Re: [avr-gcc-list] Weird optimization issue with avr-gcc 4.5.3, re "naked."

[Weddington, Eric]

How does it compile if you change the C code to this?:

int main(void) __attribute__ ((naked)) __attribute__ ((section
(".init9")));
int main()
{
  uint8_t ch;

  for (;;) {
    ch = getch();

    if(ch == STK_GET_PARAMETER) {
      register uint8_t which = getch();
      register uint8_t output = 0x03;
      verifySpace();
      if (which == 0x82) {
          output = OPTIBOOT_MINVER;
      } else if (which == 0x81) {
          output = OPTIBOOT_MAJVER;
      }
      putch(output);
    }

That should help make it somewhat immune to the vagaries of the
optimizer, but also keep the generated code small.

Let me know what you get.

Eric Weddington

> -----Original Message-----
> From: address@hidden
[mailto:avr-
> address@hidden On Behalf Of
William
> "Chops" Westfield
> Sent: Thursday, January 26, 2012 11:11 AM
> To: address@hidden
> Subject: [avr-gcc-list] Weird optimization issue with avr-gcc 4.5.3,re
> "naked."
> 
> A week or so ago I mentioned that avr-gcc 4.5.3 caused the optiboot
bootloader
> (used by Arduino and etc) to grow 22 bytes (from 500 bytes to 522
bytes,
> pushing it over the size that it needs to be.)
> 
> This took a while to track down, because it's weird.  Apparently gcc
is
> optimizing code differently depending on whether the function is
defined as
> "naked" or not.  Optiboot sets up main() as naked to save space, and
the lost
> optimization is hurting it.
> 
> I have a small test case (even smaller than optiboot :-)
> (files also attached.  foo.c, foo-naked.S, foo-clothed.S)
> 
> Consider:
> 
> int main(void) __attribute__ ((naked)) __attribute__ ((section
(".init9")));
> int main()
> {
>   uint8_t ch;
> 
>   for (;;) {
>     ch = getch();
> 
>     if(ch == STK_GET_PARAMETER) {
>       register uint8_t which = getch();
>       verifySpace();
>       if (which == 0x82) {
>       putch(OPTIBOOT_MINVER);
>       } else if (which == 0x81) {
>         putch(OPTIBOOT_MAJVER);
>       } else {
>       putch(0x03);
>       }
>     }
> 
> Without the extra attributes for main, the is optimized to fact out
the call
> to putch():
> 
>       /*
>        * Send optiboot version as "minor SW version"
>        */
>       putch(OPTIBOOT_MINVER);
>   1c: 85 e0           ldi     r24, 0x05       ; 5
>   1e: 00 c0           rjmp    .+0             ; 0x20 <main+0x20>
>       } else if (which == 0x81) {
>   20: 81 38           cpi     r24, 0x81       ; 129
>   22: 01 f4           brne    .+0             ; 0x24 <main+0x24>
>         putch(OPTIBOOT_MAJVER);
>   24: 84 e0           ldi     r24, 0x04       ; 4
>   26: 00 c0           rjmp    .+0             ; 0x28 <main+0x28>
>       } else {
>       /*
>        * GET PARAMETER returns a generic 0x03 reply for
>          * other parameters - enough to keep Avrdude happy
>        */
>       putch(0x03);
>   28: 83 e0           ldi     r24, 0x03       ; 3
>   2a: 00 d0           rcall   .+0             ; 0x2c <main+0x2c>
>   2c: 00 c0           rjmp    .+0             ; 0x2e <main+0x2e>
>       }
>     }
> 
> WITH the extra attributes, I get three separate rcall instructions:
> 
>       putch(OPTIBOOT_MINVER);
>   12: 85 e0           ldi     r24, 0x05       ; 5
>   14: 00 d0           rcall   .+0             ; 0x16 <main+0x16>
>   16: 00 c0           rjmp    .+0             ; 0x18 <main+0x18>
>       } else if (which == 0x81) {
>   18: 81 38           cpi     r24, 0x81       ; 129
>   1a: 01 f4           brne    .+0             ; 0x1c <main+0x1c>
>         putch(OPTIBOOT_MAJVER);
>   1c: 84 e0           ldi     r24, 0x04       ; 4
>   1e: 00 d0           rcall   .+0             ; 0x20 <main+0x20>
>   20: 00 c0           rjmp    .+0             ; 0x22 <main+0x22>
>       } else {
>       /*
>        * GET PARAMETER returns a generic 0x03 reply for
>          * other parameters - enough to keep Avrdude happy
>        */
>       putch(0x03);
>   22: 83 e0           ldi     r24, 0x03       ; 3
>   24: 00 d0           rcall   .+0             ; 0x26 <main+0x26>
>   26: 00 c0           rjmp    .+0             ; 0x28 <main+0x28>
>       }
>     }
> 
> Any thoughts as to what might be going on here?
> 
> Thanks
> WestfW