[stephencanon]

There are basic integer operations in the FP/SIMD units on most CPUs, so there’s no generally need to “move back and forth” unless you need to branch on the result of a comparison, use a value as an address, or do some more specialized arithmetic.

[gpderetta]

On x86 there is sometimes (depending on the specific microarchitecture) an extra cycle additional latency when using an integer operation on a xmm register last used with a float operation.

I have seen it explained as the integer and foat ALUs's being physically distant and the forwarding network needing an extra cycle to transport the operands.

[stephencanon]

This is correct, but it’s happily pretty rare for it to matter in practice (because the domain bypass penalty is small and does not directly impact throughput, only latency).

[stephencanon]

(For that matter, though, most modern FP/SIMD units have a direct approximate-reciprocal instruction that is single-cycle throughput or better and much more accurate--generally around 10-12 bits, so there's no need for this sort of thing. See, e.g. FRECPE on ARM NEON and [V]RCPP[S/D] on x86.)