| ------------------------------------------------------------------------------ |
| -- -- |
| -- GNAT RUN-TIME COMPONENTS -- |
| -- -- |
| -- S Y S T E M . B I T F I E L D _ U T I L S -- |
| -- -- |
| -- S p e c -- |
| -- -- |
| -- Copyright (C) 2019-2021, 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. -- |
| -- -- |
| -- As a special exception under Section 7 of GPL version 3, you are granted -- |
| -- additional permissions described in the GCC Runtime Library Exception, -- |
| -- version 3.1, as published by the Free Software Foundation. -- |
| -- -- |
| -- You should have received a copy of the GNU General Public License and -- |
| -- a copy of the GCC Runtime Library Exception along with this program; -- |
| -- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see -- |
| -- <http://www.gnu.org/licenses/>. -- |
| -- -- |
| -- GNAT was originally developed by the GNAT team at New York University. -- |
| -- Extensive contributions were provided by Ada Core Technologies Inc. -- |
| -- -- |
| ------------------------------------------------------------------------------ |
| |
| package System.Bitfield_Utils is |
| |
| -- This package provides a procedure for copying arbitrarily large and |
| -- arbitrarily bit-aligned bit fields. |
| |
| -- Type Val is used to represent small bit fields. Val_2 represents a |
| -- contiguous pair of Vals. Val_2'Alignment is half of its size in bytes, |
| -- which is likely not the natural alignment. This is done to ensure that |
| -- any bit field that fits in a Val can fit in an aligned Val_2, starting |
| -- somewhere in the first half, and possibly crossing over into the second |
| -- half. This allows us to isolate a Val value by shifting and masking the |
| -- Val_2. |
| -- |
| -- Val can be 8, 16, or 32 bits; larger values are more efficient. It can't |
| -- be 64 bits, because we need Val_2 to be a double-wide shiftable type, |
| -- and 128 bits is not supported. Instantiating with an 8-bit Val is useful |
| -- for testing and debugging; 32 bits should be used for production. |
| -- |
| -- We use modular types here, not because we want modular arithmetic, but |
| -- so we can do shifting and masking. The actual for Val_2 should have |
| -- pragma Provide_Shift_Operators, so that the Shift_Left and Shift_Right |
| -- intrinsics can be passed in. It is impossible to put that pragma on a |
| -- generic formal, or on a type derived from a generic formal, so they have |
| -- to be passed in. |
| -- |
| -- Endian indicates whether we're on a little- or big-endian machine. |
| |
| pragma Elaborate_Body; |
| |
| Little : constant Bit_Order := Low_Order_First; |
| Big : constant Bit_Order := High_Order_First; |
| |
| generic |
| type Val is mod <>; |
| type Val_2 is mod <>; |
| |
| with function Shift_Left |
| (Value : Val_2; |
| Amount : Natural) return Val_2 is <>; |
| |
| with function Shift_Right |
| (Value : Val_2; |
| Amount : Natural) return Val_2 is <>; |
| |
| Endian : Bit_Order := Default_Bit_Order; |
| |
| package G is |
| -- Assert that Val has one of the allowed sizes, and that Val_2 is twice |
| -- that. |
| |
| pragma Assert (Val'Size in 8 | 16 | 32); |
| pragma Assert (Val_2'Size = Val'Size * 2); |
| |
| -- Assert that both are aligned the same, to the size in bytes of Val |
| -- (not Val_2). |
| |
| pragma Assert (Val'Alignment = Val'Size / Storage_Unit); |
| pragma Assert (Val_2'Alignment = Val'Alignment); |
| |
| type Val_Array is array (Positive range <>) of Val; |
| |
| -- It might make more sense to have: |
| -- subtype Val is Val_2 range 0 .. 2**Val'Size - 1; |
| -- But then GNAT gets the component size of Val_Array wrong. |
| |
| pragma Assert (Val_Array'Alignment = Val'Alignment); |
| pragma Assert (Val_Array'Component_Size = Val'Size); |
| |
| subtype Bit_Size is Natural; -- Size in bits of a bit field |
| subtype Small_Size is Bit_Size range 1 .. Val'Size; |
| -- Size of a small one |
| subtype Bit_Offset is Small_Size'Base range 0 .. Val'Size - 1; |
| -- Starting offset |
| subtype Bit_Offset_In_Byte is Bit_Offset range 0 .. Storage_Unit - 1; |
| |
| procedure Copy_Bitfield |
| (Src_Address : Address; |
| Src_Offset : Bit_Offset_In_Byte; |
| Dest_Address : Address; |
| Dest_Offset : Bit_Offset_In_Byte; |
| Size : Bit_Size); |
| -- An Address and a Bit_Offset together form a "bit address". This |
| -- copies the source bit field to the destination. Size is the size in |
| -- bits of the bit field. The bit fields can be arbitrarily large, but |
| -- the starting offsets must be within the first byte that the Addresses |
| -- point to. The Address values need not be aligned. |
| -- |
| -- For example, a slice assignment of a packed bit field: |
| -- |
| -- D (D_First .. D_Last) := S (S_First .. S_Last); |
| -- |
| -- can be implemented using: |
| -- |
| -- Copy_Bitfield |
| -- (S (S_First)'Address, S (S_First)'Bit, |
| -- D (D_First)'Address, D (D_First)'Bit, |
| -- Size); |
| |
| function Fast_Copy_Bitfield |
| (Src : Val_2; |
| Src_Offset : Bit_Offset; |
| Dest : Val_2; |
| Dest_Offset : Bit_Offset; |
| Size : Small_Size) |
| return Val_2 with Inline; |
| -- Faster version of Copy_Bitfield, with a different calling convention. |
| -- In particular, we pass by copy rather than passing Addresses. The bit |
| -- field must fit in Val_Bits. Src and Dest must be properly aligned. |
| -- The result is supposed to be assigned back into Dest, as in: |
| -- |
| -- Dest := Fast_Copy_Bitfield (Src, ..., Dest, ..., ...); |
| |
| end G; |
| |
| end System.Bitfield_Utils; |
