alt Hacker NewsThinking about speed like this used to be necessary in C and C++ but these days you should feel free to write the most legible thing (Horner's form) and let the compiler find the optimal code for it (probably similar to Horner's form but broken up to have a shallower dependency chain).
But if you're writing in an interpreted language that doesn't have a good JIT, or for a platform with a custom compiler, it might be worth hand-tweaking expressions with an eye towards performance and precision.
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The compiler is often not allowed to rearrange such operations due to a change in intermediate results. So one would have to activate something like fastmath for this code, but that’s probably not desired for all code, so one has to introduce a small library, and so on. Debug builds may be using different compilation flags, and suddenly performance can become terrible while debugging. Performance can also tank because a new compiler version optimizes differently, etc. So in general I don’t think this advice is true.
Probably for ints unconditionally. For floats in Sesse__'s example without `-ffast-math`, I count 10 muls, 2 muladds, 1 add. With `-ffast-math`, 1 mul, 3 muladds. <https://godbolt.org/z/dPrbfjzEx>
You should never assume the compiler is allowed to reorder floating-point computations like it does with integers. Integer math is exact, within its domain. Floating-point math is not. The IEEE-754 standard knows this, and the compiler knows this.