kazuyukitanimura commented on code in PR #6288:
URL: https://github.com/apache/arrow-rs/pull/6288#discussion_r1742715271
##########
arrow-buffer/src/util/bit_mask.rs:
##########
@@ -32,33 +31,126 @@ pub fn set_bits(
) -> usize {
let mut null_count = 0;
- let mut bits_to_align = offset_write % 8;
- if bits_to_align > 0 {
- bits_to_align = std::cmp::min(len, 8 - bits_to_align);
+ let mut acc = 0;
+ while len > acc {
+ let (n, l) = set_upto_64bits(
+ write_data,
+ data,
+ offset_write + acc,
+ offset_read + acc,
+ len - acc,
+ );
+ null_count += n;
+ acc += l;
}
- let mut write_byte_index = ceil(offset_write + bits_to_align, 8);
-
- // Set full bytes provided by bit chunk iterator (which iterates in 64
bits at a time)
- let chunks = BitChunks::new(data, offset_read + bits_to_align, len -
bits_to_align);
- chunks.iter().for_each(|chunk| {
- null_count += chunk.count_zeros();
- write_data[write_byte_index..write_byte_index +
8].copy_from_slice(&chunk.to_le_bytes());
- write_byte_index += 8;
- });
-
- // Set individual bits both to align write_data to a byte offset and the
remainder bits not covered by the bit chunk iterator
- let remainder_offset = len - chunks.remainder_len();
- (0..bits_to_align)
- .chain(remainder_offset..len)
- .for_each(|i| {
- if get_bit(data, offset_read + i) {
- set_bit(write_data, offset_write + i);
+
+ null_count
+}
+
+#[inline]
+fn set_upto_64bits(
+ write_data: &mut [u8],
+ data: &[u8],
+ offset_write: usize,
+ offset_read: usize,
+ len: usize,
+) -> (usize, usize) {
+ let read_byte = offset_read / 8;
+ let read_shift = offset_read % 8;
+ let write_byte = offset_write / 8;
+ let write_shift = offset_write % 8;
+
+ if len >= 64 {
+ // SAFETY: chunk gets masked when necessary, so it is safe
+ let chunk = unsafe { read_bytes_to_u64(data, read_byte, 8) };
+ if read_shift == 0 {
+ if write_shift == 0 {
+ let len = 64;
+ let null_count = chunk.count_zeros() as usize;
+ write_u64_bytes(write_data, write_byte, chunk);
+ (null_count, len)
} else {
- null_count += 1;
+ let len = 64 - write_shift;
+ let chunk = chunk << write_shift;
+ let null_count = len - chunk.count_ones() as usize;
+ or_write_u64_bytes(write_data, write_byte, chunk);
+ (null_count, len)
}
- });
+ } else if write_shift == 0 {
+ let len = 64 - 8; // 56 bits so that write_shift == 0 for the next
iteration
+ let chunk = (chunk >> read_shift) & 0x00FFFFFFFFFFFFFF; // 56 bits
mask
+ let null_count = len - chunk.count_ones() as usize;
+ write_u64_bytes(write_data, write_byte, chunk);
+ (null_count, len)
+ } else {
+ let len = 64 - std::cmp::max(read_shift, write_shift);
+ let chunk = (chunk >> read_shift) << write_shift;
+ let null_count = len - chunk.count_ones() as usize;
+ or_write_u64_bytes(write_data, write_byte, chunk);
+ (null_count, len)
+ }
+ } else if len == 1 {
+ let c = (unsafe { *data.as_ptr().add(read_byte) } >> read_shift) & 1;
+ let ptr = write_data.as_mut_ptr();
+ unsafe { *ptr.add(write_byte) |= c << write_shift };
+ ((c ^ 1) as usize, 1)
+ } else {
+ let len = std::cmp::min(len, 64 - std::cmp::max(read_shift,
write_shift));
+ let bytes = ceil(len + read_shift, 8);
+ // SAFETY: chunk gets masked, so it is safe
+ let chunk = unsafe { read_bytes_to_u64(data, read_byte, bytes) };
+ let mask = u64::MAX >> (64 - len);
+ let chunk = (chunk >> read_shift) & mask;
+ let chunk = chunk << write_shift;
+ let null_count = len - chunk.count_ones() as usize;
+ let bytes = ceil(len + write_shift, 8);
+ let ptr = unsafe { write_data.as_mut_ptr().add(write_byte) };
+ for (i, c) in chunk.to_le_bytes().iter().enumerate().take(bytes) {
+ unsafe { *ptr.add(i) |= c };
+ }
+ (null_count, len)
+ }
+}
+
+/// # Safety
+/// The caller must ensure all arguments are within the valid range.
+/// The caller must be aware `8 - count` bytes in the returned value are
uninitialized.
+#[inline]
+#[cfg(not(miri))]
+unsafe fn read_bytes_to_u64(data: &[u8], offset: usize, count: usize) -> u64 {
+ let mut tmp = std::mem::MaybeUninit::<u64>::uninit();
+ let src = data.as_ptr().add(offset);
+ // SAFETY: the caller must not use the uninitialized `8 - count` bytes in
the returned value.
Review Comment:
The function doc already has the safety comment.
I also added `SAFETY ` here by following the Rust `read_unaligned`
https://github.com/rust-lang/rust/blob/d6c8169c186ab16a3404cd0d0866674018e8a19e/library/core/src/ptr/mod.rs#L1528
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