jhorstmann commented on code in PR #12024:
URL: https://github.com/apache/arrow-rs/pull/12024#discussion_r4437607586
##########
arrow-buffer/src/builder/boolean.rs:
##########
@@ -276,9 +276,31 @@ impl BooleanBufferBuilder {
self.advance(additional);
let offset = self.len() - additional;
- for (i, v) in slice.iter().enumerate() {
+ let buffer = self.buffer.as_slice_mut();
+
+ // Set bits individually until the write position is byte aligned
+ let head = ((8 - offset % 8) % 8).min(additional);
+ let (head_slice, rest) = slice.split_at(head);
+ for (i, v) in head_slice.iter().enumerate() {
if *v {
- unsafe { bit_util::set_bit_raw(self.buffer.as_mut_ptr(),
offset + i) }
+ bit_util::set_bit(buffer, offset + i)
+ }
+ }
+
+ // Pack 8 bools into each byte without branching: a `bool` is a byte
+ // holding 0 or 1, and the multiply moves byte `i` to bit `56 + i`
+ let byte_offset = (offset + head) / 8;
+ let (chunks, remainder) = rest.as_chunks::<8>();
+ for (dst, chunk) in buffer[byte_offset..].iter_mut().zip(chunks) {
+ let bytes: [u8; 8] = std::array::from_fn(|i| chunk[i] as u8);
+ let packed =
u64::from_le_bytes(bytes).wrapping_mul(0x0102_0408_1020_4080) >> 56;
Review Comment:
Very nice trick. On x86_64 with bmi2 feature this could also be written as
```
std::arch::x86_64::_pext_u64(u64::from_le_bytes(bytes), 0x01010101_01010101)
```
But I think that only saves a single cycle for the shift. Very cool that
there is a portable alternative.
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