[En-Nut-Discussion] Byte select macros

[Bernd Walter]

On Mon, Nov 22, 2010 at 11:37:25PM +0100, Uwe Bonnes wrote:
> >>>>> "Bernd" == Bernd Walter <enut at cicely.de> writes:
> 
>     Bernd> On Mon, Nov 22, 2010 at 08:08:45PM +0100, Uwe Bonnes wrote:
>  
>     Bernd> Is any of your variables declared volatile?
> 
> Yes, that's part of the problem.
> 
> But try
> ==
> extern unsigned char *res;
> extern unsigned short *input;
> 
> void
> foo()
> {
>         res[0] = input[0] & 0xff;
>         res[1] = input[0] >> 8;
>         res[2] = input[1] & 0xff;
>         res[3] = input[1] >> 8;
> }
> ==
> with
> /tmp> avr-gcc -Os -Wall -c test.c
> /tmp> avr-objdump -S test.o | less

Yes - this test case results in inefficient code:
void
foo()
{
        res[0] = input[0] & 0xff;
   0:   a0 91 00 00     lds     r26, 0x0000
   4:   b0 91 00 00     lds     r27, 0x0000
   8:   e0 91 00 00     lds     r30, 0x0000
   c:   f0 91 00 00     lds     r31, 0x0000
  10:   80 81           ld      r24, Z
  12:   8c 93           st      X, r24
        res[1] = input[0] >> 8;
  14:   a0 91 00 00     lds     r26, 0x0000
  18:   b0 91 00 00     lds     r27, 0x0000
  1c:   e0 91 00 00     lds     r30, 0x0000
  20:   f0 91 00 00     lds     r31, 0x0000
  24:   81 81           ldd     r24, Z+1        ; 0x01
  26:   11 96           adiw    r26, 0x01       ; 1
  28:   8c 93           st      X, r24
        res[2] = input[1] & 0xff;
  2a:   a0 91 00 00     lds     r26, 0x0000
  2e:   b0 91 00 00     lds     r27, 0x0000
  32:   e0 91 00 00     lds     r30, 0x0000
  36:   f0 91 00 00     lds     r31, 0x0000
  3a:   82 81           ldd     r24, Z+2        ; 0x02
  3c:   12 96           adiw    r26, 0x02       ; 2
  3e:   8c 93           st      X, r24
        res[3] = input[1] >> 8;
  40:   a0 91 00 00     lds     r26, 0x0000
  44:   b0 91 00 00     lds     r27, 0x0000
  48:   e0 91 00 00     lds     r30, 0x0000
  4c:   f0 91 00 00     lds     r31, 0x0000
  50:   83 81           ldd     r24, Z+3        ; 0x03
  52:   13 96           adiw    r26, 0x03       ; 3
  54:   8c 93           st      X, r24
}
  56:   08 95           ret

I have no idea why it reloads all adress registers.

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.