Feature Request: RoaringBitmap::from_lsb0_bytes

[lemolatoon]

The Feature Explanation

• Adding new creation function for RoaringBitmap

 pub fn from_lsb0_bytes(offset: u32, bytes: &[u8]) -> RoaringBitmap

Function Behavior

• Interpret bytes as little endian bytes bitmap, and construct RoaringBitmap
• offset can be used to offset the passing bitmap's index
• If offset is not aligned to # of bits of Container's Store::Bitmap (# of bits of Box<[u64; 1024]>), this function panics

use roaring::RoaringBitmap;

let bytes = [0b00000101, 0b00000010, 0b00000000, 0b10000000];
//             ^^^^^^^^    ^^^^^^^^    ^^^^^^^^    ^^^^^^^^
//             76543210          98
let rb = RoaringBitmap::from_lsb0_bytes(0, &bytes);
assert!(rb.contains(0));
assert!(!rb.contains(1));
assert!(rb.contains(2));
assert!(rb.contains(9));
assert!(rb.contains(31));

let rb = RoaringBitmap::from_lsb0_bytes(8, &bytes);
assert!(rb.contains(8));
assert!(!rb.contains(9));
assert!(rb.contains(10));
assert!(rb.contains(17));
assert!(rb.contains(39));

Motivation

Sometimes bitmap is calculated by SIMD instructions. The result of SIMD instruction is likely to be already bitmask, not the series of bitmap indicies.

Under current implementation, when you intend use RoaringBitmap with bitmask produced by SIMD instruction, you have to use RoaringBitmap::sorted_iter or just insert one by one.

To solve this problem, I implemented RoaringBitmap::from_bitmap_bytes, which can be used to construct directly from bitmask.

Example of Production of Bitmask by SIMD instructions

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=40ddd13554c171be31fe53893401d40f

use std::arch::x86_64::*;

#[target_feature(enable = "avx2")]
unsafe fn compare_u8_avx2(a: &[u8], b: &[u8]) -> u32 {
    assert!(
        a.len() == 32 && b.len() == 32,
        "Inputs must have a length of 32."
    );

    // Load the data into 256-bit AVX2 registers
    let a_vec = _mm256_loadu_si256(a.as_ptr() as *const __m256i);
    let b_vec = _mm256_loadu_si256(b.as_ptr() as *const __m256i);

    // Perform comparison (a == b)
    let cmp_result = _mm256_cmpeq_epi8(a_vec, b_vec);

    // Extract the comparison result as a bitmask
    let mask = _mm256_movemask_epi8(cmp_result);

    mask as u32
}

fn main() {
    let a: [u8; 32] = [
        1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
        26, 27, 28, 29, 30, 31, 32,
    ];
    let b: [u8; 32] = [
        1, 0, 3, 4, 5, 6, 7, 8, 0, 10, 11, 12, 13, 0, 15, 16, 0, 18, 19, 20, 21, 0, 23, 24, 0, 26,
        27, 0, 29, 0, 31, 32,
    ];

    let mask = unsafe {
        compare_u8_avx2(&a, &b)
    };
    println!("Bitmask: {:#034b}", mask);
    // Bitmask: 0b11010110110111101101111011111101
    print!("Bitmask (little endian u8): ");
    for b in mask.to_le_bytes() {
        print!("{:08b} ", b);
    }
    println!();
    // Bitmask (little endian u8): 11111101 11011110 11011110 11010110 
    
    let n = 2;
    println!("Bitmask at {n}: {}", mask & (1 << n) != 0);
    // Bitmask at 2: true
}

Benchmark Result

On my laptop (Apple M3 MacBook Air Sonoma14.3 Memory 16 GB), in most cases from_lsb0_bytes is much faster than from_sorted_iter.

Part of Results

creation/from_bitmap_bytes/census-income_srt                                                                             
                        time:   [984.25 µs 987.00 µs 990.37 µs]
                        thrpt:  [6.1521 Gelem/s 6.1731 Gelem/s 6.1904 Gelem/s]
Found 12 outliers among 100 measurements (12.00%)
  5 (5.00%) high mild
  7 (7.00%) high severe
creation/from_sorted_iter/census-income_srt                                                                            
                        time:   [23.383 ms 23.397 ms 23.413 ms]
                        thrpt:  [260.24 Melem/s 260.41 Melem/s 260.57 Melem/s]

[Kerollmops]

Hey @lemolatoon 👋

Thank you very much for these changes. The results look very good, indeed. However, could you:

• Write a better explanation of what offset means. I understand, but it needs to be clearer. Maybe talk about internal containers that are aligned around 64k values integer groups?
• Explain what kind of input is expected in plain text in the function description (endianness, size, alignment).
• Move this function and the test into the serialization module.

Thank you very much for the work!

[lemolatoon]

Hi @Kerollmops 👋

Thank you for your quick reply. I have just made changes based on your review.

Basically I did:

• Moved RoaringBitmap::from_bitmap_bytes, and its tests to serialization module.
• Added detailed decument to the RoaringBitmap::from_bitmap_bytes, including offset, bytes explanations.
• Relaxed the alignment requirement for offset.
  • Thanks to #[inline], I belive the compiler can easily optimize if offset is actually aligned to 8, or even 64Ki

[lemolatoon]

@Dr-Emann I've just fixed the documentation and made the implementation endian-aware.
You can try big endian system by running cargo +nightly miri test --target s390x-unknown-linux-gnu --package roaring --lib -- bitmap::serialization::test::test_from_bitmap_bytes.
I also added this big endian test to CI.

[lemolatoon]

I have just merged patch from @Dr-Emann (Thank you so much.) If merge commit aace6b8 is unnecessary, I'll remove it by force push.

[lemolatoon]

I applied cargo fmt, and rename from_bitmap_bytes to from_lsb0_bytes.

@Kerollmops I would like you to review this pull requests, then I can see the CI result.

[Kerollmops]

Hey @lemolatoon 👋

Sorry for the very late reply. I just approved your changes so that you can see the CI.

[lemolatoon]

Thank you @Kerollmops !
CI seems to be failing because of the change of cargo clippy on the nightly channel. I think this should be fixed on another pull request.

[Dr-Emann]

@lemolatoon The CI errors should be fixed by pulling in main, thanks to #293.

[lemolatoon]

I git rebaseed to the main branch. It seems to be required @Kerollmops approval to re-run the CI 🙏

[lemolatoon]

It seems to have the clippy warnings appeared only in 1.66.0 not in stable. I fixed the warnings. Could you re-run CI again please? @Kerollmops

[Kerollmops]

Hey @lemolatoon 👋 Would you mind rebasing on main, please? I just merged #295. Have a nice one 🐿️

[lemolatoon]

@Kerollmops
Sorry for the late fix. I have just pushed with two changes:

• git rebased upstream/main
• Fix the CI error regarding to the miri

I am afraid to bother you multiple times, but I'd like to see the CI result again 🙏

[lemolatoon]

I accidentally skipped one commit fixing the cargo clippy warning by the last rebase. I fixed the warning again and pushed.

[Kerollmops]

I am wondering if we can't just switch this offset in bits to an offset in bytes as the precision can only be expressed 8 bits at a time. This way we avoid this assert and reduce the possible errors.

If I understand correctly the offset is like a helper to increment the bitmap numbers by 8 at a time? Why is it necessary that it must be 8 at a time? Do we want to keep this? Why users can't just shift numbers themselves?

And if we want to keep it can we make it more clear (like the description I did above)? Your example helped me understand the high intent of this parameter.

[lemolatoon]

Thank you for your review and sorry for the late responce.

The reason why the offset must be 8 multiples is if the function accepts not 8 multiples offset, the copying will not so easy. If the container is Array, the implementation would be almost the same. In case Bitmap store, we need bit shifting for every bitmap byte copy.

We can switch the implementation based on the offset. In terms of the peformance, the users who use 8 multiples offset might not be impacted by this API change.
And the peformance of users with not 8 multiples offset is unknown.

I could allow any offset for this method with a little additional implementation and the document on peformances.