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//! Prevent false sharing by padding and aligning to the length of a cache line.
//!
//! In concurrent programming, sometimes it is desirable to make sure commonly accessed shared data
//! is not all placed into the same cache line. Updating an atomic value invalidates the whole cache
//! line it belongs to, which makes the next access to the same cache line slower for other CPU
//! cores. Use [`CachePadded`] to ensure updating one piece of data doesn't invalidate other cached
//! data.
//!
//! # Size and alignment
//!
//! Cache lines are assumed to be N bytes long, depending on the architecture:
//!
//! * On x86-64, aarch64, and powerpc64, N = 128.
//! * On arm, mips, mips64, and riscv64, N = 32.
//! * On s390x, N = 256.
//!
//! Note that N is just a reasonable guess and is not guaranteed to match the actual cache line
//! length of the machine the program is running on.
//!
//! The size of `CachePadded<T>` is the smallest multiple of N bytes large enough to accommodate
//! a value of type `T`.
//!
//! The alignment of `CachePadded<T>` is the maximum of N bytes and the alignment of `T`.
//!
//! # Examples
//!
//! Alignment and padding:
//!
//! ```
//! use cache_padded::CachePadded;
//!
//! let array = [CachePadded::new(1i8), CachePadded::new(2i8)];
//! let addr1 = &*array[0] as *const i8 as usize;
//! let addr2 = &*array[1] as *const i8 as usize;
//!
//! assert!(addr2 - addr1 >= 64);
//! assert_eq!(addr1 % 64, 0);
//! assert_eq!(addr2 % 64, 0);
//! ```
//!
//! When building a concurrent queue with a head and a tail index, it is wise to place indices in
//! different cache lines so that concurrent threads pushing and popping elements don't invalidate
//! each other's cache lines:
//!
//! ```
//! use cache_padded::CachePadded;
//! use std::sync::atomic::AtomicUsize;
//!
//! struct Queue<T> {
//! head: CachePadded<AtomicUsize>,
//! tail: CachePadded<AtomicUsize>,
//! buffer: *mut T,
//! }
//! ```
#![no_std]
#![forbid(unsafe_code)]
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
use core::fmt;
use core::ops::{Deref, DerefMut};
/// Pads and aligns data to the length of a cache line.
// Starting from Intel's Sandy Bridge, spatial prefetcher is now pulling pairs of 64-byte cache
// lines at a time, so we have to align to 128 bytes rather than 64.
//
// Sources:
//
// ARM's big.LITTLE architecture has asymmetric cores and "big" cores have 128-byte cache line size.
//
// Sources:
//
// powerpc64 has 128-byte cache line size.
//
// Sources:
#[cfg_attr(
any(
target_arch = "x86_64",
target_arch = "aarch64",
target_arch = "powerpc64",
),
repr(align(128))
)]
// arm, mips, mips64, and riscv64 have 32-byte cache line size.
//
// Sources:
#[cfg_attr(
any(
target_arch = "arm",
target_arch = "mips",
target_arch = "mips64",
target_arch = "riscv64",
),
repr(align(32))
)]
// s390x has 256-byte cache line size.
//
// Sources:
#[cfg_attr(target_arch = "s390x", repr(align(256)))]
// x86 and wasm have 64-byte cache line size.
//
// Sources:
//
// All others are assumed to have 64-byte cache line size.
#[cfg_attr(
not(any(
target_arch = "x86_64",
target_arch = "aarch64",
target_arch = "powerpc64",
target_arch = "arm",
target_arch = "mips",
target_arch = "mips64",
target_arch = "riscv64",
target_arch = "s390x",
)),
repr(align(64))
)]
#[derive(Clone, Copy, Default, Hash, PartialEq, Eq)]
pub struct CachePadded<T>(T);
impl<T> CachePadded<T> {
/// Pads and aligns a piece of data to the length of a cache line.
///
/// # Examples
///
/// ```
/// use cache_padded::CachePadded;
///
/// let padded = CachePadded::new(1);
/// ```
pub const fn new(t: T) -> CachePadded<T> {
CachePadded(t)
}
/// Returns the inner data.
///
/// # Examples
///
/// ```
/// use cache_padded::CachePadded;
///
/// let padded = CachePadded::new(7);
/// let data = padded.into_inner();
/// assert_eq!(data, 7);
/// ```
pub fn into_inner(self) -> T {
self.0
}
}
impl<T> Deref for CachePadded<T> {
type Target = T;
fn deref(&self) -> &T {
&self.0
}
}
impl<T> DerefMut for CachePadded<T> {
fn deref_mut(&mut self) -> &mut T {
&mut self.0
}
}
impl<T: fmt::Debug> fmt::Debug for CachePadded<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_tuple("CachePadded").field(&self.0).finish()
}
}
impl<T> From<T> for CachePadded<T> {
fn from(t: T) -> Self {
CachePadded::new(t)
}
}