Make char::is_ascii_whitespace branchless on 64-bit

Author: lnicola

library/core/src/char/methods.rs

@@ -1544,9 +1544,18 @@ impl char {
     #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
     #[inline]
     pub const fn is_ascii_whitespace(&self) -> bool {
-        match *self {
-            '\t' | '\n' | '\x0C' | '\r' | ' ' => true,
-            _ => false,
+        #[cfg(target_pointer_width = "64")]
+        {
+            // Inspired from https://pdimov.github.io/blog/2020/07/19/llvm-and-memchr/
+            const MASK: u64 = 1 << b'\t' | 1 << b'\n' | 1 << b'\x0C' | 1 << b'\r' | 1 << b' ';
+            *self <= ' ' && 1u64 << (*self as u8) & MASK != 0
+        }
+        #[cfg(not(target_pointer_width = "64"))]
+        {
+            match *self {
+                '\t' | '\n' | '\x0C' | '\r' | ' ' => true,
+                _ => false,
+            }
         }
     }