hint.rs.html -- source
Description: Source to the Rust file `src/libcore/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.

    use 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.
    ///
    /// The [`unreachable!()`] macro is the safe counterpart of this function, which
    /// will panic instead when executed.
    ///
    /// [`unreachable!()`]: ../macro.unreachable.html
    ///
    /// # 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, std::u32::MAX), 0);
    /// ```
    #[inline]
    #[stable(feature = "unreachable", since = "1.27.0")]
    pub unsafe fn unreachable_unchecked() -> ! {
        intrinsics::unreachable()
    }

    /// Signals the processor that it is entering a busy-wait spin-loop.
    ///
    /// Upon receiving spin-loop signal the processor can optimize its behavior by, for example, saving
    /// power or switching hyper-threads.
    ///
    /// This function is different than [`std::thread::yield_now`] which directly yields to the
    /// system's scheduler, whereas `spin_loop` only signals the processor that it is entering a
    /// busy-wait spin-loop without yielding control to the system's scheduler.
    ///
    /// Using a busy-wait spin-loop with `spin_loop` is ideally used in situations where a
    /// contended lock is held by another thread executed on a different CPU and where the waiting
    /// times are relatively small. Because entering busy-wait spin-loop does not trigger the system's
    /// scheduler, no overhead for switching threads occurs. However, if the thread holding the
    /// contended lock is running on the same CPU, the spin-loop is likely to occupy an entire CPU slice
    /// before switching to the thread that holds the lock. If the contending lock is held by a thread
    /// on the same CPU or if the waiting times for acquiring the lock are longer, it is often better to
    /// use [`std::thread::yield_now`].
    ///
    /// **Note**: On platforms that do not support receiving spin-loop hints this function does not
    /// do anything at all.
    ///
    /// [`std::thread::yield_now`]: ../../std/thread/fn.yield_now.html
    #[inline]
    #[unstable(feature = "renamed_spin_loop", issue = "55002")]
    pub fn spin_loop() {
        #[cfg(
            all(
                any(target_arch = "x86", target_arch = "x86_64"),
                target_feature = "sse2"
            )
        )] {
            #[cfg(target_arch = "x86")] {
                unsafe { crate::arch::x86::_mm_pause() };
            }

            #[cfg(target_arch = "x86_64")] {
                unsafe { crate::arch::x86_64::_mm_pause() };
            }
        }

        #[cfg(
            any(
                target_arch = "aarch64",
                all(target_arch = "arm", target_feature = "v6")
            )
        )] {
            #[cfg(target_arch = "aarch64")] {
                unsafe { crate::arch::aarch64::__yield() };
            }
            #[cfg(target_arch = "arm")] {
                unsafe { crate::arch::arm::__yield() };
            }
        }
    }

    /// A function that is opaque to the optimizer, to allow benchmarks to
    /// pretend to use outputs to assist in avoiding dead-code
    /// elimination.
    ///
    /// This function is a no-op, and does not even read from `dummy`.
    #[inline]
    #[unstable(feature = "test", issue = "27812")]
    pub fn black_box<T>(dummy: T) -> T {
        cfg_if! {
            if #[cfg(any(
                target_arch = "asmjs",
                all(
                    target_arch = "wasm32",
                    target_os = "emscripten"
                )
            ))] {
                #[inline]
                unsafe fn black_box_impl<T>(d: T) -> T {
                    // these targets do not support inline assembly
                    let ret = crate::ptr::read_volatile(&d);
                    crate::mem::forget(d);
                    ret
                }
            } else {
                #[inline]
                unsafe fn black_box_impl<T>(d: T) -> T {
                    // we need to "use" the argument in some way LLVM can't
                    // introspect.
                    asm!("" : : "r"(&d));
                    d
                }
            }
        }
        unsafe { black_box_impl(dummy) }
    }

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