Re: [avr-gcc-list] Confusing volatile behaviour with 4.6.2

[Weddington, Eric]

Hi Wim,

Do you have a GCC bug report filled out for this issue?

If not, could you fill out a bug report?

Thanks,
Eric Weddington

> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden On
> Behalf Of Wim Lewis
> Sent: Tuesday, November 29, 2011 1:09 AM
> To: address@hidden
> Subject: Re: [avr-gcc-list] Confusing volatile behaviour with 4.6.2
> 
> On 11/28/11 5:33 AM, Andy Warner wrote:
> > What is it about this construct that makes the compiler feel free to
> reorder ?
> > I assume this must be a very specific use case I've tripped over, if
> > volatiles were being reordered wholesale in 4.6.2, I'd expect there
to
> > be many, many problems surfacing.
> 
> Me too. Perhaps whatever optimizer rule produces the 'sbis'
instruction
> doesn't understand that it's reordering volatile operations? Turning
on
> RTL dumps suggests that the problem shows up after the combine pass.
> Maybe someone more familiar with gcc internals could comment.
> 
> Running the delta minimizer on the code you posted reduces the test
case
> to this:
> 
> typedef unsigned char uint8_t;
> #define       SEARCHING (-2)
> #define UCSR0A (*(volatile uint8_t *)((0x0B) + 0x20))
> #define UDR0 (*(volatile uint8_t *)((0x0C) + 0x20))
> void __vector_18(void)
> {
>       unsigned char status = UCSR0A;
>       unsigned char data = UDR0;
>       static volatile int slot;
>       if (status & 0x10) {
>               if (slot == SEARCHING) {
>                       slot ++;
>               }
>       }
> }
> 
> avr-gcc-4.6.2 -S -mmcu=atmega128 -Wall -fverbose-asm -O2 produces:
> [....]
> __vector_18:
> /* prologue: function */
> /* frame size = 0 */
> /* stack size = 0 */
> .L__stack_usage = 0
>         in r24,44-0x20   ;  data, MEM[(volatile uint8_t *)44B]
>         sbis 43-0x20,4   ; ,
>         rjmp .L1         ;
> [....]
> 
> 
> _______________________________________________
> AVR-GCC-list mailing list
> address@hidden
> https://lists.nongnu.org/mailman/listinfo/avr-gcc-list