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How to get GCC to optimize this inline function into one multiplcation (godbolt.org)
submitted 6 days ago by [email protected] to c/cpp
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I am trying to calculate a motor position from an encoder, however that does not really matter. I just checked godbolt.org to see if my inline function compiled into a single multiplication and was pretty disappointed that it didn't. I used -O3 for maximum optimization.

Does somebody know why it does not compile into a single function, or what I had to change to achieve that? You can find my code here: https://godbolt.org/z/qT9srfPT1 .

I know that it does not really matter, but I am curious.

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[โ€“] FizzyOrange 13 points 6 days ago (1 children)

Use -ffast-math... if you dare. Or just use

inline double position_from_steps(int steps) {
  return (2.0 * PI / 4096.0) * steps;
};

The reason it doesn't optimise it into one multiplication is because that isn't actually the same calculation. Floating point numbers aren't real numbers, so it isn't true that (a * b) / c == (a / c) * b. Since the "optimisation" would actually change the semantics, compilers don't do it by default.

-ffast-math says "hey I don't care about reliable semantics; just do whatever to make it fast". But it also does a load of stuff that you may find surprising. so you really shouldn't use it. Much better just to reorganise your code.

You can also use -funsafe-math-optimizations which is the more specific subset of -ffast-math that does this particular optimisation. See https://gcc.gnu.org/wiki/FloatingPointMath

Also I would question if it matters. This micro-optimisation will make zero difference on x86. On a microcontroller (I assume what you're actually using if you're encoding motor positions) it might matter a bit more but double check your microcontroller even has an FPU. Lots don't. Even if it does, doing it in integer space will probably be faster (though not necessarily).

[โ€“] [email protected] 2 points 6 days ago* (last edited 6 days ago)

Very interesting comment. Thanks.