These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.99 s (backtrack: 0/20).

[ebuild  N     ] sci-libs/fftw-3.3.10-r1:3.0/3::gentoo  USE="openmp threads -doc -fortran -mpi -test (-zbus)" CPU_FLAGS_X86="avx avx2 fma3 sse sse2 -fma4" 4047 KiB
[ebuild  N     ] sci-libs/nfft-3.5.3-r1::gentoo  USE="openmp -doc -test" 5512 KiB

Total: 2 packages (2 new), Size of downloads: 9559 KiB