------------------------------------------------------------------------------ -- -- -- GNAT COMPILER COMPONENTS -- -- -- -- E X P _ P A K D -- -- -- -- S p e c -- -- -- -- -- -- 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. -- -- Extensive contributions were provided by Ada Core Technologies Inc. -- -- -- ------------------------------------------------------------------------------ -- 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;