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

[Albert Seward]

On Thu, 2 Dec 2004, E. Weddington wrote:

> 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.
>

If I wanted to do it the "canonical way" would I have ask for it. But I
didn't, did I?

I do wonder how your brain work. I wish I would have taken a degree in
philosophy instead of my master degree in computer science.

But hey, thanks for the link. Here is a link so you can learn some
"Reading 101"
http://www.fisher-price.com/us/ms5/learn-to-read.asp

Enjoy.

Albert Seward
Uppsala University Sweden