Re: [avr-gcc-list] Bug or bad optimization?

[Zane D. Purvis]

Make sure you've declared command as "volatile":

volatile unsigned char command;

That will tell the compiler to reload it from memory every time it is read.



Jim Davis wrote:

> The following code generation has a problem handling
> the external variable "command":
>
> 1) I dont see the global variable "command" has been put into r16
> 2) The while loop never loads r16 with the global before the test.
>
> What is the proper way to tell the compiler what to do here?
>
> ------------------------------------------- C source
> extern unsigned char command;
> char *get_command(void)
> {
>         int i = 0;
>         while(1)
>         {
>                 command = NULL;
>                 while(command == NULL);
>                         // wait for command in uart getchar
>                 if (command == CR)
>
> 85:                     command = NULL;
> +0000096E:   9210150E    STS     0x150E,R1        Store direct to data space
> +00000970:   E000        LDI     R16,0x00         Load immediate
> 86:                     while(command == NULL);
> +00000971:   2300        TST     R16              Test for Zero or Minus
> +00000972:   F3F1        BREQ    PC-0x01          Branch if equal
> 88:                     if (command == CR)
> +00000973:   300D        CPI     R16,0x0D         Compare with immediate
>