Not necessarily. The last I checked, even Intel's compilers default to 256-bit vectorization when compiling against AVX-512 targets. And of course, AMD implemen

Not necessarily. The last I checked, even Intel's compilers default to 256-bit vectorization when compiling against AVX-512 targets. And of course, AMD implemented AVX-512 via double-pumping of 256-bit SIMD operations (so AVX-512 isn't really faster than AVX-256 there). If memory bandwidth is a significant factor in your execution time, doubling the SIMD vector size is going to give you far less than the 100% theoretical speed boost (before taking into account thermal throttling, of course). Furthermore, the execution penalties due to things like remainder loops or vector misalignment (every misaligned 512-bit vector crosses cache lines) are comparatively more painful. The fancy new features ameliorate those downsides a lot and furthermore make vectorization profitable on a larger fraction of your code. And finally, if the limiting factor of your code's execution time is the number of FMA units you can ram data through, then at this point, you've already likely switched to using a GPU to do that instead of the CPU.