From 1b7ccdcf33b744da7d8a3da9e736582867e66e57 Mon Sep 17 00:00:00 2001
From: Noah <33094578+coolreader18@users.noreply.github.com>
Date: Wed, 19 May 2021 20:32:08 -0500
Subject: [PATCH] Skip .chars().len() when str is ascii

---
 vm/src/builtins/pystr.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/vm/src/builtins/pystr.rs b/vm/src/builtins/pystr.rs
index 62d1091f6..7aee0e557 100644
--- a/vm/src/builtins/pystr.rs
+++ b/vm/src/builtins/pystr.rs
@@ -301,7 +301,13 @@ impl PyStr {
     }
     #[cold]
     fn _compute_char_len(&self) -> usize {
-        let len = self.value.chars().count();
+        // doing the check is ~10x faster for ascii, and is actually only 2% slower worst case for
+        // non-ascii; see https://github.com/RustPython/RustPython/pull/2586#issuecomment-844611532
+        let len = if self.value.is_ascii() {
+            self.value.len()
+        } else {
+            self.value.chars().count()
+        };
         // len cannot be usize::MAX, since vec.capacity() < isize::MAX
         self.char_len.store(len, atomic::Ordering::Relaxed);
         len