Rollup merge of #74066 - thomcc:optimize-is-ascii, r=nagisa

Optimize is_ascii for str and [u8].

This optimizes the `is_ascii` function for `[u8]` and `str`. I've been surprised this wasn't done for a while, so I just did it.

Benchmarks comparing before/after look like:

```
test ascii::long_readonly::is_ascii_slice_iter_all              ... bench:         174 ns/iter (+/- 79) = 40172 MB/s
test ascii::long_readonly::is_ascii_slice_libcore               ... bench:          16 ns/iter (+/- 5) = 436875 MB/s
test ascii::medium_readonly::is_ascii_slice_iter_all            ... bench:          12 ns/iter (+/- 3) = 2666 MB/s
test ascii::medium_readonly::is_ascii_slice_libcore             ... bench:           2 ns/iter (+/- 0) = 16000 MB/s
test ascii::short_readonly::is_ascii_slice_iter_all             ... bench:           3 ns/iter (+/- 0) = 2333 MB/s
test ascii::short_readonly::is_ascii_slice_libcore              ... bench:           4 ns/iter (+/- 0) = 1750 MB/s
```

(Taken on a x86_64 macbook 2.9 GHz Intel Core i9 with 6 cores)

Where `is_ascii_slice_iter_all` is the old version, and `is_ascii_slice_libcore` is the new.

I tried to document the code well, so hopefully it's understandable. It has fairly exhaustive tests ensuring size/align doesn't get violated -- because `miri` doesn't really help a lot for this sort of code right now, I tried to `debug_assert` all the safety invariants I'm depending on. (Of course, none of them are required for correctness or soundness -- just allows us to test that this sort of pointer manipulation is sound and such).

Anyway, thanks. Let me know if you have questions/desired changes.

src/libcore/benches/ascii.rs

@@ -1,3 +1,5 @@
+mod is_ascii;
+
 // Lower-case ASCII 'a' is the first byte that has its highest bit set
 // after wrap-adding 0x1F:
 //

src/libcore/benches/ascii/is_ascii.rs

@@ -0,0 +1,82 @@
+use super::{LONG, MEDIUM, SHORT};
+use test::black_box;
+use test::Bencher;
+
+macro_rules! benches {
+    ($( fn $name: ident($arg: ident: &[u8]) $body: block )+) => {
+        benches!(mod short SHORT[..] $($name $arg $body)+);
+        benches!(mod medium MEDIUM[..] $($name $arg $body)+);
+        benches!(mod long LONG[..] $($name $arg $body)+);
+        // Ensure we benchmark cases where the functions are called with strings
+        // that are not perfectly aligned or have a length which is not a
+        // multiple of size_of::<usize>() (or both)
+        benches!(mod unaligned_head MEDIUM[1..] $($name $arg $body)+);
+        benches!(mod unaligned_tail MEDIUM[..(MEDIUM.len() - 1)] $($name $arg $body)+);
+        benches!(mod unaligned_both MEDIUM[1..(MEDIUM.len() - 1)] $($name $arg $body)+);
+    };
+
+    (mod $mod_name: ident $input: ident [$range: expr] $($name: ident $arg: ident $body: block)+) => {
+        mod $mod_name {
+            use super::*;
+            $(
+                #[bench]
+                fn $name(bencher: &mut Bencher) {
+                    bencher.bytes = $input[$range].len() as u64;
+                    let mut vec = $input.as_bytes().to_vec();
+                    bencher.iter(|| {
+                        let $arg: &[u8] = &black_box(&mut vec)[$range];
+                        black_box($body)
+                    })
+                }
+            )+
+        }
+    };
+}
+
+benches! {
+    fn case00_libcore(bytes: &[u8]) {
+        bytes.is_ascii()
+    }
+
+    fn case01_iter_all(bytes: &[u8]) {
+        bytes.iter().all(|b| b.is_ascii())
+    }
+
+    fn case02_align_to(bytes: &[u8]) {
+        is_ascii_align_to(bytes)
+    }
+
+    fn case03_align_to_unrolled(bytes: &[u8]) {
+        is_ascii_align_to_unrolled(bytes)
+    }
+}
+
+// These are separate since it's easier to debug errors if they don't go through
+// macro expansion first.
+fn is_ascii_align_to(bytes: &[u8]) -> bool {
+    if bytes.len() < core::mem::size_of::<usize>() {
+        return bytes.iter().all(|b| b.is_ascii());
+    }
+    // SAFETY: transmuting a sequence of `u8` to `usize` is always fine
+    let (head, body, tail) = unsafe { bytes.align_to::<usize>() };
+    head.iter().all(|b| b.is_ascii())
+        && body.iter().all(|w| !contains_nonascii(*w))
+        && tail.iter().all(|b| b.is_ascii())
+}
+
+fn is_ascii_align_to_unrolled(bytes: &[u8]) -> bool {
+    if bytes.len() < core::mem::size_of::<usize>() {
+        return bytes.iter().all(|b| b.is_ascii());
+    }
+    // SAFETY: transmuting a sequence of `u8` to `[usize; 2]` is always fine
+    let (head, body, tail) = unsafe { bytes.align_to::<[usize; 2]>() };
+    head.iter().all(|b| b.is_ascii())
+        && body.iter().all(|w| !contains_nonascii(w[0] | w[1]))
+        && tail.iter().all(|b| b.is_ascii())
+}
+
+#[inline]
+fn contains_nonascii(v: usize) -> bool {
+    const NONASCII_MASK: usize = 0x80808080_80808080u64 as usize;
+    (NONASCII_MASK & v) != 0
+}

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

src/libcore/str/mod.rs

@@ -4348,7 +4348,7 @@ impl str {
         // We can treat each byte as character here: all multibyte characters
         // start with a byte that is not in the ascii range, so we will stop
         // there already.
-        self.bytes().all(|b| b.is_ascii())
+        self.as_bytes().is_ascii()
     }
 
     /// Checks that two strings are an ASCII case-insensitive match.

src/libcore/tests/ascii.rs

@@ -343,3 +343,59 @@ fn test_is_ascii_control() {
         " ",
     );
 }
+
+// `is_ascii` does a good amount of pointer manipulation and has
+// alignment-dependent computation. This is all sanity-checked via
+// `debug_assert!`s, so we test various sizes/alignments thoroughly versus an
+// "obviously correct" baseline function.
+#[test]
+fn test_is_ascii_align_size_thoroughly() {
+    // The "obviously-correct" baseline mentioned above.
+    fn is_ascii_baseline(s: &[u8]) -> bool {
+        s.iter().all(|b| b.is_ascii())
+    }
+
+    // Helper to repeat `l` copies of `b0` followed by `l` copies of `b1`.
+    fn repeat_concat(b0: u8, b1: u8, l: usize) -> Vec<u8> {
+        use core::iter::repeat;
+        repeat(b0).take(l).chain(repeat(b1).take(l)).collect()
+    }
+
+    // Miri is too slow for much of this, and in miri `align_offset` always
+    // returns `usize::max_value()` anyway (at the moment), so we just test
+    // lightly.
+    let iter = if cfg!(miri) { 0..5 } else { 0..100 };
+
+    for i in iter {
+        #[cfg(not(miri))]
+        let cases = &[
+            b"a".repeat(i),
+            b"\0".repeat(i),
+            b"\x7f".repeat(i),
+            b"\x80".repeat(i),
+            b"\xff".repeat(i),
+            repeat_concat(b'a', 0x80u8, i),
+            repeat_concat(0x80u8, b'a', i),
+        ];
+
+        #[cfg(miri)]
+        let cases = &[repeat_concat(b'a', 0x80u8, i)];
+
+        for case in cases {
+            for pos in 0..=case.len() {
+                // Potentially misaligned head
+                let prefix = &case[pos..];
+                assert_eq!(is_ascii_baseline(prefix), prefix.is_ascii(),);
+
+                // Potentially misaligned tail
+                let suffix = &case[..case.len() - pos];
+
+                assert_eq!(is_ascii_baseline(suffix), suffix.is_ascii(),);
+
+                // Both head and tail are potentially misaligned
+                let mid = &case[(pos / 2)..(case.len() - (pos / 2))];
+                assert_eq!(is_ascii_baseline(mid), mid.is_ascii(),);
+            }
+        }
+    }
+}