Question

If I write a line of code for a math operation, such as:

x = 109.0f*768.0f/320.0f;

Is the result (261.6f) computed at compile time or run time? In other words, does Xcode’s optimization recognize that the result of a hard-coded math operation will always be the same, and thus can be pre-computed while compiling?

Answer 1

It is computed at compile time, at least using Xcode targetting iOS. This function:

float test() {
    float x = 109.0f*768.0f/320.0f;
    return x;
}

compiles to these three instructions:

movw    r0, #52429
movt    r0, #17282
bx  lr

Computing the value at compile-time isn’t required by the C standard. In fact, if you set the FENV_ACCESS pragma, there are cases where it’s forbidden from computing it at compile-time. Turning on FENV_ACCESS didn’t affect the generated instructions in this test case.