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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() } /// Emits a machine instruction hinting to the processor that it is running in busy-wait /// spin-loop ("spin lock"). /// /// For a discussion of different locking strategies and their trade-offs, see /// [`core::sync::atomic::spin_loop_hint`]. /// /// **Note**: On platforms that do not support receiving spin-loop hints this function does not /// do anything at all. /// /// [`core::sync::atomic::spin_loop_hint`]: ../sync/atomic/fn.spin_loop_hint.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() }; } } } /// An identity function that *__hints__* to the compiler to be maximally pessimistic about what /// `black_box` could do. /// /// [`std::convert::identity`]: https://doc.rust-lang.org/core/convert/fn.identity.html /// /// Unlike [`std::convert::identity`], a Rust compiler is encouraged to assume that `black_box` can /// use `x` 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. #[inline] #[unstable(feature = "test", issue = "50297")] #[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 } }