logos-storage/plonky2
1
0
Fork 0
mirror of https://github.com/logos-storage/plonky2.git synced 2026-01-02 13:53:07 +00:00
Code Issues Packages Projects Releases Wiki Activity

Use usize::BITS and wrapping_shr in reverse_index_bits_in_place_small (#1478)

Browse Source
This commit is contained in:
Vivek Pandya 2024-01-26 20:40:45 +05:30 committed by GitHub
parent 14bb5bdb82
commit 36e62a134d
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
1 changed files with 10 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
util/src/lib.rs
View File

@ -110,9 +110,12 @@ fn reverse_index_bits_large<T: Copy>(arr: &[T], n_power: usize) -> Vec<T> {
unsafe fn reverse_index_bits_in_place_small<T>(arr: &mut [T], lb_n: usize) {
if lb_n <= 6 {
// BIT_REVERSE_6BIT holds 6-bit reverses. This shift makes them lb_n-bit reverses.
let dst_shr_amt = 6 - lb_n;
let dst_shr_amt = 6 - lb_n as u32;
for src in 0..arr.len() {
let dst = (BIT_REVERSE_6BIT[src] as usize) >> dst_shr_amt;
// `wrapping_shr` handles the case when `arr.len() == 1`. In that case `src == 0`, so
// `src.reverse_bits() == 0`. `usize::wrapping_shr` by 64 is a no-op, but it gives the
// correct result.
let dst = (BIT_REVERSE_6BIT[src] as usize).wrapping_shr(dst_shr_amt);
if src < dst {
swap(arr.get_unchecked_mut(src), arr.get_unchecked_mut(dst));
}
@ -121,11 +124,14 @@ unsafe fn reverse_index_bits_in_place_small<T>(arr: &mut [T], lb_n: usize) {
// LLVM does not know that it does not need to reverse src at each iteration (which is
// expensive on x86). We take advantage of the fact that the low bits of dst change rarely and the high
// bits of dst are dependent only on the low bits of src.
let dst_lo_shr_amt = 64 - (lb_n - 6);
let dst_lo_shr_amt = usize::BITS - (lb_n - 6) as u32;
let dst_hi_shl_amt = lb_n - 6;
for src_chunk in 0..(arr.len() >> 6) {
let src_hi = src_chunk << 6;
let dst_lo = src_chunk.reverse_bits() >> dst_lo_shr_amt;
// `wrapping_shr` handles the case when `arr.len() == 1`. In that case `src == 0`, so
// `src.reverse_bits() == 0`. `usize::wrapping_shr` by 64 is a no-op, but it gives the
// correct result.
let dst_lo = src_chunk.reverse_bits().wrapping_shr(dst_lo_shr_amt);
for src_lo in 0..(1 << 6) {
let dst_hi = (BIT_REVERSE_6BIT[src_lo] as usize) << dst_hi_shl_amt;
let src = src_hi + src_lo;