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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Odd AVR-Ada code generation issue with constant record type
Date: Tue, 09 Jul 2019 16:30:52 +0100
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Xref: reader01.eternal-september.org comp.lang.ada:56835
Date: 2019-07-09T16:30:52+01:00
List-Id: <comp.lang.ada>

I don't have access to an up-to-date AVR compiler, so I had a go with
GCC 9.1 compiling for cortex-m4. There's a big improvement using
Volatile_Full_Access rather than just Volatile.

This code
========================================================================
with System;
with Interfaces;

procedure Main is
   use Interfaces;

   type Enum_Type is (B00, B01, B10, B11);
   for Enum_Type use (B00 => 0,
                      B01 => 1,
                      B10 => 2,
                      B11 => 3);

   type List_Type is array (0 .. 4) of Boolean with Pack;

   type Reg_Type is record
      Enum : Enum_Type;
      Bool : Boolean;
      List : List_Type;
   end record;
   for Reg_Type use record
      Enum at 0 range 6 .. 7;
      Bool at 0 range 5 .. 5;
      List at 0 range 0 .. 4;
   end record;
   for Reg_Type'Size use 8;

   --  Reg_A  --
   Reg_A : Reg_Type
     with
       Volatile_Full_Access,
       Address => System'To_Address (16#7a#);

   --  Reg_B  --
   Reg_B : Reg_Type
     with
       Volatile_Full_Access,
       Address => System'To_Address (16#7b#);

   --  Reg_C  --
   Reg_C : Reg_Type
     with
       Volatile_Full_Access,
       Address => System'To_Address (16#7c#);

   --  Reg_D  --
   Reg_D : Unsigned_8
     with
       Volatile_Full_Access,
       Address => System'To_Address (16#7d#);

   --  Reg_E  --
   Reg_E : Unsigned_8
     with
       Volatile_Full_Access,
       Address => System'To_Address (16#7e#);

begin
   --  Set composite with resulting zero-byte.
   Reg_A := (Enum => B00,
             Bool => False,
             List => (others => False));

   --  Set composite with resulting non-zero-byte (any one field is set, others zero).
   Reg_B := (Enum => B00,
             Bool => True,
             List => (others => False));

   --  Set composite with resulting non-zero-byte (multiple fields is set).
   Reg_C := (Enum => B11,
             Bool => True,
             List => (others => True));

   --  Clear Unsigned_8 directly.
   Reg_D := 16#00#;

   --  Setting Unsigned_8 directly.
   Reg_E := 16#FF#;

end Main;
========================================================================

compiled with -O2 yields

========================================================================
Disassembly of section .text.startup._ada_main:

00000000 <_ada_main>:
   0:	4b0a      	ldr	r3, [pc, #40]	; (2c <_ada_main+0x2c>)
   2:	4a0b      	ldr	r2, [pc, #44]	; (30 <_ada_main+0x30>)
   4:	480b      	ldr	r0, [pc, #44]	; (34 <_ada_main+0x34>)
   6:	217a      	movs	r1, #122	; 0x7a
   8:	b410      	push	{r4}
   a:	781c      	ldrb	r4, [r3, #0]
   c:	700c      	strb	r4, [r1, #0]
   e:	237b      	movs	r3, #123	; 0x7b
  10:	7812      	ldrb	r2, [r2, #0]
  12:	701a      	strb	r2, [r3, #0]
  14:	227c      	movs	r2, #124	; 0x7c
  16:	7800      	ldrb	r0, [r0, #0]
  18:	7010      	strb	r0, [r2, #0]
  1a:	217d      	movs	r1, #125	; 0x7d
  1c:	237e      	movs	r3, #126	; 0x7e
  1e:	2000      	movs	r0, #0
  20:	22ff      	movs	r2, #255	; 0xff
  22:	7008      	strb	r0, [r1, #0]
  24:	701a      	strb	r2, [r3, #0]
  26:	f85d 4b04 	ldr.w	r4, [sp], #4
  2a:	4770      	bx	lr
========================================================================