* re-stabilize the AVX-512 features that were stabilized in Rust 1.27.0
https://github.com/rust-lang/stdarch/pull/739 added per-feature
stabilization of runtime CPU feature detection. In so doing, it
de-stabilized some detection features that had been stable since Rust
1.27.0, breaking some published crates (on nightly). This commit
re-stabilizes the subset of AVX-512 detection features that were
included in 1.27.0 (that is, the pre-Ice-Lake subset). Other instruction
sets (MMX in particular) remain de-stabilized, pending a decision about
whether should ever stabilize them.
See https://github.com/rust-lang/rust/issues/68905.
* add a comment explaining feature detection stability
* adjust stabilizations to match most recent proposal
https://github.com/rust-lang/rust/issues/68905#issuecomment-595376319
* Fix race condition in feature cache on 32 platforms
If we observe that the second word is initialized, we can't really
assume that the first is initialized as well. So check each word
separately.
* Use stronger atomic ordering
Better SeqCst than sorry!
* Use two caches on x64 for simplicity
These are needed for rustc to be able to correctly handle stability of constness of intrinsics. Without either `rustc_const_unstable` or `rustc_const_stable` an intrinsic is not const evaluable at all.
When building on aarch64, the following warning occurs.
```
warning: unused import: `mem::transmute`
--> crates/core_arch/src/arm/neon.rs:3:38
|
3 | use crate::{core_arch::simd_llvm::*, mem::transmute};
| ^^^^^^^^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default
```
We recently added a CDN in front of our CI mirrors as it's faster and
cheaper for us. This switches libc's CI to use it instead of accessing
the underlying bucket directly.
* Migrate CI to GitHub Actions
This involves less secret and user management than azure pipelines, has
more concurrency by default for repos, and in general has a bit more
modern syntax!
* Disable clippy on CI for now
Looks like it's got quite a few errors
