[xmmrm]

Surprisingly [sincosf fusion] only happened with -Ofast and not with -O3.

As noted, -Ofast turns on -ffast-math which turns on -funsafe-math-optimizations which "enables optimizations that allow arbitrary reassociations and transformations with no accuracy guarantees."[0] In this case, sincosf by itself is probably more accurate.

[0] https://gcc.gnu.org/wiki/FloatingPointMath