Optimization Problem

[caneralp 0]

Hello,

 

I have a problem with optimization. Can anyone help me?

 

my code is below which is for loop and delaying.

 

volatile unsigned int b;

 

b = 30000;

do

{

b--;

}while(b != 0);

 

normally unless I define b as a volatile variable, it doesnt work, Can anyone explain why it doesnt work?

 

Also when I debug step by step, It works. Also without this loop it works but I can not see the changes.

 

Thanks

[enl 227]

Without volatile: The compiler recognises that the loop has no net effect and that the end result will be that b==0. Therefore, the loop is replaced with an assignment.

 

With volatile: the compiler presumes that, even if it can't see it, something, somewhere, may be looking at the value of b other than the code in the loop. Therefore, it a.) compiles the code exactly as written, with b.) no optimizations, and c.) not using a register for the variable (it will always be in RAM, with the associated time penalty for access)

[roadrunner84 466]

why use delay loops if you can use timers and low power mode to achieve a similar goal?

[tripwire 139]

why use delay loops if you can use timers and low power mode to achieve a similar goal?

 

Also, if you really need a busy wait it's better to use the __delay_cycles() intrinsic function. That way the compiler definitely won't optimise it out and the length of the delay is clearly defined.

[rockets4kids 204]

Also, this is not an optimization problem.  The optimizer is doing exactly what it is supposed to do here.