The `rustc` probe done in our build scripts needs to pass `--target` to
get the correct configuration, which usually comes from the `TARGET`
environment variable. However, for targets specified via a `target.json`
file, `TARGET` gets set to the file name without an extension or path.
`rustc` will check a search path to attempt to locate the file, but this
is likely to fail since the directory where Cargo invokes build scripts
(and hence where those scripts invoke `rustc`) might not have any
relation to the JSON spec file.
Resolve this for now by leaving `f16` and `f128` disabled if the `rustc`
command fails. Result of the discussion at CARGO-14208 may eventually
provide a better solution.
A CI test is also added since custom JSON files are an edge case that
could fail in other ways. I verified this fails without the fix here.
The JSON file is the output for `thumbv7em-none-eabi`, just renamed so
`rustc` doesn't identify it.
Get performance closer to the glibc implementations by adding assembly
fma routines, with runtime feature detection so they are used even if
not compiled with `+fma` (as the distributed standard library is often
not). Glibc uses ifuncs, this implementation stores a function pointer
in an atomic.
Results of CPU flags are also cached in order to avoid repeating the
startup time in calls to different functions. The feature detection code
is a slightly simplified version of `std-detect`.
Musl sources were used as a reference [1].
Fixes: https://github.com/rust-lang/rust/issues/140452 once synced
[1]: c47ad25ea3/src/math/x32/fma.c
Update `run.sh` to start testing `libm`. Currently this is somewhat
inefficient because `builtins-test` gets run more than once on some
targets; this can be cleaned up later.
Distribute everything from `libm/` to better locations in the repo.
`libm/libm/*` has not moved yet to avoid Git seeing the move as an edit
to `Cargo.toml`.
Files that remain to be merged somehow are in `etc/libm`.
