From ab8509edaa06ea0031c421282a792dfe2a857b2e Mon Sep 17 00:00:00 2001
From: overlookmotel <557937+overlookmotel@users.noreply.github.com>
Date: Wed, 31 Jul 2024 00:28:46 +0000
Subject: [PATCH] perf(parser): use `-` not `saturating_sub` (#4561)

Follow-on after #4304. Avoid using `saturating_sub` when plain `-` will do. `-` is cheaper - it's a single assembly instruction, whereas `saturating_sub` is usually 3 instructions.

https://godbolt.org/z/fo8Tcx6bK
---
 crates/oxc_parser/src/lexer/source.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/crates/oxc_parser/src/lexer/source.rs b/crates/oxc_parser/src/lexer/source.rs
index 8c108ba6d..88b0fe6e8 100644
--- a/crates/oxc_parser/src/lexer/source.rs
+++ b/crates/oxc_parser/src/lexer/source.rs
@@ -501,9 +501,11 @@ impl<'a> Source<'a> {
     /// Peek next two bytes of source without consuming them.
     #[inline]
     pub(super) fn peek_2_bytes(&self) -> Option<[u8; 2]> {
-        if (self.end as usize).saturating_sub(self.ptr as usize) >= 2 {
+        // `end` is always >= `ptr` so `end - ptr` cannot wrap around.
+        // No need to use checked/saturating subtraction here.
+        if (self.end as usize) - (self.ptr as usize) >= 2 {
             // SAFETY: The check above ensures that there are at least 2 bytes to
-            // read from `self.ptr` without overflowing past `self.end`.
+            // read from `self.ptr` without reading past `self.end`
             let bytes = unsafe { self.position().read2() };
             Some(bytes)
         } else {