Optimize is_ascii for &str and &[u8]

1 files changed, 101 insertions, 1 deletions

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index e7a2d7adede..bed8495993f 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -2795,7 +2795,7 @@ impl [u8] {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn is_ascii(&self) -> bool {
-        self.iter().all(|b| b.is_ascii())
+        is_ascii(self)
     }
 
     /// Checks that two slices are an ASCII case-insensitive match.
@@ -2843,6 +2843,106 @@ impl [u8] {
     }
 }
 
+/// Returns `true` if any byte in the word `v` is nonascii (>= 128). Snarfed
+/// from `../str/mod.rs`, which does something similar for utf8 validation.
+#[inline]
+fn contains_nonascii(v: usize) -> bool {
+    const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;
+    (NONASCII_MASK & v) != 0
+}
+
+/// Optimized ASCII test that will use usize-at-a-time operations instead of
+/// byte-at-a-time operations (when possible).
+///
+/// The algorithm we use here is pretty simple. If `s` is too short, we just
+/// check each byte and be done with it. Otherwise:
+///
+/// - Read the first word with an unaligned load.
+/// - Align the pointer, read subsequent words until end with aligned loads.
+/// - If there's a tail, the last `usize` from `s` with an unaligned load.
+///
+/// If any of these loads produces something for which `contains_nonascii`
+/// (above) returns true, then we know the answer is false.
+#[inline]
+fn is_ascii(s: &[u8]) -> bool {
+    const USIZE_SIZE: usize = mem::size_of::<usize>();
+
+    let len = s.len();
+    let align_offset = s.as_ptr().align_offset(USIZE_SIZE);
+
+    // If we wouldn't gain anything from the word-at-a-time implementation, fall
+    // back to a scalar loop.
+    //
+    // We also do this for architectures where `size_of::<usize>()` isn't
+    // sufficient alignment for `usize`, because it's a weird edge case.
+    if len < USIZE_SIZE || len < align_offset || USIZE_SIZE < mem::align_of::<usize>() {
+        return s.iter().all(|b| b.is_ascii());
+    }
+
+    // We always read the first word unaligned, which means `align_offset` is
+    // 0, we'd read the same value again for the aligned read.
+    let offset_to_aligned = if align_offset == 0 { USIZE_SIZE } else { align_offset };
+
+    let start = s.as_ptr();
+    // SAFETY: We verify `len < USIZE_SIZE` above.
+    let first_word = unsafe { (start as *const usize).read_unaligned() };
+
+    if contains_nonascii(first_word) {
+        return false;
+    }
+    // We checked this above, somewhat implicitly. Note that `offset_to_aligned`
+    // is either `align_offset` or `USIZE_SIZE`, both of are explicitly checked
+    // above.
+    debug_assert!(offset_to_aligned <= len);
+
+    // word_ptr is the (properly aligned) usize ptr we use to read the middle chunk of the slice.
+    let mut word_ptr = unsafe { start.add(offset_to_aligned) as *const usize };
+
+    // `byte_pos` is the byte index of `word_ptr`, used for loop end checks.
+    let mut byte_pos = offset_to_aligned;
+
+    // Paranoia check about alignment, since we're about to do a bunch of
+    // unaligned loads. In practice this should be impossible barring a bug in
+    // `align_offset` though.
+    debug_assert_eq!((word_ptr as usize) % mem::align_of::<usize>(), 0);
+
+    while byte_pos <= len - USIZE_SIZE {
+        debug_assert!(
+            // Sanity check that the read is in bounds
+            (word_ptr as usize + USIZE_SIZE) <= (start.wrapping_add(len) as usize) &&
+            // And that our assumptions about `byte_pos` hold.
+            (word_ptr as usize) - (start as usize) == byte_pos
+        );
+
+        // Safety: We know `word_ptr` is properly aligned (because of
+        // `align_offset`), and we know that we have enough bytes between `word_ptr` and the end
+        let word = unsafe { word_ptr.read() };
+        if contains_nonascii(word) {
+            return false;
+        }
+
+        byte_pos += USIZE_SIZE;
+        // SAFETY: We know that `byte_pos <= len - USIZE_SIZE`, which means that
+        // after this `add`, `word_ptr` will be at most one-past-the-end.
+        word_ptr = unsafe { word_ptr.add(1) };
+    }
+
+    // If we have anything left over, it should be at-most 1 usize worth of bytes,
+    // which we check with a read_unaligned.
+    if byte_pos == len {
+        return true;
+    }
+
+    // Sanity check to ensure there really is only one `usize` left. This should
+    // be guaranteed by our loop condition.
+    debug_assert!(byte_pos < len && len - byte_pos < USIZE_SIZE);
+
+    // SAFETY: This relies on `len >= USIZE_SIZE`, which we check at the start.
+    let last_word = unsafe { (start.add(len - USIZE_SIZE) as *const usize).read_unaligned() };
+
+    !contains_nonascii(last_word)
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, I> ops::Index<I> for [T]
 where