Re: [avr-gcc-list] efficiency of assigning bits

[Dave Hansen]

From: David Willmore <address@hidden>
[...]
> Hello, all.  I'm a bit confused by many of the comments in this
> thread.  Where is there a guarentee that the two bits will be
> set at the same time in any of the shown examples?
>
> If I understand correctly, any of:
>
> PORTD &= ~_BV(PD1);
> PORTD &= ~_BV(PD4);
>
> or
>
> PORTD &= ~(_BV(PD1) | _BV(PD4));
>
> or other variants are completely the same in what kind of code
> they may generate when it gets down to machine language, no?
>
> There is no implied simultinaity in any of these statements--
> all C offers you is that the user code won't be able to tell
> the difference--unless PORTD is defined as a volatile.  Which
> may be the default and would explain my confusion.
>
> Is that the critical bit that I'm missing?

Yup.

--- begin included file ---
C:\Dave>type sfr.c
#include <avr/io.h>

void foo(void)
{
   PORTD &= 0xFC;
}

C:\Dave>avr-gcc -mmcu=atmega16 -E sfr.c > sfr.out

C:\Dave>tail sfr.out
# 176 "C:/WinAVR/bin/../lib/gcc/avr/3.4.1/../../../../avr/include/avr/io.h" 
2 3
# 241 "C:/WinAVR/bin/../lib/gcc/avr/3.4.1/../../../../avr/include/avr/io.h" 
3
# 1 
"C:/WinAVR/bin/../lib/gcc/avr/3.4.1/../../../../avr/include/avr/portpins.h"
1 3
# 242 "C:/WinAVR/bin/../lib/gcc/avr/3.4.1/../../../../avr/include/avr/io.h" 
2 3
# 2 "sfr.c" 2

void foo(void)
{
   (*(volatile uint8_t *)((0x12) + 0x20)) &= 0xFC;
}

--- end included file ---

Note: (volatile uint8_t *)

Regards,
  -=Dave