Question

I currently have code that looks like

while (very_long_loop) {
    ...
    y1 = getSomeValue();
    ...
    x1 = y1*cos(PI/2);
    x2 = y2*cos(SOME_CONSTANT);
    ...
    outputValues(x1, x2, ...);
}

the obvious optimization would be to compute the cosines ahead-of-time. I could do this by filling an array with the values but I was wondering would it be possible to make the compiler compute these at compile-time?

Edit: I know that C doesn’t have compile-time evaluation but I was hoping there would had been some weird and ugly way to do this with macros.

Answer 1

If you’re lucky, you won’t have to do anything: Modern compilers do constant propagation for functions in the same translation unit and intrinsic functions (which most likely will include the math functions).

Look at the assembly to check if that’s the case for your compiler and increase the optimization levels if necessary.