Question

I have the following looking code in VC++:

for (int i = (a - 1) * b; i < a * b && i < someObject->someFunction(); i++)
{
    // ...
}

As far as I know compilers optimize all these arithmetic operations and they won’t be executed on each loop, but I’m not sure if they can tell that the function above also returns the same value each time and it doesn’t need to be called each time.

Is it a better practice to save all calculations into variables, or just rely on compiler optimizations to have a more readable code?

int start = (a - 1) * b;
int expra = a * b;
int exprb = someObject->someFunction();
for (int i = startl i < expra && i < exprb; i++)
{
    // ...
}

Answer 1

If a function resides within the same compilation unit as its caller, the compiler can often deduce some facts about it – e.g. that its output might not change for subsequent calls. In general, however, that is not the case.

In your example, assigning variables for these simple arithmetic expressions does not really change anything with regards to the produced object code and, in my opinion, makes the code less readable. Unless you have a bunch of long expressions that cannot reasonably be put within a line or two, you should avoid using temporary variables – if for no other reason, then just to reduce namespace pollution.

Using temporary variables implies a significant management overhead for the programmer, in order to keep them separate and avoid unintended side-effects. It also makes reusing code snippets harder.

On the other hand, assigning the result of the function to a variable can help the compiler optimise your code better by explicitly avoiding multiple function calls.

Personally, I would go with this:

int expr = someObject->someFunction();
for (int i = (a - 1) * b; i < a * b && i < expr; i++)
{
    // ...
}