Re: [avr-gcc-list] Efficient I/O handling w. bitfields codegen problem

[E. Weddington]

Albert Seward wrote:

> I have some problems with code generation from GCC 3.3. Read my question
> in the main block.
>
> /*
> * bitfields.c
> *
> * GCC bitfields optimization test
> */
> typedef union {
>        unsigned char byte;
>        struct {
>                unsigned char pin0 : 1;
>                unsigned char pin1 : 1;
>                unsigned char pin2 : 1;
>                unsigned char pin3 : 1;
>                unsigned char pin4 : 1;
>                unsigned char pin5 : 1;
>                unsigned char pin6 : 1;
>                unsigned char pin7 : 1;
>        };
> } volatile * const port_t;
>
> port_t port[] = {(port_t)0x3d, (port_t)0x38, (port_t)0x35, (port_t)0x32};
>
> // Another way...
> struct  {
>        port_t portA;
>        port_t portB;
>        port_t portC;
>        port_t portD;
> } ports = {(port_t)0x3d, (port_t)0x38, (port_t)0x35, (port_t)0x32};
>
>
> int main(void) {
>        /*  This works!
>         *  It will give asm:
>         *  cbi     0x18, 7
>         */ sbi     0x18, 7
>        port[1]->pin7 = 0;
>        port[1]->pin7 = 1;
>
>        /*  Why dont I get the same asm as above?
>         *  Now I do get this:
>         *
>         *  lds     r30, 0x0068
>         *  lds     r31, 0x0069
>         *  ld      r24, Z
>         *  andi    r24, 0x7F       ; 127
>         *  st      Z, r24
>         *
>         *  lds     r30, 0x0068
>         *  lds     r31, 0x0069
>         *  ld      r24, Z
>         *  ori     r24, 0x80       ; 128
>         *  st      Z, r24
>         *
>         *  This is not as efficient, and it does not do the job
>         *  if I for example want to change GIMSK this way.
>         */
>        ports.portA->pin7 = 0;
>        ports.portA->pin7 = 1;
>
>        for(;;)
>
>        return 0;
> }
>
>
> Hope anyone has the time to help me. I am really stuck.
>
>  

The canonical way to change bits in ports in C language is not through 
the use of bitfields. You need to learn to use the C language's bitwise 
operators, in conjunction with a simple macro include in avr-libc:

#include <avr/io.h>
PORTA |= _BV(1);   // Set bit 1
PORTA &= ~_BV(1);    // Clear bit 1

See this thread on the AVR Freaks website on how to use these operators:
<http://www.avrfreaks.net/phpBB2/viewtopic.php?t=8764&highlight=programming+101>
Go to the post that starts with "Programming 101", which should be 
highlighted.