[diskann-wide] AVX512/Neon popcount

[hildebrandmw]

Adds limited support for SIMD popcounts. These instructions are weird: AVX512-BITALG has great support, Neon only supports i8/u8, and AVX2 supports nothing. The decision made here is to only implement the SIMDPopcount trait when it maps to an instruction. Trying to emulate it on an op-level basis is disastrous for performance.

This is part of a small line of work adding better AVX-512 1-bit quantization.

AI Disclosure: the author is unable to efficiently type at the moment so had an agent do most of the work. The change set is largely mechanical.

[Copilot]

Pull request overview

Adds a new SIMDPopcount trait to diskann-wide and wires it through emulated/reference paths plus selected x86_64 AVX-512 and AArch64 Neon backends, enabling lane-wise popcount where hardware instructions exist (supporting upcoming 1-bit quantization work).

Changes:

• Introduce and export SIMDPopcount, with implementations for Emulated and Doubled vector wrappers.
• Add reference scalar “integer ops” helpers including expected popcount, and new popcount correctness tests.
• Implement instruction-backed popcount for AVX-512 (avx512bitalg / avx512vpopcntdq) and Neon (vcnt*) vector types.

Reviewed changes

Copilot reviewed 28 out of 28 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
diskann-wide/src/traits.rs	Adds the public SIMDPopcount trait.
diskann-wide/src/lib.rs	Re-exports SIMDPopcount from the crate root.
diskann-wide/src/reference.rs	Renames/expands reference integer ops to include popcount expected results.
diskann-wide/src/emulated.rs	Implements SIMDPopcount for the emulated backend via reference ops.
diskann-wide/src/doubled.rs	Implements SIMDPopcount for Doubled<T> by delegating to halves.
diskann-wide/src/test_utils/ops.rs	Adds test_popcount! and updates integer test bounds to ReferenceIntegerOps.
diskann-wide/src/arch/x86_64/v4/u8x64_.rs	Adds AVX-512 byte popcount and tests.
diskann-wide/src/arch/x86_64/v4/u8x32_.rs	Adds AVX-512VL byte popcount and tests.
diskann-wide/src/arch/x86_64/v4/u8x16_.rs	Adds AVX-512VL byte popcount and tests.
diskann-wide/src/arch/x86_64/v4/i8x64_.rs	Adds AVX-512 byte popcount and tests.
diskann-wide/src/arch/x86_64/v4/i8x32_.rs	Adds AVX-512VL byte popcount and tests.
diskann-wide/src/arch/x86_64/v4/i8x16_.rs	Adds AVX-512VL byte popcount and tests.
diskann-wide/src/arch/x86_64/v4/i16x32_.rs	Adds AVX-512 word popcount and tests.
diskann-wide/src/arch/x86_64/v4/i16x16_.rs	Adds AVX-512VL word popcount and tests.
diskann-wide/src/arch/x86_64/v4/i16x8_.rs	Adds AVX-512VL word popcount and tests.
diskann-wide/src/arch/x86_64/v4/u32x16_.rs	Adds AVX-512 dword popcount and tests.
diskann-wide/src/arch/x86_64/v4/u32x8_.rs	Adds AVX-512VL dword popcount and tests.
diskann-wide/src/arch/x86_64/v4/u32x4_.rs	Adds AVX-512VL dword popcount and tests.
diskann-wide/src/arch/x86_64/v4/i32x16_.rs	Adds AVX-512 dword popcount and tests.
diskann-wide/src/arch/x86_64/v4/i32x8_.rs	Adds AVX-512VL dword popcount and tests.
diskann-wide/src/arch/x86_64/v4/i32x4_.rs	Adds AVX-512VL dword popcount and tests.
diskann-wide/src/arch/x86_64/v4/u64x4_.rs	Adds AVX-512VL qword popcount and tests.
diskann-wide/src/arch/x86_64/v4/u64x2_.rs	Adds AVX-512VL qword popcount and tests.
diskann-wide/src/arch/aarch64/u8x8_.rs	Adds Neon byte popcount and tests.
diskann-wide/src/arch/aarch64/u8x16_.rs	Adds Neon byte popcount and tests.
diskann-wide/src/arch/aarch64/i8x8_.rs	Adds Neon byte popcount and tests.
diskann-wide/src/arch/aarch64/i8x16_.rs	Adds Neon byte popcount and tests.
diskann-wide/src/arch/aarch64/double.rs	Adds popcount tests for doubled Neon vectors.

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[Copilot]

Pull request overview

Copilot reviewed 28 out of 28 changed files in this pull request and generated no new comments.

Suppressed comments (1)

diskann-wide/src/traits.rs:531

• The SIMDPopcount docs say it is only implemented for vector/architecture pairs with native popcount support, but the PR also implements it for Emulated<..> (and therefore Doubled<Emulated<..>>), which is not native SIMD popcount. Either adjust the docs to match the actual impl set, or remove the emulated impl if the intent is truly “native only”.

/// Count the set bits in each lane.
///
/// This trait is only implemented for vector/architecture pairs that have native support
/// for popcounts.
pub trait SIMDPopcount {

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.32258% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.47%. Comparing base (2ee97ff) to head (1ee7029).

Files with missing lines	Patch %	Lines
diskann-wide/src/doubled.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1310   +/-   ##
=======================================
  Coverage   91.46%   91.47%           
=======================================
  Files         516      516           
  Lines       98276    98305   +29     
=======================================
+ Hits        89891    89922   +31     
+ Misses       8385     8383    -2     

Flag	Coverage Δ
miri	91.47% <90.32%> (+<0.01%)	⬆️
unittests	91.14% <90.32%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-wide/src/arch/x86_64/v4/i16x16_.rs	57.14% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i16x32_.rs	57.14% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i16x8_.rs	57.14% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i32x16_.rs	30.76% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i32x4_.rs	57.14% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i32x8_.rs	30.76% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i8x16_.rs	40.00% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i8x32_.rs	33.33% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/i8x64_.rs	40.00% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/u32x16_.rs	42.85% <ø> (ø)
... and 13 more

... and 6 files with indirect coverage changes

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