avx – puzzling declarations in simd code of fftw

In head files of simd of fftw3, such as simd-generic128.h and simd-generic256.h, STM2, STN2, STM4, STN4 are decleared. What operation do they do in fftw?

I found that some of them lack implementations, such as STN2 and STM4 in simd-generic128.h, and STM2 and STM4 in simd-generic256.h. Does the lake of them cause problems? When I implement simd on a new architecture, I need to implement which of them?

#define STM2 ST
#define STN2(x, v0, v1, ovs) /* nop */
static inline void STN4(R *x, V v0, V v1, V v2, V v3, INT ovs)
{
  ...
}
#define STM4(x, v, ovs, aligned_like) /* no-op */