SIMD-optimized is_ascii based on rust-lang#30740

Author: mbrubeck

src/libstd/ascii.rs

@@ -164,7 +164,7 @@ impl AsciiExt for str {
 
     #[inline]
     fn is_ascii(&self) -> bool {
-        self.bytes().all(|b| b.is_ascii())
+        self.as_bytes().is_ascii()
     }
 
     #[inline]
@@ -204,7 +204,55 @@ impl AsciiExt for [u8] {
     type Owned = Vec<u8>;
     #[inline]
     fn is_ascii(&self) -> bool {
-        self.iter().all(|b| b.is_ascii())
+        // NOTE: This is largely copied from core::str::run_utf8_validation.
+
+        // use truncation to fit u64 into usize
+        const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;
+
+        /// Return `true` if any byte in the word `x` is nonascii (>= 128).
+        #[inline]
+        fn contains_nonascii(x: usize) -> bool {
+            (x & NONASCII_MASK) != 0
+        }
+
+        let mut offset = 0;
+        let len = self.len();
+        while offset < len {
+            if self[offset] >= 128 {
+                return false;
+            }
+            // When the pointer is aligned, read 2 words of data per iteration
+            // until we find a word containing a non-ascii byte.
+            let usize_bytes = mem::size_of::<usize>();
+            let bytes_per_iteration = 2 * usize_bytes;
+            let ptr = self.as_ptr();
+            let align = (ptr as usize + offset) & (usize_bytes - 1);
+            if align == 0 {
+                if len >= bytes_per_iteration {
+                    while offset <= len - bytes_per_iteration {
+                        unsafe {
+                            let u = *(ptr.offset(offset as isize) as *const usize);
+                            let v = *(ptr.offset((offset + usize_bytes) as isize) as *const usize);
+
+                            // return if there is a nonascii byte
+                            let zu = contains_nonascii(u);
+                            let zv = contains_nonascii(v);
+                            if zu || zv {
+                                return false;
+                            }
+                        }
+                        offset += bytes_per_iteration;
+                    }
+                }
+                // step from the point where the wordwise loop stopped
+                while offset < len && self[offset] < 128 {
+                    offset += 1;
+                }
+            } else {
+                offset += 1;
+            }
+        }
+        true
     }
 
     #[inline]