Re: [avr-libc-dev] [bugs #10450] missing __volatile__ in CRC functions core

[Dmitry K.]

At 20 Sep 2004 Yann GOUY wrote:

> http://savannah.nongnu.org/bugs/?func=detailitem&item_id=10450

> Original Submission:  in <avr/crc16.h>, the cores of the CRC functions use
> inline assembly language.
> the "keyword" __asm__ shall be followed by __volatile__ else compiling those 
> functions with "-Os -mcall-prologues" completly suppress the assembly
> intructions.

There is no mistake. The code is suppressed, if its result (or by-effects) is 
not used further. See old discussion about program memory reading macros.

For example:
~~~~~~~~~~~
#include <avr/crc16.h>  /* without `__volatile__' */

int foo (void)
{
    char s[10], *p;
    int sum, i;

    /* Filling s[] is omited.   */
    sum = 0;
    p = s;
    for (i = sizeof(s); i; --i)
        sum = _crc16_update (sum, *p++);
    return sum;
}

Result (avr-gcc 3.3.4, -Os -mcall-prologues) -- no suppression:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.L7:
        ld r24,Z+
        mov r23,r19
        mov r22,r18
/* #APP */
        eor r22,r24
        ...
        eor r22,r24
/* #NOAPP */
        mov r18,r22
        mov r19,r23
        subi r20,lo8(-(-1))
        sbci r21,hi8(-(-1))
        brne .L7
        mov r25,r23
        mov r24,r22
/* epilogue: frame size=10 */
        ldi r30,2
        adiw r28,10
        rjmp __epilogue_restores__+32

P.S. In practice, certainly, there is no difference.