350 lines
13 KiB
Rust
350 lines
13 KiB
Rust
#![stable(feature = "core_hint", since = "1.27.0")]
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//! Hints to compiler that affects how code should be emitted or optimized.
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//! Hints may be compile time or runtime.
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use crate::intrinsics;
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/// Informs the compiler that the site which is calling this function is not
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/// reachable, possibly enabling further optimizations.
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///
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/// # Safety
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///
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/// Reaching this function is *Undefined Behavior*.
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///
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/// As the compiler assumes that all forms of Undefined Behavior can never
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/// happen, it will eliminate all branches in the surrounding code that it can
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/// determine will invariably lead to a call to `unreachable_unchecked()`.
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///
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/// If the assumptions embedded in using this function turn out to be wrong -
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/// that is, if the site which is calling `unreachable_unchecked()` is actually
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/// reachable at runtime - the compiler may have generated nonsensical machine
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/// instructions for this situation, including in seemingly unrelated code,
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/// causing difficult-to-debug problems.
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///
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/// Use this function sparingly. Consider using the [`unreachable!`] macro,
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/// which may prevent some optimizations but will safely panic in case it is
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/// actually reached at runtime. Benchmark your code to find out if using
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/// `unreachable_unchecked()` comes with a performance benefit.
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///
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/// # Examples
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///
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/// `unreachable_unchecked()` can be used in situations where the compiler
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/// can't prove invariants that were previously established. Such situations
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/// have a higher chance of occuring if those invariants are upheld by
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/// external code that the compiler can't analyze.
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/// ```
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/// fn prepare_inputs(divisors: &mut Vec<u32>) {
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/// // Note to future-self when making changes: The invariant established
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/// // here is NOT checked in `do_computation()`; if this changes, you HAVE
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/// // to change `do_computation()`.
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/// divisors.retain(|divisor| *divisor != 0)
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/// }
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///
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/// /// # Safety
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/// /// All elements of `divisor` must be non-zero.
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/// unsafe fn do_computation(i: u32, divisors: &[u32]) -> u32 {
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/// divisors.iter().fold(i, |acc, divisor| {
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/// // Convince the compiler that a division by zero can't happen here
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/// // and a check is not needed below.
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/// if *divisor == 0 {
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/// // Safety: `divisor` can't be zero because of `prepare_inputs`,
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/// // but the compiler does not know about this. We *promise*
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/// // that we always call `prepare_inputs`.
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/// std::hint::unreachable_unchecked()
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/// }
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/// // The compiler would normally introduce a check here that prevents
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/// // a division by zero. However, if `divisor` was zero, the branch
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/// // above would reach what we explicitly marked as unreachable.
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/// // The compiler concludes that `divisor` can't be zero at this point
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/// // and removes the - now proven useless - check.
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/// acc / divisor
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/// })
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/// }
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///
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/// let mut divisors = vec![2, 0, 4];
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/// prepare_inputs(&mut divisors);
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/// let result = unsafe {
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/// // Safety: prepare_inputs() guarantees that divisors is non-zero
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/// do_computation(100, &divisors)
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/// };
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/// assert_eq!(result, 12);
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///
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/// ```
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///
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/// While using `unreachable_unchecked()` is perfectly sound in the following
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/// example, the compiler is able to prove that a division by zero is not
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/// possible. Benchmarking reveals that `unreachable_unchecked()` provides
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/// no benefit over using [`unreachable!`], while the latter does not introduce
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/// the possibility of Undefined Behavior.
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///
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/// ```
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/// fn div_1(a: u32, b: u32) -> u32 {
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/// use std::hint::unreachable_unchecked;
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///
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/// // `b.saturating_add(1)` is always positive (not zero),
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/// // hence `checked_div` will never return `None`.
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/// // Therefore, the else branch is unreachable.
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/// a.checked_div(b.saturating_add(1))
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/// .unwrap_or_else(|| unsafe { unreachable_unchecked() })
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/// }
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///
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/// assert_eq!(div_1(7, 0), 7);
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/// assert_eq!(div_1(9, 1), 4);
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/// assert_eq!(div_1(11, u32::MAX), 0);
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/// ```
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#[inline]
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#[stable(feature = "unreachable", since = "1.27.0")]
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#[rustc_const_stable(feature = "const_unreachable_unchecked", since = "1.57.0")]
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pub const unsafe fn unreachable_unchecked() -> ! {
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// SAFETY: the safety contract for `intrinsics::unreachable` must
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// be upheld by the caller.
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unsafe { intrinsics::unreachable() }
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}
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/// Emits a machine instruction to signal the processor that it is running in
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/// a busy-wait spin-loop ("spin lock").
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///
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/// Upon receiving the spin-loop signal the processor can optimize its behavior by,
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/// for example, saving power or switching hyper-threads.
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///
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/// This function is different from [`thread::yield_now`] which directly
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/// yields to the system's scheduler, whereas `spin_loop` does not interact
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/// with the operating system.
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///
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/// A common use case for `spin_loop` is implementing bounded optimistic
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/// spinning in a CAS loop in synchronization primitives. To avoid problems
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/// like priority inversion, it is strongly recommended that the spin loop is
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/// terminated after a finite amount of iterations and an appropriate blocking
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/// syscall is made.
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///
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/// **Note**: On platforms that do not support receiving spin-loop hints this
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/// function does not do anything at all.
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///
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/// # Examples
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///
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/// ```
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/// use std::sync::atomic::{AtomicBool, Ordering};
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/// use std::sync::Arc;
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/// use std::{hint, thread};
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///
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/// // A shared atomic value that threads will use to coordinate
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/// let live = Arc::new(AtomicBool::new(false));
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///
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/// // In a background thread we'll eventually set the value
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/// let bg_work = {
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/// let live = live.clone();
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/// thread::spawn(move || {
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/// // Do some work, then make the value live
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/// do_some_work();
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/// live.store(true, Ordering::Release);
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/// })
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/// };
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///
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/// // Back on our current thread, we wait for the value to be set
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/// while !live.load(Ordering::Acquire) {
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/// // The spin loop is a hint to the CPU that we're waiting, but probably
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/// // not for very long
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/// hint::spin_loop();
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/// }
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///
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/// // The value is now set
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/// # fn do_some_work() {}
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/// do_some_work();
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/// bg_work.join()?;
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/// # Ok::<(), Box<dyn core::any::Any + Send + 'static>>(())
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/// ```
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///
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/// [`thread::yield_now`]: ../../std/thread/fn.yield_now.html
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#[inline]
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#[stable(feature = "renamed_spin_loop", since = "1.49.0")]
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pub fn spin_loop() {
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#[cfg(all(any(target_arch = "x86", target_arch = "x86_64"), target_feature = "sse2"))]
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{
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#[cfg(target_arch = "x86")]
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{
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// SAFETY: the `cfg` attr ensures that we only execute this on x86 targets.
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unsafe { crate::arch::x86::_mm_pause() };
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}
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#[cfg(target_arch = "x86_64")]
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{
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// SAFETY: the `cfg` attr ensures that we only execute this on x86_64 targets.
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unsafe { crate::arch::x86_64::_mm_pause() };
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}
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}
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// RISC-V platform spin loop hint implementation
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{
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// RISC-V RV32 and RV64 share the same PAUSE instruction, but they are located in different
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// modules in `core::arch`.
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// In this case, here we call `pause` function in each core arch module.
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#[cfg(target_arch = "riscv32")]
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{
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crate::arch::riscv32::pause();
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}
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#[cfg(target_arch = "riscv64")]
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{
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crate::arch::riscv64::pause();
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}
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}
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#[cfg(any(target_arch = "aarch64", all(target_arch = "arm", target_feature = "v6")))]
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{
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#[cfg(target_arch = "aarch64")]
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{
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// SAFETY: the `cfg` attr ensures that we only execute this on aarch64 targets.
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unsafe { crate::arch::aarch64::__isb(crate::arch::aarch64::SY) };
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}
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#[cfg(target_arch = "arm")]
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{
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// SAFETY: the `cfg` attr ensures that we only execute this on arm targets
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// with support for the v6 feature.
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unsafe { crate::arch::arm::__yield() };
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}
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}
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}
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/// An identity function that *__hints__* to the compiler to be maximally pessimistic about what
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/// `black_box` could do.
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///
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/// Unlike [`std::convert::identity`], a Rust compiler is encouraged to assume that `black_box` can
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/// use `dummy` in any possible valid way that Rust code is allowed to without introducing undefined
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/// behavior in the calling code. This property makes `black_box` useful for writing code in which
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/// certain optimizations are not desired, such as benchmarks.
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///
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/// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
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/// extent to which it can block optimisations may vary depending upon the platform and code-gen
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/// backend used. Programs cannot rely on `black_box` for *correctness* in any way.
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///
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/// [`std::convert::identity`]: crate::convert::identity
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#[inline]
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#[unstable(feature = "bench_black_box", issue = "64102")]
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#[rustc_const_unstable(feature = "const_black_box", issue = "none")]
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pub const fn black_box<T>(dummy: T) -> T {
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crate::intrinsics::black_box(dummy)
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}
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/// An identity function that causes an `unused_must_use` warning to be
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/// triggered if the given value is not used (returned, stored in a variable,
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/// etc) by the caller.
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///
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/// This is primarily intended for use in macro-generated code, in which a
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/// [`#[must_use]` attribute][must_use] either on a type or a function would not
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/// be convenient.
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///
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/// [must_use]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
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///
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/// # Example
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///
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/// ```
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/// #![feature(hint_must_use)]
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///
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/// use core::fmt;
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///
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/// pub struct Error(/* ... */);
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///
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/// #[macro_export]
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/// macro_rules! make_error {
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/// ($($args:expr),*) => {
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/// core::hint::must_use({
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/// let error = $crate::make_error(core::format_args!($($args),*));
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/// error
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/// })
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/// };
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/// }
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///
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/// // Implementation detail of make_error! macro.
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/// #[doc(hidden)]
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/// pub fn make_error(args: fmt::Arguments<'_>) -> Error {
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/// Error(/* ... */)
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/// }
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///
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/// fn demo() -> Option<Error> {
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/// if true {
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/// // Oops, meant to write `return Some(make_error!("..."));`
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/// Some(make_error!("..."));
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/// }
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/// None
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/// }
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/// #
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/// # // Make rustdoc not wrap the whole snippet in fn main, so that $crate::make_error works
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/// # fn main() {}
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/// ```
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///
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/// In the above example, we'd like an `unused_must_use` lint to apply to the
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/// value created by `make_error!`. However, neither `#[must_use]` on a struct
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/// nor `#[must_use]` on a function is appropriate here, so the macro expands
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/// using `core::hint::must_use` instead.
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///
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/// - We wouldn't want `#[must_use]` on the `struct Error` because that would
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/// make the following unproblematic code trigger a warning:
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///
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/// ```
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/// # struct Error;
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/// #
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/// fn f(arg: &str) -> Result<(), Error>
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/// # { Ok(()) }
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///
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/// #[test]
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/// fn t() {
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/// // Assert that `f` returns error if passed an empty string.
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/// // A value of type `Error` is unused here but that's not a problem.
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/// f("").unwrap_err();
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/// }
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/// ```
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///
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/// - Using `#[must_use]` on `fn make_error` can't help because the return value
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/// *is* used, as the right-hand side of a `let` statement. The `let`
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/// statement looks useless but is in fact necessary for ensuring that
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/// temporaries within the `format_args` expansion are not kept alive past the
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/// creation of the `Error`, as keeping them alive past that point can cause
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/// autotrait issues in async code:
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///
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/// ```
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/// # #![feature(hint_must_use)]
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/// #
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/// # struct Error;
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/// #
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/// # macro_rules! make_error {
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/// # ($($args:expr),*) => {
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/// # core::hint::must_use({
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/// # // If `let` isn't used, then `f()` produces a non-Send future.
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/// # let error = make_error(core::format_args!($($args),*));
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/// # error
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/// # })
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/// # };
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/// # }
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/// #
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/// # fn make_error(args: core::fmt::Arguments<'_>) -> Error {
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/// # Error
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/// # }
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/// #
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/// async fn f() {
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/// // Using `let` inside the make_error expansion causes temporaries like
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/// // `unsync()` to drop at the semicolon of that `let` statement, which
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/// // is prior to the await point. They would otherwise stay around until
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/// // the semicolon on *this* statement, which is after the await point,
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/// // and the enclosing Future would not implement Send.
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/// log(make_error!("look: {:p}", unsync())).await;
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/// }
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///
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/// async fn log(error: Error) {/* ... */}
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///
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/// // Returns something without a Sync impl.
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/// fn unsync() -> *const () {
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/// 0 as *const ()
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/// }
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/// #
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/// # fn test() {
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/// # fn assert_send(_: impl Send) {}
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/// # assert_send(f());
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/// # }
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/// ```
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#[unstable(feature = "hint_must_use", issue = "94745")]
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#[rustc_const_unstable(feature = "hint_must_use", issue = "94745")]
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#[must_use] // <-- :)
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pub const fn must_use<T>(value: T) -> T {
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value
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}
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