The _fast functions (fadd_fast, fmul_fast,...) on std::intrinsics seem insufficient because they
- enable all the fast-math flags and don't allow specifying just one or a subset of the flags
- don't work on SIMD functions
- require rewriting code to call them to take advantage of the optimizations
Hence why I think a fast-math attribute is necessary (made a prototype here). I'll draft a pre-RFC if this feature is acceptable to be included in Rust.