From 2b14529a7cc106d5d808529cd14d71aa158c7789 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Gammels=C3=A6ter?= <martin@mg.am>
Date: Tue, 12 Apr 2022 16:31:03 +0200
Subject: [PATCH 1/2] Optimize line offset parsing in <SourceFile as
 Decodable>::decode

By inverting parsing loop, avoiding continually re-checking bytes_per_diff.
---
 compiler/rustc_span/src/lib.rs | 31 ++++++++++++++++++++-----------
 1 file changed, 20 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 5232c8d7006bd..68401d5ca7cba 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -1318,17 +1318,26 @@ impl<D: Decoder> Decodable<D> for SourceFile {
                 let mut line_start: BytePos = Decodable::decode(d);
                 lines.push(line_start);
 
-                for _ in 1..num_lines {
-                    let diff = match bytes_per_diff {
-                        1 => d.read_u8() as u32,
-                        2 => d.read_u16() as u32,
-                        4 => d.read_u32(),
-                        _ => unreachable!(),
-                    };
-
-                    line_start = line_start + BytePos(diff);
-
-                    lines.push(line_start);
+                match bytes_per_diff {
+                    1 => {
+                        for _ in 1..num_lines {
+                            line_start = line_start + BytePos(d.read_u8() as u32);
+                            lines.push(line_start);
+                        }
+                    }
+                    2 => {
+                        for _ in 1..num_lines {
+                            line_start = line_start + BytePos(d.read_u16() as u32);
+                            lines.push(line_start);
+                        }
+                    }
+                    4 => {
+                        for _ in 1..num_lines {
+                            line_start = line_start + BytePos(d.read_u32());
+                            lines.push(line_start);
+                        }
+                    }
+                    _ => unreachable!(),
                 }
             }
 

From 5f2c6b92d1cb2749872432448d42cd9cb05b0565 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Martin=20Gammels=C3=A6ter?= <martin@mg.am>
Date: Wed, 13 Apr 2022 08:44:20 +0200
Subject: [PATCH 2/2] Use .extend(..) instead of push()-ing in loop

A bit less readable but more compact, and maybe faster? We'll see.
---
 compiler/rustc_span/src/lib.rs | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 68401d5ca7cba..45b0e0c2dd1f8 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -1319,24 +1319,18 @@ impl<D: Decoder> Decodable<D> for SourceFile {
                 lines.push(line_start);
 
                 match bytes_per_diff {
-                    1 => {
-                        for _ in 1..num_lines {
-                            line_start = line_start + BytePos(d.read_u8() as u32);
-                            lines.push(line_start);
-                        }
-                    }
-                    2 => {
-                        for _ in 1..num_lines {
-                            line_start = line_start + BytePos(d.read_u16() as u32);
-                            lines.push(line_start);
-                        }
-                    }
-                    4 => {
-                        for _ in 1..num_lines {
-                            line_start = line_start + BytePos(d.read_u32());
-                            lines.push(line_start);
-                        }
-                    }
+                    1 => lines.extend((1..num_lines).map(|_| {
+                        line_start = line_start + BytePos(d.read_u8() as u32);
+                        line_start
+                    })),
+                    2 => lines.extend((1..num_lines).map(|_| {
+                        line_start = line_start + BytePos(d.read_u16() as u32);
+                        line_start
+                    })),
+                    4 => lines.extend((1..num_lines).map(|_| {
+                        line_start = line_start + BytePos(d.read_u32());
+                        line_start
+                    })),
                     _ => unreachable!(),
                 }
             }