this post was submitted on 10 Apr 2025
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How to get GCC to optimize this inline function into one multiplcation (godbolt.org)
submitted 1 week 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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[–] [email protected] 2 points 1 week ago

The compiler can't replace every floating point division with a multiplication with the reciprocal. And it can't assume associativity. See also -ffast-math.

You can do this: return (double)steps * (2.0 * PI / 4096.0);