From facba24926e218f996a953b0d91494a04374b217 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Fri, 7 Jan 2022 11:50:52 +1100
Subject: [PATCH] Unpeel the first iteration of the loop in
impl_read_unsigned_leb128.
---
compiler/rustc_serialize/src/leb128.rs | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)
diff --git a/compiler/rustc_serialize/src/leb128.rs b/compiler/rustc_serialize/src/leb128.rs
index de333774763..08b3c054200 100644
--- a/compiler/rustc_serialize/src/leb128.rs
+++ b/compiler/rustc_serialize/src/leb128.rs
@@ -54,8 +54,17 @@ macro_rules! impl_read_unsigned_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
- let mut result = 0;
- let mut shift = 0;
+ // The first iteration of this loop is unpeeled. This is a
+ // performance win because this code is hot and integer values less
+ // than 128 are very common, typically occurring 50-80% or more of
+ // the time, even for u64 and u128.
+ let byte = slice[*position];
+ *position += 1;
+ if (byte & 0x80) == 0 {
+ return byte as $int_ty;
+ }
+ let mut result = (byte & 0x7F) as $int_ty;
+ let mut shift = 7;
loop {
let byte = slice[*position];
*position += 1;