Cost of pushing variable onto stack

**C3Pnuts** · 04-09-2007

Does anyone have a good reference for what it costs to use a local variable versus using the return value of a method call in C++?

I hate using a string of method calls together in an equation, it makes it so unreadable. Yet if I use local variables to hold the returns of these calls, then use those variables in the equation, I draw the ire of certain guys in our department as being inefficient. Is it really that inefficient to push a few local variables on the stack simply to make a bit of code easier to read and understand?

**laserlight** · 04-09-2007

You might want to read about return value optimisations.

**Daved** · 04-09-2007

It depends on your situation, but small inefficiencies are rarely important compared to making code clear and readable.

If your department guys have profiled your application and found that the code you are modifying takes up the bulk of the time in your program, then maybe they have a point and you should use the most efficient method possible. Also, if you are being particular and picking one version over another simply based on personal preference and neither version is much clearer than the other, then again I'd say choosing the more efficient version would be better.

Otherwise, I'd say stick to the clearer version. It'll save you and them more time on code maintenance in the future.

Can you post an example of what youare talking about?

**Salem** · 04-09-2007

The cost of

Code:

int foo ( void ) {
  return f1() + f2();
}

Compared to

Code:

int foo ( void ) {
  int a = f1();
  int b = f2();
  return a + b;
}

is negligible.

For a start, the number of local variables doesn't matter. The compiler calculates the size of all of them, then in a single instruction generates the stack frame for the entire function.

Second, a good compiler will simply optimise out redundant local variables which are there purely for readability.

I agree with Daved, clarity is worth a lot in the long run, if all you're gaining is a few imaginary microseconds based on opinion rather than measurement.

**C3Pnuts** · 04-09-2007

Originally Posted by Daved

Can you post an example of what youare talking about?

This is a rather mundane instance but shows what I'm talking about.
Some variable names have been changed to protect the innocent!

This, to me, is much more clear...

Code:

float zeroOrderTerm   = calInfo.getCoeff(mDir, CalibrationInfo::ZERO_ORDER_COEFF);
float firstOrderTerm    = calInfo.getCoeff(mDir, CalibrationInfo::FIRST_ORDER_COEFF) * angleOfIncidence;
float secondOrderTerm = calInfo.getCoeff(mDir, CalibrationInfo::SECOND_ORDER_COEFF) * angleOfIncidence * angleOfIncidence;

float ref =  zeroOrderTerm + firstOrderTerm + secondOrderTerm;

than this...

Code:

float ref = calInfo.getCoeff(mDir, CalibrationInfo::ZERO_ORDER_COEFF) + 
                       (calInfo.getCoeff(mDir, CalibrationInfo::FIRST_ORDER_COEFF) * angleOfIncidence) + 
                       (calInfo.getCoeff(mDir, CalibrationInfo::SECOND_ORDER_COEFF) * angleOfIncidence * angleOfIncidence);

Plus, in debugging, it allows you to view the intermediate values of the calculation step by step.

What I would really like to understand is what are the costs (overhead) of using stack variables. There's got to be some kind of document that someone has authored on the cost of pushing local variables on the stack vs. simply using a retrieved value directly.

**Daved** · 04-09-2007

You could also use:

Code:

float ref = calInfo.getCoeff(mDir, CalibrationInfo::ZERO_ORDER_COEFF);
ref += calInfo.getCoeff(mDir, CalibrationInfo::FIRST_ORDER_COEFF) * angleOfIncidence;
ref += calInfo.getCoeff(mDir, CalibrationInfo::SECOND_ORDER_COEFF) * angleOfIncidence * angleOfIncidence;

Regardless, the difference between the efficiencies of the two versions will not be significant, so use whichever is clearer to you.

I did some timing tests for fun, but didn't spend much time on it. There was no difference in my tests, and I doubt there would be a difference if you put them through something rigorous.

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