[spider-mario]

> How is that different from any other compiler optimization?

When it can be single-handedly responsible for an 8× speedup, you might not want to rely as much on the compiler as in the case of smaller, cumulative optimisations.

> And what is the alternative? Handroll manual SIMD code for every possible architecture you may target?

Use a library like Highway? https://github.com/google/highway

[spider-mario]

Forgot to add: using Highway also means that you can detect and take advantage of e.g. AVX2 or AVX-512 in one binary that still works on CPUs without those instruction sets, whereas relying on autovectorisation would mean having to build with `-mavx2`, `-mavx512`, etc. which means that pretty much no one who relies on prebuilt binaries would get them.