Use unrolled loop for searching NULL in [u16] on Windows

src/libstd/sys/windows/mod.rs

@@ -81,10 +81,54 @@ pub fn decode_error_kind(errno: i32) -> ErrorKind {
     }
 }
 
+pub fn unrolled_find_u16s(needle: u16, haystack: &[u16]) -> Option<usize> {
+    let ptr = haystack.as_ptr();
+    let mut len = haystack.len();
+    let mut start = &haystack[..];
+
+    // For performance reasons unfold the loop eight times.
+    while len >= 8 {
+        if start[0] == needle {
+            return Some((start.as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[1] == needle {
+            return Some((start[1..].as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[2] == needle {
+            return Some((start[2..].as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[3] == needle {
+            return Some((start[3..].as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[4] == needle {
+            return Some((start[4..].as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[5] == needle {
+            return Some((start[5..].as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[6] == needle {
+            return Some((start[6..].as_ptr() as usize - ptr as usize) / 2);
+        }
+        if start[7] == needle {
+            return Some((start[7..].as_ptr() as usize - ptr as usize) / 2);
+        }
+
+        start = &start[8..];
+        len -= 8;
+    }
+
+    for (i, c) in start.iter().enumerate() {
+        if *c == needle {
+            return Some((start.as_ptr() as usize - ptr as usize) / 2 + i);
+        }
+    }
+    None
+}
+
 pub fn to_u16s<S: AsRef<OsStr>>(s: S) -> crate::io::Result<Vec<u16>> {
     fn inner(s: &OsStr) -> crate::io::Result<Vec<u16>> {
         let mut maybe_result: Vec<u16> = s.encode_wide().collect();
-        if maybe_result.iter().any(|&u| u == 0) {
+        if unrolled_find_u16s(0, &maybe_result).is_some() {
             return Err(crate::io::Error::new(
                 ErrorKind::InvalidInput,
                 "strings passed to WinAPI cannot contain NULs",
@@ -214,7 +258,7 @@ fn wide_char_to_multi_byte(
 }
 
 pub fn truncate_utf16_at_nul(v: &[u16]) -> &[u16] {
-    match v.iter().position(|c| *c == 0) {
+    match unrolled_find_u16s(0, v) {
         // don't include the 0
         Some(i) => &v[..i],
         None => v,