Re: [avr-gcc-list] Bad code generated when using volatile keyword/-O0 opt?

[Ned Konz]

On Feb 11, 2006, at 10:43 PM, Rick Mann wrote:

> Generating code for an ATmega128.
>
> I've boiled this down to a pretty simple case, I think. In the  
> following code, if compiled with -Os, if "d" is made volatile, then  
> it never seems to get past execution in the first delay loop. If  
> it's not volatile, it works correctly. If I compile with -O0,  
> neither volatile nor non-volatile works (same failure mode).
>
> I started experimenting with -O0 (and the volatile keyword) to try  
> to make sure gdb was in sync between source and assembly.

> With the volatile keyword, the .lst files shows this for the first  
> loop:
>
> ---
>   17:main.c        ****                 d = kD;
> 100                             .stabn  68,0,17,.LM4-main
> 101                     .LM4:
> 102 0016 2983                   std Y+1,r18
> 103                     .L3:
>   18:main.c        ****                 while (d--);
> 104                             .stabn  68,0,18,.LM5-main
> 105                     .LM5:
> 106 0018 8981                   ldd r24,Y+1
> 107 001a 8150                   subi r24,lo8(-(-1))
> 108 001c 8983                   std Y+1,r24
> 109 001e 8981                   ldd r24,Y+1
> 110 0020 8F3F                   cpi r24,lo8(-1)
> 111 0022 D1F7                   brne .L3
>   19:main.c        ****                 
> ---
>
> And without volatile:
>
> ---
>   99 0014 8FEF                  ldi r24,lo8(-1)
> 100                     .L3:
>   17:main.c        ****                 d = kD;
>   18:main.c        ****                 while (d--);
> 101                             .stabn  68,0,18,.LM4-main
> 102                     .LM4:
> 103 0016 8150                   subi r24,1
> 104 0018 F0F7                   brcc .L3
> ---
>
> Not sure if this is a bug, but it sure seems like it to me. Let me  
> know what other info you might need.

Why does it seem like a bug?

Is this your entire program, or might there be an interrupt handler  
that's trashing one or another registers or memory locations?

--
Ned Konz
address@hidden