rustc: Prepare the `atomics` feature for wasm
This commit adds a few changes for atomic instructions on the
`wasm32-unknown-unknown` target. Atomic instructions are not yet stable in
WebAssembly itself but there are multiple implementations and LLVM has support
for the proposed instruction set, so let's work on exposing it!
Here there are a few inclusions:
* The `atomics` feature was whitelisted for LLVM, allowing code in Rust to
enable/disable/gate on this.
* The `singlethread` option is turned off for wasm when the `atomics` feature is
enabled. This means that by default wasm won't be lowering with atomics, but
when atomics are enabled globally we'll turn off single-threaded mode to
actually codegen atomics. This probably isn't what we'll want in the long term
but for now it should work.
* Finally the maximum atomic width is increased to 64 to reflect the current
wasm spec.
This commit adds a few changes for atomic instructions on the
`wasm32-unknown-unknown` target. Atomic instructions are not yet stable in
WebAssembly itself but there are multiple implementations and LLVM has support
for the proposed instruction set, so let's work on exposing it!
Here there are a few inclusions:
* The `atomics` feature was whitelisted for LLVM, allowing code in Rust to
enable/disable/gate on this.
* The `singlethread` option is turned off for wasm when the `atomics` feature is
enabled. This means that by default wasm won't be lowering with atomics, but
when atomics are enabled globally we'll turn off single-threaded mode to
actually codegen atomics. This probably isn't what we'll want in the long term
but for now it should work.
* Finally the maximum atomic width is increased to 64 to reflect the current
wasm spec.
This fixes a regression from #53031 where specifying `-C target-cpu=native` is
printing a lot of warnings from LLVM about `native` being an unknown CPU. It
turns out that `native` is indeed an unknown CPU and we have to perform a
mapping to an actual CPU name, but this mapping is only performed in one
location rather than all locations we inform LLVM about the target CPU.
This commit centralizes the mapping of `native` to LLVM's value of the native
CPU, ensuring that all locations we inform LLVM about the `target-cpu` it's
never `native`.
Closes#53322
- `dsp`: the subtarget supports the DSP (saturating arith. and such)
instructions
- `rclass`: target is a Cortex-R
Both features are useful to support ARM MCUs on `coresimd`.
Note: Cortex-R52 is the first Armv8-R with `neon` support
