From b84942a0fe11c06a94cfe46ee787084e80c51fb9 Mon Sep 17 00:00:00 2001 From: Taras Tsugrii Date: Mon, 31 Jul 2023 12:40:11 -0700 Subject: [PATCH] [rustc_data_structures][perf] Simplify base_n::push_str. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This minor change removes the need to reverse resulting digits. Since reverse is O(|digit_num|) but bounded by 128, it's unlikely to be a noticeable in practice. At the same time, this code is also a 1 line shorter, so combined with tiny perf win, why not? I ran https://gist.github.com/ttsugriy/ed14860ef597ab315d4129d5f8adb191 on M1 macbook air and got a small improvement ``` Running benches/base_n_benchmark.rs (target/release/deps/base_n_benchmark-825fe5895b5c2693) push_str/old time: [14.180 µs 14.313 µs 14.462 µs] Performance has improved. Found 5 outliers among 100 measurements (5.00%) 4 (4.00%) high mild 1 (1.00%) high severe push_str/new time: [13.741 µs 13.839 µs 13.973 µs] Performance has improved. Found 8 outliers among 100 measurements (8.00%) 3 (3.00%) high mild 5 (5.00%) high severe ``` --- compiler/rustc_data_structures/src/base_n.rs | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/compiler/rustc_data_structures/src/base_n.rs b/compiler/rustc_data_structures/src/base_n.rs index 4567759c004..58704350706 100644 --- a/compiler/rustc_data_structures/src/base_n.rs +++ b/compiler/rustc_data_structures/src/base_n.rs @@ -16,22 +16,21 @@ const BASE_64: &[u8; MAX_BASE] = pub fn push_str(mut n: u128, base: usize, output: &mut String) { debug_assert!(base >= 2 && base <= MAX_BASE); let mut s = [0u8; 128]; - let mut index = 0; + let mut index = s.len(); let base = base as u128; loop { + index -= 1; s[index] = BASE_64[(n % base) as usize]; - index += 1; n /= base; if n == 0 { break; } } - s[0..index].reverse(); - output.push_str(str::from_utf8(&s[0..index]).unwrap()); + output.push_str(str::from_utf8(&s[index..]).unwrap()); } #[inline]