Can C++ compilers optimize “if” statements inside “for” loops?

Question

Consider an example like this:

if (flag)
  for (condition)
    do_something();
else
  for (condition)
    do_something_else();

If fla

Answer 1

I'm sure if the compiler can determine that the flag will remain constant, it can do some shufflling:

const bool flag = /* ... */;
for (..;..;..;)
{
    if (flag)
    {
        // ...
    }
    else
    {
        // ...
    }
}

If the flag is not const, the compiler cannot necessarily optimize the loop, because it can't be sure flag won't change. It can if it does static analysis, but not all compilers do, I think. const is the sure-fire way of telling the compiler the flag won't change, after that it's up to the compiler.

As usual, profile and find out if it's really a problem.