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-- --
-- GNAT COMPILER COMPONENTS --
-- --
-- E X P _ P A K D --
-- --
-- S p e c --
-- --
-- $Revision: 1.22 $
-- --
-- Copyright (C) 1992-2001 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, 59 Temple Place - Suite 330, Boston, --
-- MA 02111-1307, USA. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- It is now maintained by Ada Core Technologies Inc (http://www.gnat.com). --
-- --
------------------------------------------------------------------------------
-- 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 right 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. The special flag
-- Is_Left_Justified_Modular is set in the modular type used as the
-- packed array type in the big-endian case to ensure that this required
-- left justification occurs.
-----------------
-- 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 charactersitics
-- (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 he 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 compoment 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;