bstr/src/ascii.rs

use core::mem;

// The following ~400 lines of code exists for exactly one purpose, which is
// to optimize this code:
//
//     byte_slice.iter().position(|&b| b > 0x7F).unwrap_or(byte_slice.len())
//
// Yes... Overengineered is a word that comes to mind, but this is effectively
// a very similar problem to memchr, and virtually nobody has been able to
// resist optimizing the crap out of that (except for perhaps the BSD and MUSL
// folks). In particular, this routine makes a very common case (ASCII) very
// fast, which seems worth it. We do stop short of adding AVX variants of the
// code below in order to retain our sanity and also to avoid needing to deal
// with runtime target feature detection. RESIST!
//
// In order to understand the SIMD version below, it would be good to read this
// comment describing how my memchr routine works:
// https://github.com/BurntSushi/rust-memchr/blob/b0a29f267f4a7fad8ffcc8fe8377a06498202883/src/x86/sse2.rs#L19-L106
//
// The primary difference with memchr is that for ASCII, we can do a bit less
// work. In particular, we don't need to detect the presence of a specific
// byte, but rather, whether any byte has its most significant bit set. That
// means we can effectively skip the _mm_cmpeq_epi8 step and jump straight to
// _mm_movemask_epi8.

#[cfg(any(test, not(target_arch = "x86_64")))]
const USIZE_BYTES: usize = mem::size_of::<usize>();
#[cfg(any(test, not(target_arch = "x86_64")))]
const FALLBACK_LOOP_SIZE: usize = 2 * USIZE_BYTES;

// This is a mask where the most significant bit of each byte in the usize
// is set. We test this bit to determine whether a character is ASCII or not.
// Namely, a single byte is regarded as an ASCII codepoint if and only if it's
// most significant bit is not set.
#[cfg(any(test, not(target_arch = "x86_64")))]
const ASCII_MASK_U64: u64 = 0x8080808080808080;
#[cfg(any(test, not(target_arch = "x86_64")))]
const ASCII_MASK: usize = ASCII_MASK_U64 as usize;

/// Returns the index of the first non ASCII byte in the given slice.
///
/// If slice only contains ASCII bytes, then the length of the slice is
/// returned.
pub fn first_non_ascii_byte(slice: &[u8]) -> usize {
    #[cfg(not(target_arch = "x86_64"))]
    {
        first_non_ascii_byte_fallback(slice)
    }

    #[cfg(target_arch = "x86_64")]
    {
        first_non_ascii_byte_sse2(slice)
    }
}

#[cfg(any(test, not(target_arch = "x86_64")))]
fn first_non_ascii_byte_fallback(slice: &[u8]) -> usize {
    let align = USIZE_BYTES - 1;
    let start_ptr = slice.as_ptr();
    let end_ptr = slice[slice.len()..].as_ptr();
    let mut ptr = start_ptr;

    unsafe {
        if slice.len() < USIZE_BYTES {
            return first_non_ascii_byte_slow(start_ptr, end_ptr, ptr);
        }

        let chunk = read_unaligned_usize(ptr);
        let mask = chunk & ASCII_MASK;
        if mask != 0 {
            return first_non_ascii_byte_mask(mask);
        }

        ptr = ptr_add(ptr, USIZE_BYTES - (start_ptr as usize & align));
        debug_assert!(ptr > start_ptr);
        debug_assert!(ptr_sub(end_ptr, USIZE_BYTES) >= start_ptr);
        if slice.len() >= FALLBACK_LOOP_SIZE {
            while ptr <= ptr_sub(end_ptr, FALLBACK_LOOP_SIZE) {
                debug_assert_eq!(0, (ptr as usize) % USIZE_BYTES);

                let a = *(ptr as *const usize);
                let b = *(ptr_add(ptr, USIZE_BYTES) as *const usize);
                if (a | b) & ASCII_MASK != 0 {
                    // What a kludge. We wrap the position finding code into
                    // a non-inlineable function, which makes the codegen in
                    // the tight loop above a bit better by avoiding a
                    // couple extra movs. We pay for it by two additional
                    // stores, but only in the case of finding a non-ASCII
                    // byte.
                    #[inline(never)]
                    unsafe fn findpos(
                        start_ptr: *const u8,
                        ptr: *const u8,
                    ) -> usize {
                        let a = *(ptr as *const usize);
                        let b = *(ptr_add(ptr, USIZE_BYTES) as *const usize);

                        let mut at = sub(ptr, start_ptr);
                        let maska = a & ASCII_MASK;
                        if maska != 0 {
                            return at + first_non_ascii_byte_mask(maska);
                        }

                        at += USIZE_BYTES;
                        let maskb = b & ASCII_MASK;
                        debug_assert!(maskb != 0);
                        return at + first_non_ascii_byte_mask(maskb);
                    }
                    return findpos(start_ptr, ptr);
                }
                ptr = ptr_add(ptr, FALLBACK_LOOP_SIZE);
            }
        }
        first_non_ascii_byte_slow(start_ptr, end_ptr, ptr)
    }
}

#[cfg(target_arch = "x86_64")]
fn first_non_ascii_byte_sse2(slice: &[u8]) -> usize {
    use core::arch::x86_64::*;

    const VECTOR_SIZE: usize = mem::size_of::<__m128i>();
    const VECTOR_ALIGN: usize = VECTOR_SIZE - 1;
    const VECTOR_LOOP_SIZE: usize = 4 * VECTOR_SIZE;

    let start_ptr = slice.as_ptr();
    let end_ptr = slice[slice.len()..].as_ptr();
    let mut ptr = start_ptr;

    unsafe {
        if slice.len() < VECTOR_SIZE {
            return first_non_ascii_byte_slow(start_ptr, end_ptr, ptr);
        }

        let chunk = _mm_loadu_si128(ptr as *const __m128i);
        let mask = _mm_movemask_epi8(chunk);
        if mask != 0 {
            return mask.trailing_zeros() as usize;
        }

        ptr = ptr.add(VECTOR_SIZE - (start_ptr as usize & VECTOR_ALIGN));
        debug_assert!(ptr > start_ptr);
        debug_assert!(end_ptr.sub(VECTOR_SIZE) >= start_ptr);
        if slice.len() >= VECTOR_LOOP_SIZE {
            while ptr <= ptr_sub(end_ptr, VECTOR_LOOP_SIZE) {
                debug_assert_eq!(0, (ptr as usize) % VECTOR_SIZE);

                let a = _mm_load_si128(ptr as *const __m128i);
                let b = _mm_load_si128(ptr.add(VECTOR_SIZE) as *const __m128i);
                let c =
                    _mm_load_si128(ptr.add(2 * VECTOR_SIZE) as *const __m128i);
                let d =
                    _mm_load_si128(ptr.add(3 * VECTOR_SIZE) as *const __m128i);

                let or1 = _mm_or_si128(a, b);
                let or2 = _mm_or_si128(c, d);
                let or3 = _mm_or_si128(or1, or2);
                if _mm_movemask_epi8(or3) != 0 {
                    let mut at = sub(ptr, start_ptr);
                    let mask = _mm_movemask_epi8(a);
                    if mask != 0 {
                        return at + mask.trailing_zeros() as usize;
                    }

                    at += VECTOR_SIZE;
                    let mask = _mm_movemask_epi8(b);
                    if mask != 0 {
                        return at + mask.trailing_zeros() as usize;
                    }

                    at += VECTOR_SIZE;
                    let mask = _mm_movemask_epi8(c);
                    if mask != 0 {
                        return at + mask.trailing_zeros() as usize;
                    }

                    at += VECTOR_SIZE;
                    let mask = _mm_movemask_epi8(d);
                    debug_assert!(mask != 0);
                    return at + mask.trailing_zeros() as usize;
                }
                ptr = ptr_add(ptr, VECTOR_LOOP_SIZE);
            }
        }
        while ptr <= end_ptr.sub(VECTOR_SIZE) {
            debug_assert!(sub(end_ptr, ptr) >= VECTOR_SIZE);

            let chunk = _mm_loadu_si128(ptr as *const __m128i);
            let mask = _mm_movemask_epi8(chunk);
            if mask != 0 {
                return sub(ptr, start_ptr) + mask.trailing_zeros() as usize;
            }
            ptr = ptr.add(VECTOR_SIZE);
        }
        first_non_ascii_byte_slow(start_ptr, end_ptr, ptr)
    }
}

#[inline(always)]
unsafe fn first_non_ascii_byte_slow(
    start_ptr: *const u8,
    end_ptr: *const u8,
    mut ptr: *const u8,
) -> usize {
    debug_assert!(start_ptr <= ptr);
    debug_assert!(ptr <= end_ptr);

    while ptr < end_ptr {
        if *ptr > 0x7F {
            return sub(ptr, start_ptr);
        }
        ptr = ptr.offset(1);
    }
    sub(end_ptr, start_ptr)
}

/// Compute the position of the first ASCII byte in the given mask.
///
/// The mask should be computed by `chunk & ASCII_MASK`, where `chunk` is
/// 8 contiguous bytes of the slice being checked where *at least* one of those
/// bytes is not an ASCII byte.
///
/// The position returned is always in the inclusive range [0, 7].
#[cfg(any(test, not(target_arch = "x86_64")))]
fn first_non_ascii_byte_mask(mask: usize) -> usize {
    #[cfg(target_endian = "little")]
    {
        mask.trailing_zeros() as usize / 8
    }
    #[cfg(target_endian = "big")]
    {
        mask.leading_zeros() as usize / 8
    }
}

/// Increment the given pointer by the given amount.
unsafe fn ptr_add(ptr: *const u8, amt: usize) -> *const u8 {
    debug_assert!(amt < ::core::isize::MAX as usize);
    ptr.offset(amt as isize)
}

/// Decrement the given pointer by the given amount.
unsafe fn ptr_sub(ptr: *const u8, amt: usize) -> *const u8 {
    debug_assert!(amt < ::core::isize::MAX as usize);
    ptr.offset((amt as isize).wrapping_neg())
}

#[cfg(any(test, not(target_arch = "x86_64")))]
unsafe fn read_unaligned_usize(ptr: *const u8) -> usize {
    use core::ptr;

    let mut n: usize = 0;
    ptr::copy_nonoverlapping(ptr, &mut n as *mut _ as *mut u8, USIZE_BYTES);
    n
}

/// Subtract `b` from `a` and return the difference. `a` should be greater than
/// or equal to `b`.
fn sub(a: *const u8, b: *const u8) -> usize {
    debug_assert!(a >= b);
    (a as usize) - (b as usize)
}

#[cfg(test)]
mod tests {
    use super::*;

    // Our testing approach here is to try and exhaustively test every case.
    // This includes the position at which a non-ASCII byte occurs in addition
    // to the alignment of the slice that we're searching.

    #[test]
    fn positive_fallback_forward() {
        for i in 0..517 {
            let s = "a".repeat(i);
            assert_eq!(
                i,
                first_non_ascii_byte_fallback(s.as_bytes()),
                "i: {:?}, len: {:?}, s: {:?}",
                i,
                s.len(),
                s
            );
        }
    }

    #[test]
    #[cfg(target_arch = "x86_64")]
    fn positive_sse2_forward() {
        for i in 0..517 {
            let b = "a".repeat(i).into_bytes();
            assert_eq!(b.len(), first_non_ascii_byte_sse2(&b));
        }
    }

    #[test]
    fn negative_fallback_forward() {
        for i in 0..517 {
            for align in 0..65 {
                let mut s = "a".repeat(i);
                s.push_str("☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃");
                let s = s.get(align..).unwrap_or("");
                assert_eq!(
                    i.saturating_sub(align),
                    first_non_ascii_byte_fallback(s.as_bytes()),
                    "i: {:?}, align: {:?}, len: {:?}, s: {:?}",
                    i,
                    align,
                    s.len(),
                    s
                );
            }
        }
    }

    #[test]
    #[cfg(target_arch = "x86_64")]
    fn negative_sse2_forward() {
        for i in 0..517 {
            for align in 0..65 {
                let mut s = "a".repeat(i);
                s.push_str("☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃☃");
                let s = s.get(align..).unwrap_or("");
                assert_eq!(
                    i.saturating_sub(align),
                    first_non_ascii_byte_sse2(s.as_bytes()),
                    "i: {:?}, align: {:?}, len: {:?}, s: {:?}",
                    i,
                    align,
                    s.len(),
                    s
                );
            }
        }
    }
}