval_Shift);

   pragma Inline (Eval_Slice);

   pragma Inline (Eval_String_Literal);

   pragma Inline (Eval_Unchecked_Conversion);

   pragma Inline (Is_OK_Static_Expression);

end Sem_Eval;