Thread: evaluation of const variables

Originally Posted by laserlight

And the point of all this is to...?

I'm making a program to read data from USB Devices (gamepads and joysticks).

Code:

while(!poll_interrupt)
{
 switch(dev_type)
 {
 case 1:
 fetch data from dev1;
 break
 case 2:
 fetch data from dev2;
 break
 etc..
 }
 translate information
 send_it_somewhere
}

I would like to turn the code as efficient as possible to improve sampling times.

The dev_type is an input parameter, but it doesn't change during runtime.

Originally Posted by frs

I would like to turn the code as efficient as possible to improve sampling times.

So you have this in mind?

Code:

switch (dev_type)
{
case 1:
    while (!poll_interrupt)
    {
        /* fetch data from dev1 */
        /* translate information */
        /* send it somewhere */
    }
    break;
case 2:
    while (!poll_interrupt)
    {
        /* fetch data from dev2 */
        /* translate information */
        /* send it somewhere */
    }
    break;
}

But you are concerned about the code duplication?

Personally, I would recommend that you test. Does any attempted optimisation really make a tangible difference to begin with, after compiler optimisations have been applied?

Originally Posted by frs

The dev_type is an input parameter, but it doesn't change during runtime.

As in it is actually a macro?

Originally Posted by laserlight

So you have this in mind?

Code:

switch (dev_type)
{
case 1:
    while (!poll_interrupt)
    {
        /* fetch data from dev1 */
        /* translate information */
        /* send it somewhere */
    }
    break;
case 2:
    while (!poll_interrupt)
    {
        /* fetch data from dev2 */
        /* translate information */
        /* send it somewhere */
    }
    break;
}

But you are concerned about the code duplication?

Yes yes. That's it.

Personally, I would recommend that you test. Does any attempted optimisation really make a tangible difference to begin with, after compiler optimisations have been applied?

That is what I was going to do, I never did compiler optimizations..
Probably I won't see any difference, but anyway is unnecessary to evaluate the switch statement in every poll cycle, and I can't see a solution without replicating the code over and over again.

As in it is actually a macro?

I know what a macro is but I didn't understand what you mean. sorry.

Originally Posted by frs

I know what a macro is but I didn't understand what you mean.

I am asking if dev_type is a preprocessor macro. More explicit, I am asking if it is input to the compiler (e.g., by using a -D or similiar option when invoking the compiler) rather than input to the program.