Is Loop Hoisting still a valid manual optimization for C code?

Question

Using the latest gcc compiler, do I still have to think about these types of manual loop optimizations, or will the compiler take care of them for me well enough?

Answer 1

A good rule of thumb is usually that the compiler performs the optimizations it is able to. Does the optimization require any knowledge about your code that isn't immediately obvious to the compiler? Then it is hard for the compiler to apply the optimization automatically, and you may want to do it yourself

In most cases, lop hoisting is a fully automatic process requiring no high-level knowledge of the code -- just a lot of lifetime and dependency analysis, which is what the compiler excels at in the first place.

It is possible to write code where the compiler is unable to determine whether something can be hoisted out safely though -- and in those cases, you may want to do it yourself, as it is a very efficient optimization.

As an example, take the snippet posted by Steve Jessop:

for (int i = 0; i < 10; ++i) {
    std::cout << "The number of primes less than 1 billion is: ";
    std::cout << countPrimesLessThan(1000*1000*1000);
    std::cout << std::endl;
}

Is it safe to hoist out the call to countPrimesLessThan? That depends on how and where the function is defined. What if it has side effects? It may make an important difference whether it is called once or ten times, as well as when it is called. If we don't know how the function is defined, we can't move it outside the loop. And the same is true if the compiler is to perform the optimization.

Is the function definition visible to the compiler? And is the function short enough that we can trust the compiler to inline it, or at least analyze the function for side effects? If so, then yes, it will hoist it outside the loop.

If the definition is not visible, or if the function is very big and complicated, then the compiler will probably assume that the function call can not be moved safely, and then it won't automatically hoist it out.