Re: [avr-libc-dev] RFC: avr/bits.h

[E. Weddington]

Erik Walthinsen wrote:

> As per E. Weddinton's suggestion a month ago, I'd like to start the 
> process of getting avr/bits.h included in avr-libc.  A first pass at 
> the file and its documentation are attached.
>
> Comments, enhancements, and additional documentation requested.  Let's 
> get this finished and the sbi/cbi mess behind us.

IIRC, additional macros are also needed according to the size of the 
operands. For example this is really what should be done:

=================================================================
#define bit_set_8(var, mask)                 ((uint8_t)(var) |= 
(uint8_t)(mask))
#define bit_clear_8(var, mask)               ((uint8_t)(var) &= 
(uint8_t)~(mask))
#define bit_toggle_8(var, mask)              ((uint8_t)(var) ^= 
(uint8_t)(mask))
#define bit_read_8(var, mask)                ((uint8_t)(var) & 
(uint8_t)(mask))

#define bit_set_16(var, mask)                 ((uint16_t)(var) |= 
(uint16_t)(mask))
#define bit_clear_16(var, mask)               ((uint16_t)(var) &= 
(uint16_t)~(mask))
#define bit_toggle_16(var, mask)              ((uint16_t)(var) ^= 
(uint16_t)(mask))
#define bit_read_16(var, mask)                ((uint16_t)(var) & 
(uint16_t)(mask))

#define bit_set_32(var, mask)                ((uint32_t)(var) |= 
(uint32_t)(mask))
#define bit_clear_32(var, mask)              ((uint32_t)(var) &= 
(uint32_t)~(mask))
#define bit_toggle_32(var, mask)             ((uint32_t)(var) ^= 
(uint32_t)(mask))
#define bit_read_32(var, mask)               ((uint32_t)(var) & 
(uint32_t)(mask))

#define bit_set(var, mask)                      bit_set_8(var, mask)
#define bit_clear(var, mask)                    bit_clear_8(var, mask)
#define bit_toggle(var, mask)                   bit_toggle_8(var, mask)
#define bit_read(var, mask)                     bit_read_8(var, mask)
=================================================================

The reason why this should be done is to get around the automatic 
integer promotion. The C language bit operators automatically promote 
it's operands to an int, 16 bits. This is unacceptable for operations on 
8 bit values which are commonly used when operating on the AVR 
registers. Typecasting is necessary to tell the compiler to optimize the 
generated assembly. There's a FAQ item in the avr-libc user manual about 
this issue.

Eric