In my experience, compiling C with -ffast-math will tremendously improve floating point autovectorization and optimizations to SIMD (C vector extensions, which are similar to Rust std::simd) code in general.

This obviously has a lot of caveats, and should only be enabled on a per function or per file basis.

Unfortunately Rust does not currently have options for adjusting per-function compiler optimization parameters. This is possible in some C compilers using function attributes.