018616e78b ("Always have math functions but with `weak` linking
attribute if we can") made all math symbols available on platforms that
support weak linkage. This caused some unexpected regressions, however,
because our less accurate and sometimes slow routines were being
selected over the system `libm`, which also tends to be weak [1]. Thus,
0fab77e8d7 ("Don't include `math` for `unix` and `wasi` targets") was
applied to undo these changes on many platforms.
Now that some improvements have been made to `libm`, add back a subset
of these functions:
* cbrt
* ceil
* copysign
* fabs
* fdim
* floor
* fma
* fmax
* fmaximum
* fmin
* fminimum
* fmod
* rint
* round
* roundeven
* sqrt
* trunc
This list includes only functions that produce exact results (verified
with exhaustive / extensive tests, and also required by IEEE in most
cases), and for which benchmarks indicate performance similar to or
better than Musl's soft float math routines [^1]. All except `cbrt` also
have `f16` and `f128` implementations. Once more routines meet these
criteria, we can move them from platform-specific availability to always
available.
Once this change makes it to rust-lang/rust, we will also be able to
move the relevant functions from `std` to `core`.
[^1]: We still rely on the backend to provide optimized assmebly
routines when available.
[1]: https://github.com/rust-lang/rust/issues/128386
55bbda8ff8