Function black_box

pub fn black_box<T>(dummy: T) -> T

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 needle and haystack do not change, move the call to contains outside the loop and delete the loop
• Inline contains
• needle and haystack have values known at compile time, contains is always true. Remove the call and replace with true
• Nothing is done with the result of contains: delete this function call entirely
• benchmark now 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 {
        // Adjust our benchmark loop contents
        black_box(contains(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 contains as unpredictable: the body of contains can no longer be optimized based on argument values
• Treats the call to contains and its result as volatile: the body of benchmark cannot 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.