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

[HutchinsonAndy]

Nice one!

I have been looking at the new register allocator. This produces much better 
code in many cases BUT unfortunately it seems to have a bug that leaves space 
on the stack - ie a frame pointer when one is not reqd.

Your change mitigates that effect.



"Dmitry K." <address@hidden> wrote:

>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.
>

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