rustc_stable_hash/

sip128.rs

//! This is a copy of `core::hash::sip` adapted to providing 128 bit hashes.

// This code is very hot and uses lots of arithmetic, avoid overflow checks for performance.
// See https://github.com/rust-lang/rust/pull/119440#issuecomment-1874255727
use crate::int_overflow::{DebugStrictAdd, DebugStrictSub};
use crate::ExtendedHasher;

use std::hash::Hasher;
use std::mem::{self, MaybeUninit};
use std::ptr;

#[cfg(test)]
mod tests;

// The SipHash algorithm operates on 8-byte chunks.
const ELEM_SIZE: usize = mem::size_of::<u64>();

// Size of the buffer in number of elements, not including the spill.
//
// The selection of this size was guided by rustc-perf benchmark comparisons of
// different buffer sizes. It should be periodically reevaluated as the compiler
// implementation and input characteristics change.
//
// Using the same-sized buffer for everything we hash is a performance versus
// complexity tradeoff. The ideal buffer size, and whether buffering should even
// be used, depends on what is being hashed. It may be worth it to size the
// buffer appropriately (perhaps by making SipHasher128 generic over the buffer
// size) or disable buffering depending on what is being hashed. But at this
// time, we use the same buffer size for everything.
const BUFFER_CAPACITY: usize = 8;

// Size of the buffer in bytes, not including the spill.
const BUFFER_SIZE: usize = BUFFER_CAPACITY * ELEM_SIZE;

// Size of the buffer in number of elements, including the spill.
const BUFFER_WITH_SPILL_CAPACITY: usize = BUFFER_CAPACITY + 1;

// Size of the buffer in bytes, including the spill.
const BUFFER_WITH_SPILL_SIZE: usize = BUFFER_WITH_SPILL_CAPACITY * ELEM_SIZE;

// Index of the spill element in the buffer.
const BUFFER_SPILL_INDEX: usize = BUFFER_WITH_SPILL_CAPACITY - 1;

/// Hashing result of [`SipHasher128`]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SipHasher128Hash(pub [u64; 2]);

#[derive(Debug, Clone)]
#[repr(C)]
pub struct SipHasher128 {
    // The access pattern during hashing consists of accesses to `nbuf` and
    // `buf` until the buffer is full, followed by accesses to `state` and
    // `processed`, and then repetition of that pattern until hashing is done.
    // This is the basis for the ordering of fields below. However, in practice
    // the cache miss-rate for data access is extremely low regardless of order.
    nbuf: usize, // how many bytes in buf are valid
    buf: [MaybeUninit<u64>; BUFFER_WITH_SPILL_CAPACITY], // unprocessed bytes le
    state: State, // hash State
    processed: usize, // how many bytes we've processed
}

#[derive(Debug, Clone, Copy)]
#[repr(C)]
struct State {
    // v0, v2 and v1, v3 show up in pairs in the algorithm,
    // and simd implementations of SipHash will use vectors
    // of v02 and v13. By placing them in this order in the struct,
    // the compiler can pick up on just a few simd optimizations by itself.
    v0: u64,
    v2: u64,
    v1: u64,
    v3: u64,
}

macro_rules! compress {
    ($state:expr) => {{
        compress!($state.v0, $state.v1, $state.v2, $state.v3)
    }};
    ($v0:expr, $v1:expr, $v2:expr, $v3:expr) => {{
        $v0 = $v0.wrapping_add($v1);
        $v2 = $v2.wrapping_add($v3);
        $v1 = $v1.rotate_left(13);
        $v1 ^= $v0;
        $v3 = $v3.rotate_left(16);
        $v3 ^= $v2;
        $v0 = $v0.rotate_left(32);

        $v2 = $v2.wrapping_add($v1);
        $v0 = $v0.wrapping_add($v3);
        $v1 = $v1.rotate_left(17);
        $v1 ^= $v2;
        $v3 = $v3.rotate_left(21);
        $v3 ^= $v0;
        $v2 = $v2.rotate_left(32);
    }};
}

// Copies up to 8 bytes from source to destination. This performs better than
// `ptr::copy_nonoverlapping` on microbenchmarks and may perform better on real
// workloads since all of the copies have fixed sizes and avoid calling memcpy.
//
// This is specifically designed for copies of up to 8 bytes, because that's the
// maximum of number bytes needed to fill an 8-byte-sized element on which
// SipHash operates. Note that for variable-sized copies which are known to be
// less than 8 bytes, this function will perform more work than necessary unless
// the compiler is able to optimize the extra work away.
#[inline]
unsafe fn copy_nonoverlapping_small(src: *const u8, dst: *mut u8, count: usize) {
    debug_assert!(count <= 8);

    unsafe {
        if count == 8 {
            ptr::copy_nonoverlapping(src, dst, 8);
            return;
        }

        let mut i = 0;
        if i.debug_strict_add(3) < count {
            ptr::copy_nonoverlapping(src.add(i), dst.add(i), 4);
            i = i.debug_strict_add(4);
        }

        if i.debug_strict_add(1) < count {
            ptr::copy_nonoverlapping(src.add(i), dst.add(i), 2);
            i = i.debug_strict_add(2)
        }

        if i < count {
            *dst.add(i) = *src.add(i);
            i = i.debug_strict_add(1);
        }

        debug_assert_eq!(i, count);
    }
}

// # Implementation
//
// This implementation uses buffering to reduce the hashing cost for inputs
// consisting of many small integers. Buffering simplifies the integration of
// integer input--the integer write function typically just appends to the
// buffer with a statically sized write, updates metadata, and returns.
//
// Buffering also prevents alternating between writes that do and do not trigger
// the hashing process. Only when the entire buffer is full do we transition
// into hashing. This allows us to keep the hash state in registers for longer,
// instead of loading and storing it before and after processing each element.
//
// When a write fills the buffer, a buffer processing function is invoked to
// hash all of the buffered input. The buffer processing functions are marked
// `#[inline(never)]` so that they aren't inlined into the append functions,
// which ensures the more frequently called append functions remain inlineable
// and don't include register pushing/popping that would only be made necessary
// by inclusion of the complex buffer processing path which uses those
// registers.
//
// The buffer includes a "spill"--an extra element at the end--which simplifies
// the integer write buffer processing path. The value that fills the buffer can
// be written with a statically sized write that may spill over into the spill.
// After the buffer is processed, the part of the value that spilled over can be
// written from the spill to the beginning of the buffer with another statically
// sized write. This write may copy more bytes than actually spilled over, but
// we maintain the metadata such that any extra copied bytes will be ignored by
// subsequent processing. Due to the static sizes, this scheme performs better
// than copying the exact number of bytes needed into the end and beginning of
// the buffer.
//
// The buffer is uninitialized, which improves performance, but may preclude
// efficient implementation of alternative approaches. The improvement is not so
// large that an alternative approach should be disregarded because it cannot be
// efficiently implemented with an uninitialized buffer. On the other hand, an
// uninitialized buffer may become more important should a larger one be used.
//
// # Platform Dependence
//
// The SipHash algorithm operates on byte sequences. It parses the input stream
// as 8-byte little-endian integers. Therefore, given the same byte sequence, it
// produces the same result on big- and little-endian hardware.
//
// However, the Hasher trait has methods which operate on multi-byte integers.
// How they are converted into byte sequences can be endian-dependent (by using
// native byte order) or independent (by consistently using either LE or BE byte
// order). It can also be `isize` and `usize` size dependent (by using the
// native size), or independent (by converting to a common size), supposing the
// values can be represented in 32 bits.
//
// In order to make `SipHasher128` consistent with `SipHasher` in libstd, we
// choose to do the integer to byte sequence conversion in the platform-
// dependent way. Clients can achieve platform-independent hashing by widening
// `isize` and `usize` integers to 64 bits on 32-bit systems and byte-swapping
// integers on big-endian systems before passing them to the writing functions.
// This causes the input byte sequence to look identical on big- and little-
// endian systems (supposing `isize` and `usize` values can be represented in 32
// bits), which ensures platform-independent results.
impl SipHasher128 {
    #[inline]
    pub fn new_with_keys(key0: u64, key1: u64) -> SipHasher128 {
        let mut hasher = SipHasher128 {
            nbuf: 0,
            // HACK: Manual MaybeUninit::uninit_array, use inline const with Rust 1.79
            buf: unsafe {
                MaybeUninit::<[MaybeUninit<_>; BUFFER_WITH_SPILL_CAPACITY]>::uninit().assume_init()
            },
            state: State {
                v0: key0 ^ 0x736f6d6570736575,
                // The XOR with 0xee is only done on 128-bit algorithm version.
                v1: key1 ^ (0x646f72616e646f6d ^ 0xee),
                v2: key0 ^ 0x6c7967656e657261,
                v3: key1 ^ 0x7465646279746573,
            },
            processed: 0,
        };

        unsafe {
            // Initialize spill because we read from it in `short_write_process_buffer`.
            *hasher.buf.get_unchecked_mut(BUFFER_SPILL_INDEX) = MaybeUninit::zeroed();
        }

        hasher
    }

    // A specialized write function for values with size <= 8 that should only
    // be called when the write would cause the buffer to fill.
    //
    // SAFETY: the write of `x` into `self.buf` starting at byte offset
    // `self.nbuf` must cause `self.buf` to become fully initialized (and not
    // overflow) if it wasn't already.
    #[inline(never)]
    unsafe fn short_write_process_buffer<const LEN: usize>(&mut self, bytes: [u8; LEN]) {
        unsafe {
            let nbuf = self.nbuf;
            debug_assert!(LEN <= 8);
            debug_assert!(nbuf < BUFFER_SIZE);
            debug_assert!(nbuf + LEN >= BUFFER_SIZE);
            debug_assert!(nbuf + LEN < BUFFER_WITH_SPILL_SIZE);

            // Copy first part of input into end of buffer, possibly into spill
            // element. The memcpy call is optimized away because the size is known.
            let dst = (self.buf.as_mut_ptr() as *mut u8).add(nbuf);
            ptr::copy_nonoverlapping(bytes.as_ptr(), dst, LEN);

            // Process buffer.
            for i in 0..BUFFER_CAPACITY {
                let elem = self.buf.get_unchecked(i).assume_init().to_le();
                self.state.v3 ^= elem;
                Sip13Rounds::c_rounds(&mut self.state);
                self.state.v0 ^= elem;
            }

            // Copy remaining input into start of buffer by copying LEN - 1
            // elements from spill (at most LEN - 1 bytes could have overflowed
            // into the spill). The memcpy call is optimized away because the size
            // is known. And the whole copy is optimized away for LEN == 1.
            let dst = self.buf.as_mut_ptr() as *mut u8;
            let src = self.buf.get_unchecked(BUFFER_SPILL_INDEX) as *const _ as *const u8;
            ptr::copy_nonoverlapping(src, dst, LEN - 1);

            // This function should only be called when the write fills the buffer.
            // Therefore, when LEN == 1, the new `self.nbuf` must be zero.
            // LEN is statically known, so the branch is optimized away.
            self.nbuf = if LEN == 1 {
                0
            } else {
                nbuf.debug_strict_add(LEN).debug_strict_sub(BUFFER_SIZE)
            };
            self.processed = self.processed.debug_strict_add(BUFFER_SIZE);
        }
    }

    // A write function for byte slices.
    #[inline]
    fn slice_write(&mut self, msg: &[u8]) {
        let length = msg.len();
        let nbuf = self.nbuf;
        debug_assert!(nbuf < BUFFER_SIZE);

        if nbuf.debug_strict_add(length) < BUFFER_SIZE {
            unsafe {
                let dst = (self.buf.as_mut_ptr() as *mut u8).add(nbuf);

                if length <= 8 {
                    copy_nonoverlapping_small(msg.as_ptr(), dst, length);
                } else {
                    // This memcpy is *not* optimized away.
                    ptr::copy_nonoverlapping(msg.as_ptr(), dst, length);
                }
            }

            self.nbuf = nbuf.debug_strict_add(length);

            return;
        }

        unsafe { self.slice_write_process_buffer(msg) }
    }

    // A write function for byte slices that should only be called when the
    // write would cause the buffer to fill.
    //
    // SAFETY: `self.buf` must be initialized up to the byte offset `self.nbuf`,
    // and `msg` must contain enough bytes to initialize the rest of the element
    // containing the byte offset `self.nbuf`.
    #[inline(never)]
    unsafe fn slice_write_process_buffer(&mut self, msg: &[u8]) {
        unsafe {
            let length = msg.len();
            let nbuf = self.nbuf;
            debug_assert!(nbuf < BUFFER_SIZE);
            debug_assert!(nbuf + length >= BUFFER_SIZE);

            // Always copy first part of input into current element of buffer.
            // This function should only be called when the write fills the buffer,
            // so we know that there is enough input to fill the current element.
            let valid_in_elem = nbuf % ELEM_SIZE;
            let needed_in_elem = ELEM_SIZE.debug_strict_sub(valid_in_elem);

            let src = msg.as_ptr();
            let dst = (self.buf.as_mut_ptr() as *mut u8).add(nbuf);
            copy_nonoverlapping_small(src, dst, needed_in_elem);

            // Process buffer.

            // Using `nbuf / ELEM_SIZE + 1` rather than `(nbuf + needed_in_elem) /
            // ELEM_SIZE` to show the compiler that this loop's upper bound is > 0.
            // We know that is true, because last step ensured we have a full
            // element in the buffer.
            let last = (nbuf / ELEM_SIZE).debug_strict_add(1);

            for i in 0..last {
                let elem = self.buf.get_unchecked(i).assume_init().to_le();
                self.state.v3 ^= elem;
                Sip13Rounds::c_rounds(&mut self.state);
                self.state.v0 ^= elem;
            }

            // Process the remaining element-sized chunks of input.
            let mut processed = needed_in_elem;
            let input_left = length.debug_strict_sub(processed);
            let elems_left = input_left / ELEM_SIZE;
            let extra_bytes_left = input_left % ELEM_SIZE;

            for _ in 0..elems_left {
                let elem = (msg.as_ptr().add(processed) as *const u64)
                    .read_unaligned()
                    .to_le();
                self.state.v3 ^= elem;
                Sip13Rounds::c_rounds(&mut self.state);
                self.state.v0 ^= elem;
                processed = processed.debug_strict_add(ELEM_SIZE);
            }

            // Copy remaining input into start of buffer.
            let src = msg.as_ptr().add(processed);
            let dst = self.buf.as_mut_ptr() as *mut u8;
            copy_nonoverlapping_small(src, dst, extra_bytes_left);

            self.nbuf = extra_bytes_left;
            self.processed = self
                .processed
                .debug_strict_add(nbuf.debug_strict_add(processed));
        }
    }

    // A function for finishing the hashing.
    //
    // SAFETY: `buf` must be initialized up to the byte offset `nbuf`.
    #[inline]
    unsafe fn finish128_inner(
        nbuf: usize,
        buf: &mut [MaybeUninit<u64>; BUFFER_WITH_SPILL_CAPACITY],
        mut state: State,
        processed: usize,
    ) -> [u64; 2] {
        debug_assert!(nbuf < BUFFER_SIZE);

        // Process full elements in buffer.
        let last = nbuf / ELEM_SIZE;

        for i in 0..last {
            let elem = unsafe { buf.get_unchecked(i).assume_init().to_le() };
            state.v3 ^= elem;
            Sip13Rounds::c_rounds(&mut state);
            state.v0 ^= elem;
        }

        // Get remaining partial element.
        let elem = if nbuf % ELEM_SIZE != 0 {
            unsafe {
                // Ensure element is initialized by writing zero bytes. At most
                // `ELEM_SIZE - 1` are required given the above check. It's safe
                // to write this many because we have the spill and we maintain
                // `self.nbuf` such that this write will start before the spill.
                let dst = (buf.as_mut_ptr() as *mut u8).add(nbuf);
                ptr::write_bytes(dst, 0, ELEM_SIZE - 1);
                buf.get_unchecked(last).assume_init().to_le()
            }
        } else {
            0
        };

        // Finalize the hash.
        let length = processed.debug_strict_add(nbuf);
        let b: u64 = ((length as u64 & 0xff) << 56) | elem;

        state.v3 ^= b;
        Sip13Rounds::c_rounds(&mut state);
        state.v0 ^= b;

        state.v2 ^= 0xee;
        Sip13Rounds::d_rounds(&mut state);
        let l = state.v0 ^ state.v1 ^ state.v2 ^ state.v3;

        state.v1 ^= 0xdd;
        Sip13Rounds::d_rounds(&mut state);
        let h = state.v0 ^ state.v1 ^ state.v2 ^ state.v3;

        [l, h]
    }
}

impl Default for SipHasher128 {
    fn default() -> SipHasher128 {
        SipHasher128::new_with_keys(0, 0)
    }
}

impl ExtendedHasher for SipHasher128 {
    type Hash = SipHasher128Hash;

    #[inline]
    fn short_write<const LEN: usize>(&mut self, bytes: [u8; LEN]) {
        let nbuf = self.nbuf;
        debug_assert!(LEN <= 8);
        debug_assert!(nbuf < BUFFER_SIZE);
        debug_assert!(nbuf + LEN < BUFFER_WITH_SPILL_SIZE);

        if nbuf.debug_strict_add(LEN) < BUFFER_SIZE {
            unsafe {
                // The memcpy call is optimized away because the size is known.
                let dst = (self.buf.as_mut_ptr() as *mut u8).add(nbuf);
                ptr::copy_nonoverlapping(bytes.as_ptr(), dst, LEN);
            }

            self.nbuf = nbuf.debug_strict_add(LEN);

            return;
        }

        unsafe { self.short_write_process_buffer(bytes) }
    }

    #[inline(always)]
    fn finish(mut self) -> SipHasher128Hash {
        SipHasher128Hash(unsafe {
            SipHasher128::finish128_inner(self.nbuf, &mut self.buf, self.state, self.processed)
        })
    }
}

impl Hasher for SipHasher128 {
    #[inline]
    fn write_u8(&mut self, i: u8) {
        self.short_write(i.to_ne_bytes());
    }

    #[inline]
    fn write_u16(&mut self, i: u16) {
        self.short_write(i.to_ne_bytes());
    }

    #[inline]
    fn write_u32(&mut self, i: u32) {
        self.short_write(i.to_ne_bytes());
    }

    #[inline]
    fn write_u64(&mut self, i: u64) {
        self.short_write(i.to_ne_bytes());
    }

    #[inline]
    fn write_usize(&mut self, i: usize) {
        self.short_write(i.to_ne_bytes());
    }

    #[inline]
    fn write_i8(&mut self, i: i8) {
        self.short_write((i as u8).to_ne_bytes());
    }

    #[inline]
    fn write_i16(&mut self, i: i16) {
        self.short_write((i as u16).to_ne_bytes());
    }

    #[inline]
    fn write_i32(&mut self, i: i32) {
        self.short_write((i as u32).to_ne_bytes());
    }

    #[inline]
    fn write_i64(&mut self, i: i64) {
        self.short_write((i as u64).to_ne_bytes());
    }

    #[inline]
    fn write_isize(&mut self, i: isize) {
        self.short_write((i as usize).to_ne_bytes());
    }

    #[inline]
    fn write(&mut self, msg: &[u8]) {
        self.slice_write(msg);
    }

    #[cfg(feature = "nightly")]
    #[inline]
    fn write_str(&mut self, s: &str) {
        // This hasher works byte-wise, and `0xFF` cannot show up in a `str`,
        // so just hashing the one extra byte is enough to be prefix-free.
        self.write(s.as_bytes());
        self.write_u8(0xFF);
    }

    fn finish(&self) -> u64 {
        let mut buf = self.buf;
        let [a, b] = unsafe {
            SipHasher128::finish128_inner(self.nbuf, &mut buf, self.state, self.processed)
        };

        // Combining the two halves makes sure we get a good quality hash.
        a.wrapping_mul(3).wrapping_add(b)
    }
}

#[derive(Debug, Clone, Default)]
struct Sip13Rounds;

impl Sip13Rounds {
    #[inline]
    fn c_rounds(state: &mut State) {
        compress!(state);
    }

    #[inline]
    fn d_rounds(state: &mut State) {
        compress!(state);
        compress!(state);
        compress!(state);
    }
}