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optimize unaligned string reads

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Robin Appelman 2019-12-25 23:08:39 +01:00
commit 4a888e685c
1 changed files with 45 additions and 26 deletions
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65
src/buffer.rs
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@ -442,42 +442,39 @@ where
#[inline]
fn read_string_bytes(&self, position: usize) -> Result<Vec<u8>> {
if false && position & 7 == 0 {
let shift = position & 7;
if shift == 0 {
let byte_index = position / 8;
Ok(self.bytes[byte_index..self.find_null_byte(byte_index)].to_vec())
} else {
let mut acc = Vec::with_capacity(32);
let mut pos = position;
loop {
let read = min((USIZE_SIZE - 1) * 8, self.bit_len() - pos);
let raw_bytes = self.read_usize(pos, read);
let bytes: [u8; USIZE_SIZE] = if E::is_le() {
raw_bytes.to_le_bytes()
} else {
raw_bytes.to_be_bytes()
};
// note: if less then a usize worth of data is left in the buffer, read_usize_bytes
// will automatically pad with null bytes, triggering the loop termination
// thus no separate logic for dealing with the end of the bytes is required
let bytes_read = read / 8;
let byte_index = pos / 8;
let raw_bytes: [u8; USIZE_SIZE] = self.read_usize_bytes(byte_index);
let raw_usize: usize = usize::from_le_bytes(raw_bytes);
let shifted = raw_usize >> shift;
let (start, end) = if E::is_le() {
(0usize, bytes_read)
} else {
(USIZE_SIZE - bytes_read, USIZE_SIZE)
};
let has_null = contains_zero_byte_non_top(shifted);
let bytes: [u8; USIZE_SIZE] = shifted.to_le_bytes();
let usable_bytes = &bytes[0..USIZE_SIZE - 1];
for i in start..end {
if bytes[i] == 0 {
acc.extend_from_slice(&bytes[start..i]);
return Ok(acc);
if has_null {
for i in 0..USIZE_SIZE - 1 {
if usable_bytes[i] == 0 {
acc.extend_from_slice(&usable_bytes[0..i]);
return Ok(acc);
}
}
}
acc.extend_from_slice(&bytes[start..end]);
if bytes_read < (USIZE_SIZE - 1) {
return Ok(acc);
}
acc.extend_from_slice(&usable_bytes[0..USIZE_SIZE - 1]);
pos += read;
pos += (USIZE_SIZE - 1) * 8;
}
}
}
@ -586,3 +583,25 @@ impl<E: Endianness> Debug for BitBuffer<E> {
)
}
}
/// Return `true` if `x` contains any zero byte.
///
/// From *Matters Computational*, J. Arndt
///
/// "The idea is to subtract one from each of the bytes and then look for
/// bytes where the borrow propagated all the way to the most significant
/// bit."
#[inline(always)]
fn contains_zero_byte_non_top(x: usize) -> bool {
#[cfg(target_pointer_width = "64")]
const LO_USIZE: usize = 0x0001010101010101;
#[cfg(target_pointer_width = "64")]
const HI_USIZE: usize = 0x0080808080808080;
#[cfg(target_pointer_width = "32")]
const LO_USIZE: usize = 0x00010101;
#[cfg(target_pointer_width = "32")]
const HI_USIZE: usize = 0x00808080;
x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0
}