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--                                                                          --

--                         GNAT COMPILER COMPONENTS                         --

--                                                                          --

--                             E X P _ P A K D                              --

--                                                                          --

--                                 S p e c                                  --

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--          Copyright (C) 1992-2008, 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.          --

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-- GNAT was originally developed  by the GNAT team at  New York University. --

-- Extensive contributions were provided by Ada Core Technologies Inc.      --

--                                                                          --

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--  Expand routines for manipulation of packed arrays

with Types; use Types;

package Exp_Pakd is

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

   -- Implementation of Packed Arrays --

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

   --  When a packed array (sub)type is frozen, we create a corresponding

   --  type that will be used to hold the bits of the packed value, and

   --  store the entity for this type in the Packed_Array_Type field of the

   --  E_Array_Type or E_Array_Subtype entity for the packed array.

   --  This packed array type has the name xxxPn, where xxx is the name

   --  of the packed type, and n is the component size. The expanded

   --  declaration declares a type that is one of the following:

   --    For an unconstrained array with component size 1,2,4 or any other

   --    odd component size. These are the cases in which we do not need

   --    to align the underlying array.

   --      type xxxPn is new Packed_Bytes1;

   --    For an unconstrained array with component size that is divisible

   --    by 2, but not divisible by 4 (other than 2 itself). These are the

   --    cases in which we can generate better code if the underlying array

   --    is 2-byte aligned (see System.Pack_14 in file s-pack14 for example).

   --      type xxxPn is new Packed_Bytes2;

   --    For an unconstrained array with component size that is divisible

   --    by 4, other than powers of 2 (which either come under the 1,2,4

   --    exception above, or are not packed at all). These are cases where

   --    we can generate better code if the underlying array is 4-byte

   --    aligned (see System.Pack_20 in file s-pack20 for example).

   --      type xxxPn is new Packed_Bytes4;

   --    For a constrained array with a static index type where the number

   --    of bits does not exceed the size of Unsigned:

   --      type xxxPn is new Unsigned range 0 .. 2 ** nbits - 1;

   --    For a constrained array with a static index type where the number

   --    of bits is greater than the size of Unsigned, but does not exceed

   --    the size of Long_Long_Unsigned:

   --       type xxxPn is new Long_Long_Unsigned range 0 .. 2 ** nbits - 1;

   --    For all other constrained arrays, we use one of

   --       type xxxPn is new Packed_Bytes1 (0 .. m);

   --       type xxxPn is new Packed_Bytes2 (0 .. m);

   --       type xxxPn is new Packed_Bytes4 (0 .. m);

   --    where m is calculated (from the length of the original packed array)

   --    to hold the required number of bits, and the choice of the particular

   --    Packed_Bytes{1,2,4} type is made on the basis of alignment needs as

   --    described above for the unconstrained case.

   --  When a variable of packed array type is allocated, gigi will allocate

   --  the amount of space indicated by the corresponding packed array type.

   --  However, we do NOT attempt to rewrite the types of any references or

   --  to retype the variable itself, since this would cause all kinds of

   --  semantic problems in the front end (remember that expansion proceeds

   --  at the same time as analysis).

   --  For an indexed reference to a packed array, we simply convert the

   --  reference to the appropriate equivalent reference to the object

   --  of the packed array type (using unchecked conversion).

   --  In some cases (for internally generated types, and for the subtypes

   --  for record fields that depend on a discriminant), the corresponding

   --  packed type cannot be easily generated in advance. In these cases,

   --  we generate the required subtype on the fly at the reference point.

   --  For the modular case, any unused bits are initialized to zero, and

   --  all operations maintain these bits as zero (where necessary all

   --  unchecked conversions from corresponding array values require

   --  these bits to be clear, which is done automatically by gigi).

   --  For the array cases, there can be unused bits in the last byte, and

   --  these are neither initialized, nor treated specially in operations

   --  (i.e. it is allowable for these bits to be clobbered, e.g. by not).

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

   -- Endian Considerations --

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

   --  The standard does not specify the way in which bits are numbered in

   --  a packed array. There are two reasonable rules for deciding this:

   --    Store the first bit at right end (low order) word. This means

   --    that the scaled subscript can be used directly as a left shift

   --    count (if we put bit 0 at the left end, then we need an extra

   --    subtract to compute the shift count).

   --    Layout the bits so that if the packed boolean array is overlaid on

   --    a record, using unchecked conversion, then bit 0 of the array is

   --    the same as the bit numbered bit 0 in a record representation

   --    clause applying to the record. For example:

   --       type Rec is record

   --          C : Bits4;

   --          D : Bits7;

   --          E : Bits5;

   --       end record;

   --       for Rec use record

   --          C at 0 range  0  .. 3;

   --          D at 0 range  4 .. 10;

   --          E at 0 range 11 .. 15;

   --       end record;

   --       type P16 is array (0 .. 15) of Boolean;

   --       pragma Pack (P16);

   --    Now if we use unchecked conversion to convert a value of the record

   --    type to the packed array type, according to this second criterion,

   --    we would expect field D to occupy bits 4..10 of the Boolean array.

   --  Although not required, this correspondence seems a highly desirable

   --  property, and is one that GNAT decides to guarantee. For a little

   --  endian machine, we can also meet the first requirement, but for a

   --  big endian machine, it will be necessary to store the first bit of

   --  a Boolean array in the left end (most significant) bit of the word.

   --  This may cost an extra instruction on some machines, but we consider

   --  that a worthwhile price to pay for the consistency.

   --  One more important point arises in the case where we have a constrained

   --  subtype of an unconstrained array. Take the case of 20 bits. For the

   --  unconstrained representation, we would use an array of bytes:

   --     Little-endian case

   --       8-7-6-5-4-3-2-1  16-15-14-13-12-11-10-9  x-x-x-x-20-19-18-17

   --     Big-endian case

   --       1-2-3-4-5-6-7-8  9-10-11-12-13-14-15-16  17-18-19-20-x-x-x-x

   --   For the constrained case, we use a 20-bit modular value, but in

   --   general this value may well be stored in 32 bits. Let's look at

   --   what it looks like:

   --     Little-endian case

   --       x-x-x-x-x-x-x-x-x-x-x-x-20-19-18-17-...-10-9-8-7-6-5-4-3-2-1

   --         which stored in memory looks like

   --       8-7-...-2-1  16-15-...-10-9  x-x-x-x-20-19-18-17  x-x-x-x-x-x-x

   --   An important rule is that the constrained and unconstrained cases

   --   must have the same bit representation in memory, since we will often

   --   convert from one to the other (e.g. when calling a procedure whose

   --   formal is unconstrained). As we see, that criterion is met for the

   --   little-endian case above. Now let's look at the big-endian case:

   --     Big-endian case

   --       x-x-x-x-x-x-x-x-x-x-x-x-1-2-3-4-5-6-7-8-9-10-...-17-18-19-20

   --         which stored in memory looks like

   --       x-x-x-x-x-x-x-x  x-x-x-x-1-2-3-4  5-6-...11-12  13-14-...-19-20

   --   That won't do, the representation value in memory is NOT the same in

   --   the constrained and unconstrained case. The solution is to store the

   --   modular value left-justified:

   --       1-2-3-4-5-6-7-8-9-10-...-17-18-19-20-x-x-x-x-x-x-x-x-x-x-x

   --         which stored in memory looks like

   --       1-2-...-7-8  9-10-...15-16  17-18-19-20-x-x-x-x  x-x-x-x-x-x-x-x

   --   and now, we do indeed have the same representation for the memory

   --   version in the constrained and unconstrained cases.

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

   -- Subprograms --

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

   procedure Create_Packed_Array_Type (Typ  : Entity_Id);

   --  Typ is a array type or subtype to which pragma Pack applies. If the

   --  Packed_Array_Type field of Typ is already set, then the call has no

   --  effect, otherwise a suitable type or subtype is created and stored

   --  in the Packed_Array_Type field of Typ. This created type is an Itype

   --  so that Gigi will simply elaborate and freeze the type on first use

   --  (which is typically the definition of the corresponding array type).

   --

   --  Note: although this routine is included in the expander package for

   --  packed types, it is actually called unconditionally from Freeze,

   --  whether or not expansion (and code generation) is enabled. We do this

   --  since we want gigi to be able to properly compute type characteristics

   --  (for the Data Decomposition Annex of ASIS, and possible other future

   --  uses) even if code generation is not active. Strictly this means that

   --  this procedure is not part of the expander, but it seems appropriate

   --  to keep it together with the other expansion routines that have to do

   --  with packed array types.

   procedure Expand_Packed_Boolean_Operator (N : Node_Id);

   --  N is an N_Op_And, N_Op_Or or N_Op_Xor node whose operand type is a

   --  packed boolean array. This routine expands the appropriate operations

   --  to carry out the logical operation on the packed arrays. It handles

   --  both the modular and array representation cases.

   procedure Expand_Packed_Element_Reference (N : Node_Id);

   --  N is an N_Indexed_Component node whose prefix is a packed array. In

   --  the bit packed case, this routine can only be used for the expression

   --  evaluation case, not the assignment case, since the result is not a

   --  variable. See Expand_Bit_Packed_Element_Set for how the assignment case

   --  is handled in the bit packed case. For the enumeration case, the result

   --  of this call is always a variable, so the call can be used for both the

   --  expression evaluation and assignment cases.

   procedure Expand_Bit_Packed_Element_Set (N : Node_Id);

   --  N is an N_Assignment_Statement node whose name is an indexed

   --  component of a bit-packed array. This procedure rewrites the entire

   --  assignment statement with appropriate code to set the referenced

   --  bits of the packed array type object. Note that this procedure is

   --  used only for the bit-packed case, not for the enumeration case.

   procedure Expand_Packed_Eq (N : Node_Id);

   --  N is an N_Op_Eq node where the operands are packed arrays whose

   --  representation is an array-of-bytes type (the case where a modular

   --  type is used for the representation does not require any special

   --  handling, because in the modular case, unused bits are zeroes.

   procedure Expand_Packed_Not (N : Node_Id);

   --  N is an N_Op_Not node where the operand is packed array of Boolean

   --  in standard representation (i.e. component size is one bit). This

   --  procedure expands the corresponding not operation. Note that the

   --  non-standard representation case is handled by using a loop through

   --  elements generated by the normal non-packed circuitry.

   function Involves_Packed_Array_Reference (N : Node_Id) return Boolean;

   --  N is the node for a name. This function returns true if the name

   --  involves a packed array reference. A node involves a packed array

   --  reference if it is itself an indexed component referring to a bit-

   --  packed array, or it is a selected component whose prefix involves

   --  a packed array reference.

   procedure Expand_Packed_Address_Reference (N : Node_Id);

   --  The node N is an attribute reference for the 'Address reference, where

   --  the prefix involves a packed array reference. This routine expands the

   --  necessary code for performing the address reference in this case.

end Exp_Pakd;