firefox-main/third_party/rust/core_detect/src
Name Description Size Coverage
arch -
cache.rs Caches run-time feature detection so that it only needs to be computed once. 4846 -
lib.rs This crate provides a `no_std` version of std's [`is_x86_feature_detected!`] macro. This is possible because x86 chips can just use the `cpuid` instruction to detect CPU features, whereas most other architectures require either reading files or querying the OS. # Usage ``` # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] # fn main() { if core_detect::is_x86_feature_detected!("ssse3") { println!("SSSE3 is available"); } # } # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] # fn main() {} ``` Note that like the [equivalent macro in `std`][stddetect], this will error on architectures other than x86/x86_64, so you should put the code behind a `#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]` check. [stddetect]: https://doc.rust-lang.org/nightly/std/macro.is_x86_feature_detected.html (In the future, this crate may provide another macro which returns false in these cases instead, and supports testing multiple features simultaneously). # Caveats The `cpuid` instruction doesn't exist on all x86 machines, it was added around 1994. (It's also not available on SGX, but this doesn't cause any issues since we can check that with `cfg(target_env = "sgx")`). If you run `cpuid` on a machine older than that, it causes an illegal instruction fault (SIGILL). Unfortunately, there's no good stable way to reliably determine if `cpuid` will fault in stable rust: A [`core::arch::x86::has_cpuid`](https://doc.rust-lang.org/nightly/core/arch/x86/fn.has_cpuid.html) function exists, but didn't stabilize with the rest of `core::arch::x86`, and the only way to implement it ourselves is with inline asm, which... is also still unstable. For what it's worth, it's actually pretty uncommon that we'd need to call `has_cpuid` on common rust targets, since we perform the following compile time checks: - We never have cpuid on `target_env = "sgx"` (as mentioned). - We always have cpuid on `target_arch = "x86_64"`. - And we always have cpuid if `target_feature = "sse"` (which covers the `i686-*` targets). Unfortunately, if none of those applies... we don't know if calling CPUID will crash the process. This library has a few ways it can handle this: 1. Cautiously (the default): In this mode, we conceptually swap `has_cpuid` out with a function that always returns false. That is: we never call it unless we're sure it wont crash the process. 2. Recklessly (`feature = "assume_has_cpuid"`): This is essentially the opposite of the last one — assume `has_cpuid` would have returned true, and call `cpuid` anyway. In practice, this should be fine. These machines are rare now (they're over 30 years old...), and pretty only are common through QEMU, and even then, usually after a misconfiguration. If you do happen to run the instruction, the process crashes, but in a controlled manner — Executing an illegal instruction to tringger a SIGILL is what `core::intrinsics::abort` does on x86, so it's not dangerous or anything. 3. Using unstable nightly features (`feature = "unstable_has_cpuid"`): This approach requires a nightly compiler, but has no other major downsides, besides the fact that the `has_cpuid` function could vanish at any time. Eventually, inline asm will stabilize and we can solve this problem more cleanly. 5525 -
macros.rs 3203 -
os -