Re: [avr-gcc-list] Strange inline asm bug or silly user?

[David Carr]

Georg and Niklas,

Thanks for the help!
You're both right in that adding "clr r1"  to the end fixed the 
problem.   I guess I figured that simply adding R1 to the clobber list 
would cause gcc to do that automatically.  Silly me.

-DC

David Carr wrote:
> I'm just learning to use inline assembly with avr-gcc, and I've 
> encountered what seems to me to be a very strange bug (but may be user 
> silliness).
>
> In the test case below, I repeatedly change the first entry of the 
> data array between 0xFF and 0x00.  A short piece of assembly then 
> moves data[0] into r0 and pushes it out to PORTB.  This causes an LED 
> to flash.  This works.
>
> However, when I change:
>
> "ldd r0, %a1+0 \n\t"
> "out %0, r0 \n\r"
>
> to:
>
> ldd r1, %a1+0 \n\t"
> "out %0, r1 \n\r"
>
> IE: replace r0 with r1, the LED just stays lit continuously.
>
> Fine, so I look at the assembly listing generated by avr-objdump -h -S 
> ads-tx.elf > ads-tx.lst
> This is the diff between the working and non-working versions:
>
> 99,102c99,102
> <   ae:    10 80           ld    r1, Z
> <   b0:    15 b8           out    0x05, r1    ; 5
> <            "ldd r1, %a1+0 \n\t"
> <            "out %0, r1 \n\r"
> ---
> >   ae:    00 80           ld    r0, Z
> >   b0:    05 b8           out    0x05, r0    ; 5
> >            "ldd r0, %a1+0 \n\t"
> >            "out %0, r0 \n\r"
> 170,171c170,171
> <   e4:    10 80           ld    r1, Z
> <   e6:    15 b8           out    0x05, r1    ; 5
> ---
> >   e4:    00 80           ld    r0, Z
> >   e6:    05 b8           out    0x05, r0    ; 5
> 188,189c188,189
> <   f0:    10 80           ld    r1, Z
> <   f2:    15 b8           out    0x05, r1    ; 5
> ---
> >   f0:    00 80           ld    r0, Z
> >   f2:    05 b8           out    0x05, r0    ; 5
>
> It looks to me like the only thing that has changed is R0/R1.
>
> Even more strange is that using R2, R3, R13 all work.  Just R1 fails.  
> I am baffled.
>
> Test code:
>
> #include <stdint.h>
> #include <avr/io.h>
>
> #define F_CPU 8000000UL
> #include <util/delay.h>
>
> void send_packet(volatile uint8_t* data)
> {
>  asm volatile(
>           //first load data in to RX
>           "ldd r0, %a1+0 \n\t"
>           "out %0, r0 \n\r"
>           :
>           :"I" (_SFR_IO_ADDR(PORTB)), "e" (data)
>           : "r0", "r1", "r2", "r3", "r13");
> }
>
> void main(void)
> {
>  //this shouldn't need volatile, but gcc optimizes it out otherwise?
>  volatile uint8_t data[14];
>  DDRB = 0xFF;
>
>  while (1)
>    {
>        data[0] = 0x00;
>      send_packet(data);
>      _delay_ms(20);
>          data[0] = 0xFF;
>      send_packet(data);
>      _delay_ms(20);
>    }
> }
>
> Thanks for the help,
> -DC
>
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