[diskann-wide] Add u64x8

[Mark Hildebrand (hildebrandmw)]

Another step towards better 1-bit ops on AVX512.

[Copilot]

Pull request overview

Adds a new 8-lane u64 SIMD vector (u64x8) across backends to support wider 64-bit unsigned operations, aligning with ongoing work toward improved 1-bit ops on AVX-512.

Changes:

• Extend the Architecture trait with u64x8 and its associated mask type (mask_u64x8).
• Implement u64x8 for x86_64 V3 (via Doubled<u64x4>) and x86_64 V4 (native __m512i AVX-512).
• Add emulated and AArch64 double-wide support plus reduction/split-join coverage for u64x8.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
diskann-wide/src/emulated.rs	Adds Emulated<{i64,u64}, 8> sum-tree support and Emulated<u64, 8> split/join plumbing with tests.
diskann-wide/src/arch/x86_64/v4/u64x8_.rs	New native AVX-512 u64x8 implementation (ops, load/store, comparisons, sum-tree) with tests.
diskann-wide/src/arch/x86_64/v4/mod.rs	Registers the new V4 u64x8 module via maybe_miri!.
diskann-wide/src/arch/x86_64/v3/u64x8_.rs	Adds V3 u64x8 as a doubled vector over u64x4 with tests.
diskann-wide/src/arch/x86_64/v3/mod.rs	Exposes the new V3 u64x8 module/type.
diskann-wide/src/arch/x86_64/v3/masks.rs	Adds doubled-mask support needed for u64x8 masks in the doubled backend.
diskann-wide/src/arch/mod.rs	Extends Architecture with mask_u64x8 and u64x8 vector definition.
diskann-wide/src/arch/emulated/mod.rs	Exposes u64x8 for the emulated/scalar backend.
diskann-wide/src/arch/aarch64/mod.rs	Re-exports AArch64 u64x8 from the double-wide module.
diskann-wide/src/arch/aarch64/double.rs	Adds AArch64 u64x8 double-double alias, mask support, and tests.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 69.23077% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.26%. Comparing base (16c7451) to head (d50d555).

Files with missing lines	Patch %	Lines
diskann-wide/src/arch/x86_64/v4/u64x8_.rs	50.00%	12 Missing ⚠️

❌ Your patch status has failed because the patch coverage (69.23%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1312      +/-   ##
==========================================
- Coverage   91.26%   91.26%   -0.01%     
==========================================
  Files         517      519       +2     
  Lines       98511    98550      +39     
==========================================
+ Hits        89910    89944      +34     
- Misses       8601     8606       +5     

Flag	Coverage Δ
miri	91.26% <69.23%> (-0.01%)	⬇️
unittests	90.94% <69.23%> (-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/emulated/mod.rs	100.00% <ø> (ø)
diskann-wide/src/arch/mod.rs	83.79% <ø> (ø)
diskann-wide/src/arch/x86_64/v3/masks.rs	98.19% <ø> (ø)
diskann-wide/src/arch/x86_64/v3/mod.rs	98.22% <ø> (ø)
diskann-wide/src/arch/x86_64/v3/u64x8_.rs	100.00% <100.00%> (ø)
diskann-wide/src/arch/x86_64/v4/mod.rs	24.02% <ø> (ø)
diskann-wide/src/emulated.rs	98.29% <ø> (ø)
diskann-wide/src/arch/x86_64/v4/u64x8_.rs	50.00% <50.00%> (ø)

... and 4 files with indirect coverage changes

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