get_unchecked Benchmark
This page was last reviewed on Apr 9, 2022.
Dot Net Perls
Get unchecked. Bounds checks on slice element accesses are not usually a performance problem in programs. But these checks can slow down programs slightly.
In Rust, a direct access of a slice may have bounds checks that are not optimized out. A call to get_unchecked, though, never has bounds checks.
Example benchmark. Consider this Rust program. It creates an vector of 10000 elements, and then assigns a value somewhere in the middle of the vector.
Version 1 We access many elements in the vector's memory and test for the value 5 that was set in the vector.
Version 2 We use get_unchecked() in an unsafe block of code to test elements in the vector. No bounds checks are used in this loop.
Result The performance advantage of using get_unchecked is clear and reproducible. The bounds checks cause a significant performance reduction.
fn main() { // Vector for testing. let mut x = vec![]; for _ in 0..10000 { x.push(0); } x[5555] = 5; // Version 1: use checked slice accesses. let t = Instant::now(); let mut count = 0; for _ in 0..100000 { for j in 10..9900 { if x[j] == 5 { count += 1; } } } println!("{}", t.elapsed().as_millis()); // Version 2: use get_unchecked. let t = Instant::now(); for _ in 0..100000 { for j in 10..9900 { unsafe { if *x.get_unchecked(j) == 5 { count += 1; } } } } println!("{}", t.elapsed().as_millis()); println!("{}", count); }
468 ms x[j] 314 ms *x.get_unchecked(j) 200000
Some discussion. Calling get_unchecked in Rust program always will involve some risk. It should only be used when you are absolutely certain it is correct.
Info You can call get_unchecked on invalid memory addresses, and unexpected values will be returned. The program may not terminate.
A summary. Using get_checked in Rust can significantly speed up some functions that repeatedly read elements from a slice. But the optimization is not safe, and must be carefully considered.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Apr 9, 2022 (edit link).
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