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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 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. /// /// [`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")] #[allow(unreachable_code)] // this makes #[cfg] a bit easier below. pub fn black_box<T>(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 intepretation 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(any( target_arch = "asmjs", all( target_arch = "wasm32", target_os = "emscripten" ) )))] unsafe { asm!("" : : "r"(&dummy)); return dummy; } // Not all platforms support inline assembly so try to do something without // inline assembly which in theory still hinders at least some optimizations // on those targets. This is the "best effort" scenario. unsafe { let ret = crate::ptr::read_volatile(&dummy); crate::mem::forget(dummy); ret } }