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//! Caches run-time feature detection so that it only needs to be computed
//! once.
#![allow(dead_code)] // not used on all platforms
use crate::sync::atomic::Ordering;
#[cfg(target_pointer_width = "64")]
use crate::sync::atomic::AtomicU64;
#[cfg(target_pointer_width = "32")]
use crate::sync::atomic::AtomicU32;
/// Sets the `bit` of `x`.
#[inline]
const fn set_bit(x: u64, bit: u32) -> u64 {
x | 1 << bit
}
/// Tests the `bit` of `x`.
#[inline]
const fn test_bit(x: u64, bit: u32) -> bool {
x & (1 << bit) != 0
}
/// Maximum number of features that can be cached.
const CACHE_CAPACITY: u32 = 63;
/// This type is used to initialize the cache
#[derive(Copy, Clone)]
pub(crate) struct Initializer(u64);
#[allow(clippy::use_self)]
impl Default for Initializer {
fn default() -> Self {
Initializer(0)
}
}
impl Initializer {
/// Tests the `bit` of the cache.
#[allow(dead_code)]
#[inline]
pub(crate) fn test(self, bit: u32) -> bool {
// FIXME: this way of making sure that the cache is large enough is
// brittle.
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) {
// FIXME: this way of making sure that the cache is large enough is
// brittle.
debug_assert!(
bit < CACHE_CAPACITY,
"too many features, time to increase the cache size!"
);
let v = self.0;
self.0 = set_bit(v, bit);
}
}
/// This global variable is a cache of the features supported by the CPU.
static CACHE: Cache = Cache::uninitialized();
/// Feature cache with capacity for `CACHE_CAPACITY` features.
///
/// Note: the last feature bit is used to represent an
/// uninitialized cache.
#[cfg(target_pointer_width = "64")]
struct Cache(AtomicU64);
#[cfg(target_pointer_width = "64")]
#[allow(clippy::use_self)]
impl Cache {
/// Creates an uninitialized cache.
#[allow(clippy::declare_interior_mutable_const)]
const fn uninitialized() -> Self {
Cache(AtomicU64::new(u64::max_value()))
}
/// Is the cache uninitialized?
#[inline]
pub(crate) fn is_uninitialized(&self) -> bool {
self.0.load(Ordering::Relaxed) == u64::max_value()
}
/// Is the `bit` in the cache set?
#[inline]
pub(crate) fn test(&self, bit: u32) -> bool {
test_bit(CACHE.0.load(Ordering::Relaxed), bit)
}
/// Initializes the cache.
#[inline]
pub(crate) fn initialize(&self, value: Initializer) {
self.0.store(value.0, Ordering::Relaxed);
}
}
/// Feature cache with capacity for `CACHE_CAPACITY` features.
///
/// Note: the last feature bit is used to represent an
/// uninitialized cache.
#[cfg(target_pointer_width = "32")]
struct Cache(AtomicU32, AtomicU32);
#[cfg(target_pointer_width = "32")]
impl Cache {
/// Creates an uninitialized cache.
const fn uninitialized() -> Self {
Cache(
AtomicU32::new(u32::max_value()),
AtomicU32::new(u32::max_value()),
)
}
/// Is the cache uninitialized?
#[inline]
pub(crate) fn is_uninitialized(&self) -> bool {
self.1.load(Ordering::Relaxed) == u32::max_value()
}
/// Is the `bit` in the cache set?
#[inline]
pub(crate) fn test(&self, bit: u32) -> bool {
if bit < 32 {
test_bit(CACHE.0.load(Ordering::Relaxed) as u64, bit)
} else {
test_bit(CACHE.1.load(Ordering::Relaxed) as u64, bit - 32)
}
}
/// Initializes the cache.
#[inline]
pub(crate) fn initialize(&self, value: Initializer) {
let lo: u32 = value.0 as u32;
let hi: u32 = (value.0 >> 32) as u32;
self.0.store(lo, Ordering::Relaxed);
self.1.store(hi, Ordering::Relaxed);
}
}
/// Tests the `bit` of the storage. If the storage has not been initialized,
/// initializes it with the result of `f()`.
///
/// 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.
#[inline]
pub(crate) fn test<F>(bit: u32, f: F) -> bool
where
F: FnOnce() -> Initializer,
{
if CACHE.is_uninitialized() {
CACHE.initialize(f());
}
CACHE.test(bit)
}
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