alamb commented on code in PR #2278: URL: https://github.com/apache/arrow-rs/pull/2278#discussion_r935887698
########## parquet/src/util/bit_pack.rs: ########## @@ -0,0 +1,231 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +//! Vectorised bit-packing utilities + +macro_rules! unroll_impl { + (1, $offset:expr, $v:ident, $c:block) => {{ + const $v: usize = $offset; + $c + }}; + (2, $offset:expr, $v:ident, $c:block) => {{ + unroll_impl!(1, $offset, $v, $c); + unroll_impl!(1, $offset + 1, $v, $c); + }}; + (4, $offset:expr, $v:ident, $c:block) => {{ + unroll_impl!(2, 0, __v4, { unroll_impl!(2, __v4 * 2 + $offset, $v, $c) }); + }}; + (8, $offset:expr, $v:ident, $c:block) => {{ + unroll_impl!(2, 0, __v8, { unroll_impl!(4, __v8 * 4 + $offset, $v, $c) }); + }}; + (16, $offset:expr, $v:ident, $c:block) => {{ + unroll_impl!(4, 0, __v16, { + unroll_impl!(4, __v16 * 4 + $offset, $v, $c) + }); + }}; + (32, $offset:expr, $v:ident, $c:block) => {{ + unroll_impl!(2, 0, __v32, { + unroll_impl!(16, __v32 * 16 + $offset, $v, $c) + }); + }}; + (64, $offset:expr, $v:ident, $c:block) => {{ + unroll_impl!(2, 0, __v64, { + unroll_impl!(32, __v64 * 32 + $offset, $v, $c) + }); + }}; +} + +/// Manually unrolls a loop body, this is useful for two reasons +/// +/// - force unrolling of a loop so that LLVM optimises it properly +/// - call a const generic function with the loop value +macro_rules! unroll { + (for $v:ident in 0..$end:tt $c:block) => { + #[allow(non_upper_case_globals)] + { + unroll_impl!($end, 0, $v, $c) + } + }; +} + +/// Macro that generates an unpack function taking the number of bits as a const generic +macro_rules! unpack_impl { + ($t:ty, $bytes:literal, $bits:tt) => { + pub fn unpack<const NUM_BITS: usize>(input: &[u8], output: &mut [$t; $bits]) { + if NUM_BITS == 0 { + for out in output { + *out = 0; + } + return; + } + + assert!(NUM_BITS <= $bytes * 8); + + let mask = match NUM_BITS { + $bits => <$t>::MAX, + _ => ((1 << NUM_BITS) - 1), + }; + + assert!(input.len() >= NUM_BITS * $bytes); + + let r = |output_idx: usize| { + <$t>::from_le_bytes( + input[output_idx * $bytes..output_idx * $bytes + $bytes] + .try_into() + .unwrap(), + ) + }; + + unroll!(for i in 0..$bits { + let start_bit = i * NUM_BITS; + let end_bit = start_bit + NUM_BITS; + + let start_bit_offset = start_bit % $bits; + let end_bit_offset = end_bit % $bits; + let start_byte = start_bit / $bits; + let end_byte = end_bit / $bits; + if start_byte != end_byte && end_bit_offset != 0 { + let val = r(start_byte); + let a = val >> start_bit_offset; + let val = r(end_byte); + let b = val << (NUM_BITS - end_bit_offset); + + output[i] = a | (b & mask); Review Comment: ah good call, I was confusing that with `input: &[u8]` 🤦 -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: github-unsubscr...@arrow.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
