HashMap slow compared to hashbrown implementation

[stefanhoelzl]

Since Rust 1.36 the HashMap implementation in the standard library should be ported from hashbrown (see #56241)).

But when benchmarking both, hashbrown is a lot faster (3x) on inserts than the HashMap in the standard library:

$ cargo bench
running 2 tests
test tests::brown ... bench:           4 ns/iter (+/- 3)
test tests::std   ... bench:          14 ns/iter (+/- 4)

Here is my benchmarking code

#![feature(test)]
extern crate test;
extern crate hashbrown;

#[cfg(test)]
mod tests {
    use test::Bencher;
    use hashbrown::HashMap as BrownMap;
    use std::collections::HashMap as StdMap;

    #[bench]
    fn brown(b: &mut Bencher) {
        let mut map = BrownMap::new();
        b.iter(|| map.insert(0, 0));
    }

    #[bench]
    fn std(b: &mut Bencher) {
        let mut map = StdMap::new();
        b.iter(|| map.insert(0, 0));
    }
}

Meta

$ rustc --version --verbose
rustc 1.39.0-nightly (b9de4ef89 2019-09-03)
binary: rustc
commit-hash: b9de4ef89e0e53099a084001b26ec3207c5f8391
commit-date: 2019-09-03
host: x86_64-pc-windows-msvc
release: 1.39.0-nightly
LLVM version: 9.0

on Windows 10

[Mark-Simulacrum]

This is expected, as std has a different (slower) hasher due to wanting default DDOS protection and such.