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To: gcc@gcc.gnu.org
From: "Paulo J. Matos" <pocmatos@gmail.com>
Subject: avr compilation
Date: Fri, 18 Mar 2011 08:48:00 -0000
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Hi all,

I am looking at the avr backend in order to try to sort some things out 
on my own backend.

One of the tests I am doing is by compiling the following:
int x = 0x1010;
int y = 0x0101;

int add(void)
{
   return x+y;
}

It compiles to (in gcc-4.3.5_avr with -Os)
add:
/* prologue: function */
/* frame size = 0 */
         lds r18,y
         lds r19,(y)+1
         lds r24,x
         lds r25,(x)+1
         add r18,r24
         adc r19,r25
         mov r24,r18
         mov r25,r19
/* epilogue start */
         ret

I don't know much avr assembler so bear with me but I would expect this 
to be written:
add:
/* prologue: function */
/* frame size = 0 */
         lds r18,y
         lds r19,(y)+1
         lds r24,x
         lds r25,(x)+1
         add r24,r18
         adc r25,r19
/* epilogue start */
         ret

By inverting the add arguments we save two mov instructions.

If it can be written like this any ideas on why GCC is avoiding it?

Cheers,

--
PMatos