1#![stable(feature = "core_hint", since = "1.27.0")]
2
3//! Hints to compiler that affects how code should be emitted or optimized.
4//! Hints may be compile time or runtime.
5
6use crate::intrinsics;
7
8/// Informs the compiler that the site which is calling this function is not
9/// reachable, possibly enabling further optimizations.
10///
11/// # Safety
12///
13/// Reaching this function is *Undefined Behavior*.
14///
15/// As the compiler assumes that all forms of Undefined Behavior can never
16/// happen, it will eliminate all branches in the surrounding code that it can
17/// determine will invariably lead to a call to `unreachable_unchecked()`.
18///
19/// If the assumptions embedded in using this function turn out to be wrong -
20/// that is, if the site which is calling `unreachable_unchecked()` is actually
21/// reachable at runtime - the compiler may have generated nonsensical machine
22/// instructions for this situation, including in seemingly unrelated code,
23/// causing difficult-to-debug problems.
24///
25/// Use this function sparingly. Consider using the [`unreachable!`] macro,
26/// which may prevent some optimizations but will safely panic in case it is
27/// actually reached at runtime. Benchmark your code to find out if using
28/// `unreachable_unchecked()` comes with a performance benefit.
29///
30/// # Examples
31///
32/// `unreachable_unchecked()` can be used in situations where the compiler
33/// can't prove invariants that were previously established. Such situations
34/// have a higher chance of occurring if those invariants are upheld by
35/// external code that the compiler can't analyze.
36/// ```
37/// fn prepare_inputs(divisors: &mut Vec<u32>) {
38/// // Note to future-self when making changes: The invariant established
39/// // here is NOT checked in `do_computation()`; if this changes, you HAVE
40/// // to change `do_computation()`.
41/// divisors.retain(|divisor| *divisor != 0)
42/// }
43///
44/// /// # Safety
45/// /// All elements of `divisor` must be non-zero.
46/// unsafe fn do_computation(i: u32, divisors: &[u32]) -> u32 {
47/// divisors.iter().fold(i, |acc, divisor| {
48/// // Convince the compiler that a division by zero can't happen here
49/// // and a check is not needed below.
50/// if *divisor == 0 {
51/// // Safety: `divisor` can't be zero because of `prepare_inputs`,
52/// // but the compiler does not know about this. We *promise*
53/// // that we always call `prepare_inputs`.
54/// std::hint::unreachable_unchecked()
55/// }
56/// // The compiler would normally introduce a check here that prevents
57/// // a division by zero. However, if `divisor` was zero, the branch
58/// // above would reach what we explicitly marked as unreachable.
59/// // The compiler concludes that `divisor` can't be zero at this point
60/// // and removes the - now proven useless - check.
61/// acc / divisor
62/// })
63/// }
64///
65/// let mut divisors = vec![2, 0, 4];
66/// prepare_inputs(&mut divisors);
67/// let result = unsafe {
68/// // Safety: prepare_inputs() guarantees that divisors is non-zero
69/// do_computation(100, &divisors)
70/// };
71/// assert_eq!(result, 12);
72///
73/// ```
74///
75/// While using `unreachable_unchecked()` is perfectly sound in the following
76/// example, as the compiler is able to prove that a division by zero is not
77/// possible, benchmarking reveals that `unreachable_unchecked()` provides
78/// no benefit over using [`unreachable!`], while the latter does not introduce
79/// the possibility of Undefined Behavior.
80///
81/// ```
82/// fn div_1(a: u32, b: u32) -> u32 {
83/// use std::hint::unreachable_unchecked;
84///
85/// // `b.saturating_add(1)` is always positive (not zero),
86/// // hence `checked_div` will never return `None`.
87/// // Therefore, the else branch is unreachable.
88/// a.checked_div(b.saturating_add(1))
89/// .unwrap_or_else(|| unsafe { unreachable_unchecked() })
90/// }
91///
92/// assert_eq!(div_1(7, 0), 7);
93/// assert_eq!(div_1(9, 1), 4);
94/// assert_eq!(div_1(11, u32::MAX), 0);
95/// ```
96#[inline]
97#[stable(feature = "unreachable", since = "1.27.0")]
98#[rustc_const_stable(feature = "const_unreachable_unchecked", since = "1.57.0")]
99#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
100pub const unsafe fn unreachable_unchecked() -> ! {
101 // SAFETY: the safety contract for `intrinsics::unreachable` must
102 // be upheld by the caller.
103 unsafe {
104 intrinsics::assert_unsafe_precondition!("hint::unreachable_unchecked must never be reached", () => false);
105 intrinsics::unreachable()
106 }
107}
108
109/// Makes a *soundness* promise to the compiler that `cond` holds.
110///
111/// This may allow the optimizer to simplify things,
112/// but it might also make the generated code slower.
113/// Either way, calling it will most likely make compilation take longer.
114///
115/// This is a situational tool for micro-optimization, and is allowed to do nothing.
116/// Any use should come with a repeatable benchmark to show the value
117/// and allow removing it later should the optimizer get smarter and no longer need it.
118///
119/// The more complicated the condition the less likely this is to be fruitful.
120/// For example, `assert_unchecked(foo.is_sorted())` is a complex enough value
121/// that the compiler is unlikely to be able to take advantage of it.
122///
123/// There's also no need to `assert_unchecked` basic properties of things. For
124/// example, the compiler already knows the range of `count_ones`, so there's no
125/// benefit to `let n = u32::count_ones(x); assert_unchecked(n <= u32::BITS);`.
126///
127/// If ever you're tempted to write `assert_unchecked(false)`, then you're
128/// actually looking for [`unreachable_unchecked()`].
129///
130/// You may know this from other places
131/// as [`llvm.assume`](https://llvm.org/docs/LangRef.html#llvm-assume-intrinsic)
132/// or [`__builtin_assume`](https://clang.llvm.org/docs/LanguageExtensions.html#builtin-assume).
133///
134/// This promotes a correctness requirement to a soundness requirement.
135/// Don't do that without very good reason.
136///
137/// # Safety
138///
139/// `cond` must be `true`. It's immediate UB to call this with `false`.
140///
141#[inline(always)]
142#[doc(alias = "assume")]
143#[track_caller]
144#[unstable(feature = "hint_assert_unchecked", issue = "119131")]
145#[rustc_const_unstable(feature = "const_hint_assert_unchecked", issue = "119131")]
146pub const unsafe fn assert_unchecked(cond: bool) {
147 // SAFETY: The caller promised `cond` is true.
148 unsafe {
149 intrinsics::assert_unsafe_precondition!(
150 "hint::assert_unchecked must never be called when the condition is false",
151 (cond: bool) => cond,
152 );
153 crate::intrinsics::assume(cond);
154 }
155}
156
157/// Emits a machine instruction to signal the processor that it is running in
158/// a busy-wait spin-loop ("spin lock").
159///
160/// Upon receiving the spin-loop signal the processor can optimize its behavior by,
161/// for example, saving power or switching hyper-threads.
162///
163/// This function is different from [`thread::yield_now`] which directly
164/// yields to the system's scheduler, whereas `spin_loop` does not interact
165/// with the operating system.
166///
167/// A common use case for `spin_loop` is implementing bounded optimistic
168/// spinning in a CAS loop in synchronization primitives. To avoid problems
169/// like priority inversion, it is strongly recommended that the spin loop is
170/// terminated after a finite amount of iterations and an appropriate blocking
171/// syscall is made.
172///
173/// **Note**: On platforms that do not support receiving spin-loop hints this
174/// function does not do anything at all.
175///
176/// # Examples
177///
178/// ```
179/// use std::sync::atomic::{AtomicBool, Ordering};
180/// use std::sync::Arc;
181/// use std::{hint, thread};
182///
183/// // A shared atomic value that threads will use to coordinate
184/// let live = Arc::new(AtomicBool::new(false));
185///
186/// // In a background thread we'll eventually set the value
187/// let bg_work = {
188/// let live = live.clone();
189/// thread::spawn(move || {
190/// // Do some work, then make the value live
191/// do_some_work();
192/// live.store(true, Ordering::Release);
193/// })
194/// };
195///
196/// // Back on our current thread, we wait for the value to be set
197/// while !live.load(Ordering::Acquire) {
198/// // The spin loop is a hint to the CPU that we're waiting, but probably
199/// // not for very long
200/// hint::spin_loop();
201/// }
202///
203/// // The value is now set
204/// # fn do_some_work() {}
205/// do_some_work();
206/// bg_work.join()?;
207/// # Ok::<(), Box<dyn core::any::Any + Send + 'static>>(())
208/// ```
209///
210/// [`thread::yield_now`]: ../../std/thread/fn.yield_now.html
211#[inline(always)]
212#[stable(feature = "renamed_spin_loop", since = "1.49.0")]
213pub fn spin_loop() {
214 #[cfg(target_arch = "x86")]
215 {
216 // SAFETY: the `cfg` attr ensures that we only execute this on x86 targets.
217 unsafe { crate::arch::x86::_mm_pause() };
218 }
219
220 #[cfg(target_arch = "x86_64")]
221 {
222 // SAFETY: the `cfg` attr ensures that we only execute this on x86_64 targets.
223 unsafe { crate::arch::x86_64::_mm_pause() };
224 }
225
226 #[cfg(target_arch = "riscv32")]
227 {
228 crate::arch::riscv32::pause();
229 }
230
231 #[cfg(target_arch = "riscv64")]
232 {
233 crate::arch::riscv64::pause();
234 }
235
236 #[cfg(target_arch = "aarch64")]
237 {
238 // SAFETY: the `cfg` attr ensures that we only execute this on aarch64 targets.
239 unsafe { crate::arch::aarch64::__isb(crate::arch::aarch64::SY) };
240 }
241
242 #[cfg(all(target_arch = "arm", target_feature = "v6"))]
243 {
244 // SAFETY: the `cfg` attr ensures that we only execute this on arm targets
245 // with support for the v6 feature.
246 unsafe { crate::arch::arm::__yield() };
247 }
248}
249
250/// An identity function that *__hints__* to the compiler to be maximally pessimistic about what
251/// `black_box` could do.
252///
253/// Unlike [`std::convert::identity`], a Rust compiler is encouraged to assume that `black_box` can
254/// use `dummy` in any possible valid way that Rust code is allowed to without introducing undefined
255/// behavior in the calling code. This property makes `black_box` useful for writing code in which
256/// certain optimizations are not desired, such as benchmarks.
257///
258/// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
259/// extent to which it can block optimisations may vary depending upon the platform and code-gen
260/// backend used. Programs cannot rely on `black_box` for *correctness*, beyond it behaving as the
261/// identity function. As such, it **must not be relied upon to control critical program behavior.**
262/// This _immediately_ precludes any direct use of this function for cryptographic or security
263/// purposes.
264///
265/// [`std::convert::identity`]: crate::convert::identity
266///
267/// # When is this useful?
268///
269/// While not suitable in those mission-critical cases, `black_box`'s functionality can generally be
270/// relied upon for benchmarking, and should be used there. It will try to ensure that the
271/// compiler doesn't optimize away part of the intended test code based on context. For
272/// example:
273///
274/// ```
275/// fn contains(haystack: &[&str], needle: &str) -> bool {
276/// haystack.iter().any(|x| x == &needle)
277/// }
278///
279/// pub fn benchmark() {
280/// let haystack = vec!["abc", "def", "ghi", "jkl", "mno"];
281/// let needle = "ghi";
282/// for _ in 0..10 {
283/// contains(&haystack, needle);
284/// }
285/// }
286/// ```
287///
288/// The compiler could theoretically make optimizations like the following:
289///
290/// - The `needle` and `haystack` do not change, move the call to `contains` outside the loop and
291/// delete the loop
292/// - Inline `contains`
293/// - `needle` and `haystack` have values known at compile time, `contains` is always true. Remove
294/// the call and replace with `true`
295/// - Nothing is done with the result of `contains`: delete this function call entirely
296/// - `benchmark` now has no purpose: delete this function
297///
298/// It is not likely that all of the above happens, but the compiler is definitely able to make some
299/// optimizations that could result in a very inaccurate benchmark. This is where `black_box` comes
300/// in:
301///
302/// ```
303/// use std::hint::black_box;
304///
305/// // Same `contains` function
306/// fn contains(haystack: &[&str], needle: &str) -> bool {
307/// haystack.iter().any(|x| x == &needle)
308/// }
309///
310/// pub fn benchmark() {
311/// let haystack = vec!["abc", "def", "ghi", "jkl", "mno"];
312/// let needle = "ghi";
313/// for _ in 0..10 {
314/// // Adjust our benchmark loop contents
315/// black_box(contains(black_box(&haystack), black_box(needle)));
316/// }
317/// }
318/// ```
319///
320/// This essentially tells the compiler to block optimizations across any calls to `black_box`. So,
321/// it now:
322///
323/// - Treats both arguments to `contains` as unpredictable: the body of `contains` can no longer be
324/// optimized based on argument values
325/// - Treats the call to `contains` and its result as volatile: the body of `benchmark` cannot
326/// optimize this away
327///
328/// This makes our benchmark much more realistic to how the function would actually be used, where
329/// arguments are usually not known at compile time and the result is used in some way.
330#[inline]
331#[stable(feature = "bench_black_box", since = "1.66.0")]
332#[rustc_const_unstable(feature = "const_black_box", issue = "none")]
333pub const fn black_box<T>(dummy: T) -> T {
334 crate::intrinsics::black_box(dummy)
335}
336
337/// An identity function that causes an `unused_must_use` warning to be
338/// triggered if the given value is not used (returned, stored in a variable,
339/// etc) by the caller.
340///
341/// This is primarily intended for use in macro-generated code, in which a
342/// [`#[must_use]` attribute][must_use] either on a type or a function would not
343/// be convenient.
344///
345/// [must_use]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
346///
347/// # Example
348///
349/// ```
350/// #![feature(hint_must_use)]
351///
352/// use core::fmt;
353///
354/// pub struct Error(/* ... */);
355///
356/// #[macro_export]
357/// macro_rules! make_error {
358/// ($($args:expr),*) => {
359/// core::hint::must_use({
360/// let error = $crate::make_error(core::format_args!($($args),*));
361/// error
362/// })
363/// };
364/// }
365///
366/// // Implementation detail of make_error! macro.
367/// #[doc(hidden)]
368/// pub fn make_error(args: fmt::Arguments<'_>) -> Error {
369/// Error(/* ... */)
370/// }
371///
372/// fn demo() -> Option<Error> {
373/// if true {
374/// // Oops, meant to write `return Some(make_error!("..."));`
375/// Some(make_error!("..."));
376/// }
377/// None
378/// }
379/// #
380/// # // Make rustdoc not wrap the whole snippet in fn main, so that $crate::make_error works
381/// # fn main() {}
382/// ```
383///
384/// In the above example, we'd like an `unused_must_use` lint to apply to the
385/// value created by `make_error!`. However, neither `#[must_use]` on a struct
386/// nor `#[must_use]` on a function is appropriate here, so the macro expands
387/// using `core::hint::must_use` instead.
388///
389/// - We wouldn't want `#[must_use]` on the `struct Error` because that would
390/// make the following unproblematic code trigger a warning:
391///
392/// ```
393/// # struct Error;
394/// #
395/// fn f(arg: &str) -> Result<(), Error>
396/// # { Ok(()) }
397///
398/// #[test]
399/// fn t() {
400/// // Assert that `f` returns error if passed an empty string.
401/// // A value of type `Error` is unused here but that's not a problem.
402/// f("").unwrap_err();
403/// }
404/// ```
405///
406/// - Using `#[must_use]` on `fn make_error` can't help because the return value
407/// *is* used, as the right-hand side of a `let` statement. The `let`
408/// statement looks useless but is in fact necessary for ensuring that
409/// temporaries within the `format_args` expansion are not kept alive past the
410/// creation of the `Error`, as keeping them alive past that point can cause
411/// autotrait issues in async code:
412///
413/// ```
414/// # #![feature(hint_must_use)]
415/// #
416/// # struct Error;
417/// #
418/// # macro_rules! make_error {
419/// # ($($args:expr),*) => {
420/// # core::hint::must_use({
421/// # // If `let` isn't used, then `f()` produces a non-Send future.
422/// # let error = make_error(core::format_args!($($args),*));
423/// # error
424/// # })
425/// # };
426/// # }
427/// #
428/// # fn make_error(args: core::fmt::Arguments<'_>) -> Error {
429/// # Error
430/// # }
431/// #
432/// async fn f() {
433/// // Using `let` inside the make_error expansion causes temporaries like
434/// // `unsync()` to drop at the semicolon of that `let` statement, which
435/// // is prior to the await point. They would otherwise stay around until
436/// // the semicolon on *this* statement, which is after the await point,
437/// // and the enclosing Future would not implement Send.
438/// log(make_error!("look: {:p}", unsync())).await;
439/// }
440///
441/// async fn log(error: Error) {/* ... */}
442///
443/// // Returns something without a Sync impl.
444/// fn unsync() -> *const () {
445/// 0 as *const ()
446/// }
447/// #
448/// # fn test() {
449/// # fn assert_send(_: impl Send) {}
450/// # assert_send(f());
451/// # }
452/// ```
453#[unstable(feature = "hint_must_use", issue = "94745")]
454#[rustc_const_unstable(feature = "hint_must_use", issue = "94745")]
455#[must_use] // <-- :)
456#[inline(always)]
457pub const fn must_use<T>(value: T) -> T {
458 value
459}
460