pub const fn black_box<T>(dummy: T) -> TExpand description
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, beyond it behaving as the
identity function. As such, it must not be relied upon to control critical program behavior.
This also means that this function does not offer any guarantees for cryptographic or security
purposes.
§When is this useful?
While not suitable in those mission-critical cases, black_box’s functionality can generally be
relied upon for benchmarking, and should be used there. It will try to ensure that the
compiler doesn’t optimize away part of the intended test code based on context. For
example:
fn contains(haystack: &[&str], needle: &str) -> bool {
haystack.iter().any(|x| x == &needle)
}
pub fn benchmark() {
let haystack = vec!["abc", "def", "ghi", "jkl", "mno"];
let needle = "ghi";
for _ in 0..10 {
contains(&haystack, needle);
}
}The compiler could theoretically make optimizations like the following:
- The
needleandhaystackdo not change, move the call tocontainsoutside the loop and delete the loop - Inline
contains needleandhaystackhave values known at compile time,containsis always true. Remove the call and replace withtrue- Nothing is done with the result of
contains: delete this function call entirely benchmarknow has no purpose: delete this function
It is not likely that all of the above happens, but the compiler is definitely able to make some
optimizations that could result in a very inaccurate benchmark. This is where black_box comes
in:
use std::hint::black_box;
// Same `contains` function.
fn contains(haystack: &[&str], needle: &str) -> bool {
haystack.iter().any(|x| x == &needle)
}
pub fn benchmark() {
let haystack = vec!["abc", "def", "ghi", "jkl", "mno"];
let needle = "ghi";
for _ in 0..10 {
// Force the compiler to run `contains`, even though it is a pure function whose
// results are unused.
black_box(contains(
// Prevent the compiler from making assumptions about the input.
black_box(&haystack),
black_box(needle),
));
}
}This essentially tells the compiler to block optimizations across any calls to black_box. So,
it now:
- Treats both arguments to
containsas unpredictable: the body ofcontainscan no longer be optimized based on argument values - Treats the call to
containsand its result as volatile: the body ofbenchmarkcannot optimize this away
This makes our benchmark much more realistic to how the function would actually be used, where arguments are usually not known at compile time and the result is used in some way.
§How to use this
In practice, black_box serves two purposes:
- It prevents the compiler from making optimizations related to the value returned by
black_box - It forces the value passed to
black_boxto be calculated, even if the return value ofblack_boxis unused
use std::hint::black_box;
let zero = 0;
let five = 5;
// The compiler will see this and remove the `* five` call, because it knows that multiplying
// any integer by 0 will result in 0.
let c = zero * five;
// Adding `black_box` here disables the compiler's ability to reason about the first operand in the multiplication.
// It is forced to assume that it can be any possible number, so it cannot remove the `* five`
// operation.
let c = black_box(zero) * five;While most cases will not be as clear-cut as the above example, it still illustrates how
black_box can be used. When benchmarking a function, you usually want to wrap its inputs in
black_box so the compiler cannot make optimizations that would be unrealistic in real-life
use.
use std::hint::black_box;
// This is a simple function that increments its input by 1. Note that it is pure, meaning it
// has no side-effects. This function has no effect if its result is unused. (An example of a
// function *with* side-effects is `println!()`.)
fn increment(x: u8) -> u8 {
x + 1
}
// Here, we call `increment` but discard its result. The compiler, seeing this and knowing that
// `increment` is pure, will eliminate this function call entirely. This may not be desired,
// though, especially if we're trying to track how much time `increment` takes to execute.
let _ = increment(black_box(5));
// Here, we force `increment` to be executed. This is because the compiler treats `black_box`
// as if it has side-effects, and thus must compute its input.
let _ = black_box(increment(black_box(5)));There may be additional situations where you want to wrap the result of a function in
black_box to force its execution. This is situational though, and may not have any effect
(such as when the function returns a zero-sized type such as () unit).
Note that black_box has no effect on how its input is treated, only its output. As such,
expressions passed to black_box may still be optimized:
use std::hint::black_box;
// The compiler sees this...
let y = black_box(5 * 10);
// ...as this. As such, it will likely simplify `5 * 10` to just `50`.
let _0 = 5 * 10;
let y = black_box(_0);In the above example, the 5 * 10 expression is considered distinct from the black_box call,
and thus is still optimized by the compiler. You can prevent this by moving the multiplication
operation outside of black_box:
use std::hint::black_box;
// No assumptions can be made about either operand, so the multiplication is not optimized out.
let y = black_box(5) * black_box(10);During constant evaluation, black_box is treated as a no-op.