AVR-Chat

--- In AVR-Chat@yahoogroups.com, "bayramdavies" <Yahoo37849@...> wrote:
>> ... unless it can determine conclusively otherwise.
> Which, I think, it generally can't for the C language.
For most languages, C included, it is conceptually easy: look at a function and see what global variables it might modify and then do the same for any function that it might call and so on.  Different compilers may go to varying lengths to try to optimize out reloading of globals and there are situations that complicate matters (e.g. writing through a pointer having an unknown value).  

If you compile the example code below with WinAVR_20100110 using -Os you'll see that the compiler inlines the foo() code into main() and determines that the variable b1 is loop invariant.  Granted, it is a simple example but it shows that it can be and is done.

uint8_t b1;
uint8_t b2;

void foo(void)
{
    b2++;
}

int main(void)
{
    while (b1)
        foo();
    for (;;)
        ;
}


Don Kinzer
ZBasic Microcontrollers
http://www.zbasic.net