[Ada] Add the System.Bitfield_Utils runtime unit

2019-08-21  Bob Duff  <duff@adacore.com>

gcc/ada/

	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Add s-bitutil.o and
	s-biutin.o.
	* exp_ch5.adb (Expand_Assign_Array_Bitfield): New function to
	generate a call to Copy_Bitfield. This is disabled for now.
	(Expand_Assign_Array_Loop_Or_Bitfield): New function to decide
	whether to call Expand_Assign_Array_Bitfield.
	(Expand_Assign_Array): Call Expand_Assign_Array_Loop_Or_Bitfield
	instead of Expand_Assign_Array_Loop.
	* libgnat/s-bitfie.ads, libgnat/s-bituti.adb,
	libgnat/s-bituti.ads: New units.
	* rtsfind.ads: Add enum literals for accessing Copy_Bitfield.

From-SVN: r274785

1 files changed, 132 insertions, 0 deletions

diff --git a/gcc/ada/libgnat/s-bituti.ads b/gcc/ada/libgnat/s-bituti.ads
new file mode 100644
index 0000000..1e446c1
--- /dev/null
+++ b/gcc/ada/libgnat/s-bituti.ads
@@ -0,0 +1,132 @@
+------------------------------------------------------------------------------
+--                                                                          --
+--                         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, 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 little-endian 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 0 .. Val'Size;
+      --  Size of a small one
+      subtype Bit_Offset is Small_Size 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);
+
+   end G;
+
+end System.Bitfield_Utils;