[avr-gcc-list] Small frame optimization: Butterfly -= 90 bytes

[Dmitry K.]

Hi all.

Avr-gcc use list of pop's to restore stack in body of function.
(After call with parameters in stack, small size).
The same can be made for prologues and epilogues.
It has reduced the size of code Butterfly by 90 bytes (with
a simultaneous prize in speed).

See example:

   extern void foo (char *);
   void foo_2 (void)
   {
       char s[2];
       foo(s);
   }

Original (3.3.2) compilation:

   foo_2:
   /* prologue: frame size=2 */
        push r28
        push r29
        in r28,__SP_L__
        in r29,__SP_H__
        sbiw r28,2
        in __tmp_reg__,__SREG__
        cli
        out __SP_H__,r29
        out __SREG__,__tmp_reg__
        out __SP_L__,r28
   /* prologue end (size=10) */
        mov r24,r28
        mov r25,r29
        adiw r24,1
        rcall foo
   /* epilogue: frame size=2 */
        adiw r28,2
        in __tmp_reg__,__SREG__
        cli
        out __SP_H__,r29
        out __SREG__,__tmp_reg__
        out __SP_L__,r28
        pop r29
        pop r28
        ret
   /* epilogue end (size=9) */

After optimization:

   foo_2:
   /* prologue: frame size=2 */
        push r28
        push r29
        rcall .
        in r28,__SP_L__
        in r29,__SP_H__
   /* prologue end (size=5) */
        mov r24,r28
        mov r25,r29
        adiw r24,1
        rcall foo
   /* epilogue: frame size=2 */
        pop __tmp_reg__
        pop __tmp_reg__
        pop r29
        pop r28
        ret
   /* epilogue end (size=5) */

Patch for avr.c in attachment:
----------------------------------------------------------------------
    FOR LOOCKING OF IDEA ONLY, NOT USE FOR WORK.
  THERE ARE IMPERFECTIONS AND MISTAKES ARE POSSIBLE, DID NOT TEST.
----------------------------------------------------------------------

Best wishes.