alt Hacker News> They specifies a constant SIMD width so it's non-portable.
This is incorrect, you can use vectors wider than native SIMD width and the compiler will break them down to register size of the target cpu.
In fact it's sometimes better to used wider than native width, in some applications I see 20% better throughput with f32x16 (512 bits) on an AVX2 CPU (256 bits). It is kinda like loop unrolling it.
Except you can't use this in actual code, because either, as is the case in this example with f32x32, you run out of registers and spill all over the place. Or you aren't using your full vector register or could've gotten better performance by "unrolling" more often for the larger vectors.
If you use f32x16 (the avx-512 wisth), SSE now effectively has 4 registers to work with and will spill when doing anything beyond the most simple stuff.
The default should imo be relative to the native register width, so you can do 1x, 2x or sometimes 4x the native width, depensing on your register preasure.