// the one fitting our cache.
impl Initializer {
/// Tests the `bit` of the cache.
- #[allow(dead_code)]
#[inline]
pub(crate) fn test(self, bit: u32) -> bool {
debug_assert!(
// Note: on x64, we only use the first slot
static CACHE: [Cache; 2] = [Cache::uninitialized(), Cache::uninitialized()];
-/// Feature cache with capacity for `usize::MAX - 1` features.
+/// Feature cache with capacity for `size_of::<usize::MAX>() * 8 - 1` features.
///
-/// Note: the last feature bit is used to represent an
-/// uninitialized cache.
+/// 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 can 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". However,
-/// we use `SeqCst` just to be on the safe side.
+/// 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(usize::MAX))
- }
- /// Is the cache uninitialized?
- #[inline]
- pub(crate) fn is_uninitialized(&self) -> bool {
- self.0.load(Ordering::SeqCst) == usize::MAX
+ Cache(AtomicUsize::new(0))
}
- /// Is the `bit` in the cache set?
+ /// Is the `bit` in the cache set? Returns `None` if the cache has not been initialized.
#[inline]
- pub(crate) fn test(&self, bit: u32) -> bool {
- test_bit(self.0.load(Ordering::SeqCst) as u64, bit)
+ 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 u64, bit))
+ }
}
/// Initializes the cache.
#[inline]
- fn initialize(&self, value: usize) {
- self.0.store(value, Ordering::SeqCst);
+ 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(never)]
- fn initialize(mut value: Initializer) {
+ #[inline]
+ fn initialize(mut value: Initializer) -> Initializer {
if let Ok(disable) = crate::env::var("RUST_STD_DETECT_UNSTABLE") {
for v in disable.split(" ") {
let _ = super::Feature::from_str(v).map(|v| value.unset(v as u32));
}
}
do_initialize(value);
+ value
}
} else {
#[inline]
- fn initialize(value: Initializer) {
+ fn initialize(value: Initializer) -> Initializer {
do_initialize(value);
+ value
}
}
}
CACHE[1].initialize((value.0 >> 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 `f()`.
+/// 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`.
/// variable `RUST_STD_DETECT_UNSTABLE` and uses its its content to disable
/// Features that would had been otherwise detected.
#[inline]
-pub(crate) fn test<F>(bit: u32, f: F) -> bool
-where
- F: FnOnce() -> Initializer,
-{
- let (bit, idx) = if bit < Cache::CAPACITY {
+pub(crate) fn test(bit: u32) -> bool {
+ let (relative_bit, idx) = if bit < Cache::CAPACITY {
(bit, 0)
} else {
(bit - Cache::CAPACITY, 1)
};
-
- if CACHE[idx].is_uninitialized() {
- initialize(f())
- }
- CACHE[idx].test(bit)
+ CACHE[idx]
+ .test(relative_bit)
+ .unwrap_or_else(|| detect_and_initialize().test(bit))
}
projects / rustc.git / blobdiff