hint.rs - source
Description: Source of the Rust file `library/core/src/hint.rs`.
Keywords: rust, rustlang, rust-lang

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    #![stable(feature = "core_hint", since = "1.27.0")]

    //! Hints to compiler that affects how code should be emitted or optimized.
    //! Hints may be compile time or runtime.

    use crate::intrinsics;

    /// Informs the compiler that this point in the code is not reachable, enabling
    /// further optimizations.
    ///
    /// # Safety
    ///
    /// Reaching this function is completely *undefined behavior* (UB). In
    /// particular, the compiler assumes that all UB must never happen, and
    /// therefore will eliminate all branches that reach to a call to
    /// `unreachable_unchecked()`.
    ///
    /// Like all instances of UB, if this assumption turns out to be wrong, i.e., the
    /// `unreachable_unchecked()` call is actually reachable among all possible
    /// control flow, the compiler will apply the wrong optimization strategy, and
    /// may sometimes even corrupt seemingly unrelated code, causing
    /// difficult-to-debug problems.
    ///
    /// Use this function only when you can prove that the code will never call it.
    /// Otherwise, consider using the [`unreachable!`] macro, which does not allow
    /// optimizations but will panic when executed.
    ///
    /// # Example
    ///
    /// ```
    /// fn div_1(a: u32, b: u32) -> u32 {
    ///     use std::hint::unreachable_unchecked;
    ///
    ///     // `b.saturating_add(1)` is always positive (not zero),
    ///     // hence `checked_div` will never return `None`.
    ///     // Therefore, the else branch is unreachable.
    ///     a.checked_div(b.saturating_add(1))
    ///         .unwrap_or_else(|| unsafe { unreachable_unchecked() })
    /// }
    ///
    /// assert_eq!(div_1(7, 0), 7);
    /// assert_eq!(div_1(9, 1), 4);
    /// assert_eq!(div_1(11, u32::MAX), 0);
    /// ```
    #[inline]
    #[stable(feature = "unreachable", since = "1.27.0")]
    #[rustc_const_unstable(feature = "const_unreachable_unchecked", issue = "53188")]
    pub const unsafe fn unreachable_unchecked() -> ! {
        // SAFETY: the safety contract for `intrinsics::unreachable` must
        // be upheld by the caller.
        unsafe { intrinsics::unreachable() }
    }

    /// Emits a machine instruction to signal the processor that it is running in
    /// a busy-wait spin-loop ("spin lock").
    ///
    /// Upon receiving the spin-loop signal the processor can optimize its behavior by,
    /// for example, saving power or switching hyper-threads.
    ///
    /// This function is different from [`thread::yield_now`] which directly
    /// yields to the system's scheduler, whereas `spin_loop` does not interact
    /// with the operating system.
    ///
    /// A common use case for `spin_loop` is implementing bounded optimistic
    /// spinning in a CAS loop in synchronization primitives. To avoid problems
    /// like priority inversion, it is strongly recommended that the spin loop is
    /// terminated after a finite amount of iterations and an appropriate blocking
    /// syscall is made.
    ///
    /// **Note**: On platforms that do not support receiving spin-loop hints this
    /// function does not do anything at all.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::atomic::{AtomicBool, Ordering};
    /// use std::sync::Arc;
    /// use std::{hint, thread};
    ///
    /// // A shared atomic value that threads will use to coordinate
    /// let live = Arc::new(AtomicBool::new(false));
    ///
    /// // In a background thread we'll eventually set the value
    /// let bg_work = {
    ///     let live = live.clone();
    ///     thread::spawn(move || {
    ///         // Do some work, then make the value live
    ///         do_some_work();
    ///         live.store(true, Ordering::Release);
    ///     })
    /// };
    ///
    /// // Back on our current thread, we wait for the value to be set
    /// while live.load(Ordering::Acquire) {
    ///     // The spin loop is a hint to the CPU that we're waiting, but probably
    ///     // not for very long
    ///     hint::spin_loop();
    /// }
    ///
    /// // The value is now set
    /// # fn do_some_work() {}
    /// do_some_work();
    /// bg_work.join()?;
    /// # Ok::<(), Box<dyn core::any::Any + Send + 'static>>(())
    /// ```
    ///
    /// [`thread::yield_now`]: ../../std/thread/fn.yield_now.html
    #[inline]
    #[stable(feature = "renamed_spin_loop", since = "1.49.0")]
    pub fn spin_loop() {
        #[cfg(all(any(target_arch = "x86", target_arch = "x86_64"), target_feature = "sse2"))]
        {
            #[cfg(target_arch = "x86")]
            {
                // SAFETY: the `cfg` attr ensures that we only execute this on x86 targets.
                unsafe { crate::arch::x86::_mm_pause() };
            }

            #[cfg(target_arch = "x86_64")]
            {
                // SAFETY: the `cfg` attr ensures that we only execute this on x86_64 targets.
                unsafe { crate::arch::x86_64::_mm_pause() };
            }
        }

        #[cfg(any(target_arch = "aarch64", all(target_arch = "arm", target_feature = "v6")))]
        {
            #[cfg(target_arch = "aarch64")]
            {
                // SAFETY: the `cfg` attr ensures that we only execute this on aarch64 targets.
                unsafe { crate::arch::aarch64::__yield() };
            }
            #[cfg(target_arch = "arm")]
            {
                // SAFETY: the `cfg` attr ensures that we only execute this on arm targets
                // with support for the v6 feature.
                unsafe { crate::arch::arm::__yield() };
            }
        }
    }

    /// An identity function that *__hints__* to the compiler to be maximally pessimistic about what
    /// `black_box` could do.
    ///
    /// Unlike [`std::convert::identity`], a Rust compiler is encouraged to assume that `black_box` can
    /// use `dummy` in any possible valid way that Rust code is allowed to without introducing undefined
    /// behavior in the calling code. This property makes `black_box` useful for writing code in which
    /// certain optimizations are not desired, such as benchmarks.
    ///
    /// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
    /// extent to which it can block optimisations may vary depending upon the platform and code-gen
    /// backend used. Programs cannot rely on `black_box` for *correctness* in any way.
    ///
    /// [`std::convert::identity`]: crate::convert::identity
    #[cfg_attr(not(miri), inline)]
    #[cfg_attr(miri, inline(never))]
    #[unstable(feature = "test", issue = "50297")]
    #[cfg_attr(miri, allow(unused_mut))]
    pub fn black_box<T>(mut dummy: T) -> T {
        // We need to "use" the argument in some way LLVM can't introspect, and on
        // targets that support it we can typically leverage inline assembly to do
        // this. LLVM's interpretation of inline assembly is that it's, well, a black
        // box. This isn't the greatest implementation since it probably deoptimizes
        // more than we want, but it's so far good enough.

        #[cfg(not(miri))] // This is just a hint, so it is fine to skip in Miri.
        // SAFETY: the inline assembly is a no-op.
        unsafe {
            // FIXME: Cannot use `asm!` because it doesn't support MIPS and other architectures.
            llvm_asm!("" : : "r"(&mut dummy) : "memory" : "volatile");
        }

        dummy
    }