Auto merge of #141899 - Kobzol:stdarch-josh, r=Amanieu

Turn `stdarch` into a Josh subtree

In a similar vein as https://github.com/rust-lang/rust/pull/141229, this PR makes the `stdarch` repository a Josh subtree (it was previously a submodule). The initial commit of `stdarch` upon this is based is `5a7342fc16b208b1b16624e886937ed8509a6506`, which is the previous commit SHA of the `stdarch` submodule. The sync was performed according to https://hackmd.io/7pOuxnkdQDaL1Y1FQr65xg.

This was decided in https://github.com/rust-lang/stdarch/issues/1655.

Test pull PR on my fork: https://github.com/Kobzol/stdarch/pull/1
Test push PR on my fork: https://github.com/Kobzol/rust/pull/59

I plan to use the same Rust (miri-inspired) tooling that we use for `rustc-dev-guide` to enable pulls/pushes on stdarch.

Note that this repository currently doesn't have any stdarch-specific tests, so before that, the subtree should only be modified through this repository only when dealing with changes that contain "cyclical dependencies" between stdarch and rustc. The long term vision is to integrate stdarch into rust-lang/rust completely.

CC `@Amanieu`

try-job: aarch64-apple
try-job: aarch64-gnu
try-job: `x86_64-msvc-*`
try-job: x86_64-gnu
try-job: x86_64-gnu-aux

1 files changed, 223 insertions, 0 deletions

diff --git a/library/stdarch/crates/std_detect/src/detect/cache.rs b/library/stdarch/crates/std_detect/src/detect/cache.rs
new file mode 100644
index 00000000000..83bcedea612
--- /dev/null
+++ b/library/stdarch/crates/std_detect/src/detect/cache.rs
@@ -0,0 +1,223 @@
+//! Caches run-time feature detection so that it only needs to be computed
+//! once.
+
+#![allow(dead_code)] // not used on all platforms
+
+use core::sync::atomic::Ordering;
+
+use core::sync::atomic::AtomicUsize;
+
+/// Sets the `bit` of `x`.
+#[inline]
+const fn set_bit(x: u128, bit: u32) -> u128 {
+    x | 1 << bit
+}
+
+/// Tests the `bit` of `x`.
+#[inline]
+const fn test_bit(x: u128, bit: u32) -> bool {
+    x & (1 << bit) != 0
+}
+
+/// Unset the `bit of `x`.
+#[inline]
+const fn unset_bit(x: u128, bit: u32) -> u128 {
+    x & !(1 << bit)
+}
+
+/// Maximum number of features that can be cached.
+const CACHE_CAPACITY: u32 = 93;
+
+/// This type is used to initialize the cache
+// The derived `Default` implementation will initialize the field to zero,
+// which is what we want.
+#[derive(Copy, Clone, Default, PartialEq, Eq)]
+pub(crate) struct Initializer(u128);
+
+// NOTE: the `debug_assert!` would catch that we do not add more Features than
+// the one fitting our cache.
+impl Initializer {
+    /// Tests the `bit` of the cache.
+    #[inline]
+    pub(crate) fn test(self, bit: u32) -> bool {
+        debug_assert!(
+            bit < CACHE_CAPACITY,
+            "too many features, time to increase the cache size!"
+        );
+        test_bit(self.0, bit)
+    }
+
+    /// Sets the `bit` of the cache.
+    #[inline]
+    pub(crate) fn set(&mut self, bit: u32) {
+        debug_assert!(
+            bit < CACHE_CAPACITY,
+            "too many features, time to increase the cache size!"
+        );
+        let v = self.0;
+        self.0 = set_bit(v, bit);
+    }
+
+    /// Unsets the `bit` of the cache.
+    #[inline]
+    pub(crate) fn unset(&mut self, bit: u32) {
+        debug_assert!(
+            bit < CACHE_CAPACITY,
+            "too many features, time to increase the cache size!"
+        );
+        let v = self.0;
+        self.0 = unset_bit(v, bit);
+    }
+}
+
+/// This global variable is a cache of the features supported by the CPU.
+// Note: the third slot is only used in x86
+// Another Slot can be added if needed without any change to `Initializer`
+static CACHE: [Cache; 3] = [
+    Cache::uninitialized(),
+    Cache::uninitialized(),
+    Cache::uninitialized(),
+];
+
+/// Feature cache with capacity for `size_of::<usize>() * 8 - 1` features.
+///
+/// Note: 0 is used to represent an uninitialized cache, and (at least) the most
+/// significant bit is set on any cache which has been initialized.
+///
+/// Note: we use `Relaxed` atomic operations, because we are only interested in
+/// the effects of operations on a single memory location. That is, we only need
+/// "modification order", and not the full-blown "happens before".
+struct Cache(AtomicUsize);
+
+impl Cache {
+    const CAPACITY: u32 = (core::mem::size_of::<usize>() * 8 - 1) as u32;
+    const MASK: usize = (1 << Cache::CAPACITY) - 1;
+    const INITIALIZED_BIT: usize = 1usize << Cache::CAPACITY;
+
+    /// Creates an uninitialized cache.
+    #[allow(clippy::declare_interior_mutable_const)]
+    const fn uninitialized() -> Self {
+        Cache(AtomicUsize::new(0))
+    }
+
+    /// Is the `bit` in the cache set? Returns `None` if the cache has not been initialized.
+    #[inline]
+    pub(crate) fn test(&self, bit: u32) -> Option<bool> {
+        let cached = self.0.load(Ordering::Relaxed);
+        if cached == 0 {
+            None
+        } else {
+            Some(test_bit(cached as u128, bit))
+        }
+    }
+
+    /// Initializes the cache.
+    #[inline]
+    fn initialize(&self, value: usize) -> usize {
+        debug_assert_eq!((value & !Cache::MASK), 0);
+        self.0
+            .store(value | Cache::INITIALIZED_BIT, Ordering::Relaxed);
+        value
+    }
+}
+
+cfg_if::cfg_if! {
+    if #[cfg(feature = "std_detect_env_override")] {
+        #[inline]
+        fn disable_features(disable: &[u8], value: &mut Initializer) {
+            if let Ok(disable) = core::str::from_utf8(disable) {
+                for v in disable.split(" ") {
+                    let _ = super::Feature::from_str(v).map(|v| value.unset(v as u32));
+                }
+            }
+        }
+
+        #[inline]
+        fn initialize(mut value: Initializer) -> Initializer {
+            use core::ffi::CStr;
+            const RUST_STD_DETECT_UNSTABLE: &CStr = c"RUST_STD_DETECT_UNSTABLE";
+            cfg_if::cfg_if! {
+                if #[cfg(windows)] {
+                    use alloc::vec;
+                    #[link(name = "kernel32")]
+                    unsafe extern "system" {
+                        fn GetEnvironmentVariableA(name: *const u8, buffer: *mut u8, size: u32) -> u32;
+                    }
+                    let len = unsafe { GetEnvironmentVariableA(RUST_STD_DETECT_UNSTABLE.as_ptr().cast::<u8>(), core::ptr::null_mut(), 0) };
+                    if len > 0 {
+                        // +1 to include the null terminator.
+                        let mut env = vec![0; len as usize + 1];
+                        let len = unsafe { GetEnvironmentVariableA(RUST_STD_DETECT_UNSTABLE.as_ptr().cast::<u8>(), env.as_mut_ptr(), len + 1) };
+                        if len > 0 {
+                            disable_features(&env[..len as usize], &mut value);
+                        }
+                    }
+                } else {
+                    let env = unsafe {
+                        libc::getenv(RUST_STD_DETECT_UNSTABLE.as_ptr())
+                    };
+                    if !env.is_null() {
+                        let len = unsafe { libc::strlen(env) };
+                        let env = unsafe { core::slice::from_raw_parts(env as *const u8, len) };
+                        disable_features(env, &mut value);
+                    }
+                }
+            }
+            do_initialize(value);
+            value
+        }
+    } else {
+        #[inline]
+        fn initialize(value: Initializer) -> Initializer {
+            do_initialize(value);
+            value
+        }
+    }
+}
+
+#[inline]
+fn do_initialize(value: Initializer) {
+    CACHE[0].initialize((value.0) as usize & Cache::MASK);
+    CACHE[1].initialize((value.0 >> Cache::CAPACITY) as usize & Cache::MASK);
+    CACHE[2].initialize((value.0 >> (2 * Cache::CAPACITY)) as usize & Cache::MASK);
+}
+
+// We only have to detect features once, and it's fairly costly, so hint to LLVM
+// that it should assume that cache hits are more common than misses (which is
+// the point of caching). It's possibly unfortunate that this function needs to
+// reach across modules like this to call `os::detect_features`, but it produces
+// the best code out of several attempted variants.
+//
+// The `Initializer` that the cache was initialized with is returned, so that
+// the caller can call `test()` on it without having to load the value from the
+// cache again.
+#[cold]
+fn detect_and_initialize() -> Initializer {
+    initialize(super::os::detect_features())
+}
+
+/// Tests the `bit` of the storage. If the storage has not been initialized,
+/// initializes it with the result of `os::detect_features()`.
+///
+/// On its first invocation, it detects the CPU features and caches them in the
+/// `CACHE` global variable as an `AtomicU64`.
+///
+/// It uses the `Feature` variant to index into this variable as a bitset. If
+/// the bit is set, the feature is enabled, and otherwise it is disabled.
+///
+/// If the feature `std_detect_env_override` is enabled looks for the env
+/// variable `RUST_STD_DETECT_UNSTABLE` and uses its content to disable
+/// Features that would had been otherwise detected.
+#[inline]
+pub(crate) fn test(bit: u32) -> bool {
+    let (relative_bit, idx) = if bit < Cache::CAPACITY {
+        (bit, 0)
+    } else if bit < 2 * Cache::CAPACITY {
+        (bit - Cache::CAPACITY, 1)
+    } else {
+        (bit - 2 * Cache::CAPACITY, 2)
+    };
+    CACHE[idx]
+        .test(relative_bit)
+        .unwrap_or_else(|| detect_and_initialize().test(bit))
+}