Simplify leftover output writes

No perf impact

src/engine/general_purpose/decode_suffix.rs

@@ -123,9 +123,9 @@ pub(crate) fn decode_suffix(
  // TODO how do we know this?
  debug_assert!(morsels_in_leftover != 1 && morsels_in_leftover != 5);
  let leftover_bytes_to_append = morsels_in_leftover * 6 / 8;
- let leftover_bits_to_append = leftover_bytes_to_append * 8;
- // A couple percent speedup from nudging these ORs to use more ILP with a two-way split
- let leftover_bits = ((u64::from(morsels[0]) << 58)
+ // Put the up to 6 complete bytes as the high bytes.
+ // Gain a couple percent speedup from nudging these ORs to use more ILP with a two-way split.
+ let mut leftover_num = ((u64::from(morsels[0]) << 58)
  | (u64::from(morsels[1]) << 52)
  | (u64::from(morsels[2]) << 46)
  | (u64::from(morsels[3]) << 40))
@@ -136,24 +136,22 @@ pub(crate) fn decode_suffix(
 
  // if there are bits set outside the bits we care about, last symbol encodes trailing bits that
  // will not be included in the output
- let mask = !0 >> leftover_bits_to_append;
- if !decode_allow_trailing_bits && (leftover_bits & mask) != 0 {
+ let mask = !0 >> (leftover_bytes_to_append * 8);
+ if !decode_allow_trailing_bits && (leftover_num & mask) != 0 {
  // last morsel is at `morsels_in_leftover` - 1
  return Err(DecodeError::InvalidLastSymbol(
  start_of_leftovers + morsels_in_leftover - 1,
  last_symbol,
  ));
  }
 
- // TODO benchmark simply converting to big endian bytes
- let mut leftover_bits_appended_to_buf = 0;
- while leftover_bits_appended_to_buf < leftover_bits_to_append {
- // `as` simply truncates the higher bits, which is what we want here
- let selected_bits = (leftover_bits >> (56 - leftover_bits_appended_to_buf)) as u8;
- output[output_index] = selected_bits;
+ // Strangely, this approach benchmarks better than writing bytes one at a time,
+ // or copy_from_slice into output.
+ for _ in 0..leftover_bytes_to_append {
+ let hi_byte = (leftover_num >> 56) as u8;
+ leftover_num <<= 8;
+ output[output_index] = hi_byte;
  output_index += 1;
-
- leftover_bits_appended_to_buf += 8;
  }
 
  Ok(DecodeMetadata::new(