Slight perf improvement on char::to_ascii_lowercase

library/core/benches/ascii.rs

@@ -66,6 +66,8 @@ macro_rules! benches {
 use test::black_box;
 use test::Bencher;
 
+const ASCII_CASE_MASK: u8 = 0b0010_0000;
+
 benches! {
     fn case00_alloc_only(_bytes: &mut [u8]) {}
 
@@ -204,7 +206,7 @@ benches! {
             }
         }
         for byte in bytes {
-            *byte &= !((is_ascii_lowercase(*byte) as u8) << 5)
+            *byte &= !((is_ascii_lowercase(*byte) as u8) * ASCII_CASE_MASK)
         }
     }
 
@@ -216,7 +218,7 @@ benches! {
             }
         }
         for byte in bytes {
-            *byte -= (is_ascii_lowercase(*byte) as u8) << 5
+            *byte -= (is_ascii_lowercase(*byte) as u8) * ASCII_CASE_MASK
         }
     }
 

library/core/benches/char/methods.rs

@@ -35,3 +35,13 @@ fn bench_to_digit_radix_var(b: &mut Bencher) {
             .min()
     })
 }
+
+#[bench]
+fn bench_to_ascii_uppercase(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_ascii_uppercase()).min())
+}
+
+#[bench]
+fn bench_to_ascii_lowercase(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_ascii_lowercase()).min())
+}

library/core/src/char/methods.rs

@@ -3,7 +3,7 @@
 use crate::slice;
 use crate::str::from_utf8_unchecked_mut;
 use crate::unicode::printable::is_printable;
-use crate::unicode::{self, conversions};
+use crate::unicode::{self, conversions, ASCII_CASE_MASK};
 
 use super::*;
 
@@ -1090,7 +1090,7 @@ impl char {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_uppercase(&self) -> char {
-        if self.is_ascii() { (*self as u8).to_ascii_uppercase() as char } else { *self }
+        if self.is_ascii_lowercase() { ((*self as u8) & !ASCII_CASE_MASK) as char } else { *self }
     }
 
     /// Makes a copy of the value in its ASCII lower case equivalent.
@@ -1118,7 +1118,7 @@ impl char {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_lowercase(&self) -> char {
-        if self.is_ascii() { (*self as u8).to_ascii_lowercase() as char } else { *self }
+        if self.is_ascii_uppercase() { ((*self as u8) | ASCII_CASE_MASK) as char } else { *self }
     }
 
     /// Checks that two values are an ASCII case-insensitive match.

library/core/src/num/mod.rs

@@ -5,6 +5,7 @@
 use crate::intrinsics;
 use crate::mem;
 use crate::str::FromStr;
+use crate::unicode::ASCII_CASE_MASK;
 
 // Used because the `?` operator is not allowed in a const context.
 macro_rules! try_opt {
@@ -195,7 +196,7 @@ impl u8 {
     #[inline]
     pub fn to_ascii_uppercase(&self) -> u8 {
         // Unset the fifth bit if this is a lowercase letter
-        *self & !((self.is_ascii_lowercase() as u8) << 5)
+        *self & !((self.is_ascii_lowercase() as u8) * ASCII_CASE_MASK)
     }
 
     /// Makes a copy of the value in its ASCII lower case equivalent.
@@ -218,7 +219,7 @@ impl u8 {
     #[inline]
     pub fn to_ascii_lowercase(&self) -> u8 {
         // Set the fifth bit if this is an uppercase letter
-        *self | ((self.is_ascii_uppercase() as u8) << 5)
+        *self | (self.is_ascii_uppercase() as u8 * ASCII_CASE_MASK)
     }
 
     /// Checks that two values are an ASCII case-insensitive match.

library/core/src/unicode/mod.rs

@@ -17,6 +17,9 @@ mod unicode_data;
 #[stable(feature = "unicode_version", since = "1.45.0")]
 pub const UNICODE_VERSION: (u8, u8, u8) = unicode_data::UNICODE_VERSION;
 
+/// If 6th bit set ascii is upper case.
+pub(crate) const ASCII_CASE_MASK: u8 = 0b0010_0000;
+
 // For use in liballoc, not re-exported in libstd.
 pub use unicode_data::{
     case_ignorable::lookup as Case_Ignorable, cased::lookup as Cased, conversions,