Fix flaky CodSpeed microbenchmarks

[connortsui20]

A small pool of microbenchmarks flips ±10-35% between two fixed values on unrelated PRs (including docs-only and lockfile-only changes), spamming every CodSpeed report. This PR fixes them (and removes only 1 benchmark). No #[cfg(codspeed)] gating; CI and local runs stay identical. One commit per benchmark.

Changes

• mimalloc as global allocator in the 5 flaky vortex-array bench files: chunk_array_builder, dict_compress, varbinview_compact, compare, binary_ops
• bitwise_not_vortex_buffer_mut: drop the 128/1024/2048 sizes (measured only harness overhead)
• slice_empty_vortex: rewrite as a 1024-iteration tight loop, renamed slice_empty_tight_loop_vortex
• rebuild_naive (vortex-zstd): the one benchmark removed instead of fixed — its cost is dominated by zstd-internal copies (glibc ifunc-resolved memcpy) that no bench-level change can stabilize, its fixture is degenerate (a 4-string dictionary with all-zero offsets), and ListView rebuild is already benchmarked across element types and list shapes in vortex-array/benches/listview_rebuild.rs. The crate's now-unused divan dev-dependency goes with it.

Which benchmarks were flaky, and the evidence

Identified by reading the CodSpeed comments on the ~47 PRs merged since June 25 (post-#8490). The tell: the same benchmark flipping between the same two values, in both directions, on PRs that can't have affected it — including deny.toml-only (#8712, #8716), uv.lock-only (#8732), docs-only (#8737, #8728, #8685), and CI-YAML-only (#8660, #8683) changes.

Benchmark	Evidence
bitwise_not_vortex_buffer_mut[128/1024/2048]	~half of all PRs — worst offender
chunked_varbinview_* ×4	~20 PRs, both directions
chunked_bool_canonical_into[(1000,10)]	~2× flips (16µs ↔ 35µs) on 4 PRs
encode_varbin, encode_varbinview	~12 PRs; encode_varbinview[(10000,4)] also flipped on an earlier revision of this PR
compact, compact_sliced (90%-utilization args)	~8 PRs
compare_int_constant	±11.1% verbatim on ≥9 PRs
eq_i64_constant	same ±11% signature incl. docs-only PR
slice_empty_vortex	-14.66% verbatim on ~13 PRs
rebuild_naive (vortex-zstd)	~10 PRs, both directions

Watch list (left alone, below the ≥3-independent-sightings bar): copy_nullable/copy_non_nullable[65536] in cast_decimal.rs, true_count_vortex_buffer[128].

Root causes and why each fix matches

• Allocation in the timed region → glibc malloc's code differs across CodSpeed runner images, so alloc-heavy benchmarks trace differently for byte-identical Vortex code. Vendored mimalloc removes glibc malloc from the trace. Empirical support: the only three bench files already using mimalloc (single_encoding_throughput, common_encoding_tree_throughput, row_encode) are the most alloc-heavy suites in the repo and were never flagged once in the 47-PR window.
• Sub-microsecond work → the measurement is fixed harness overhead plus binary code layout, which shifts with any unrelated change. Fix by making the operation dominate: the in-place NOT (no alloc, no copy) keeps only sizes where the loop dominates; the empty slice runs 1024× per iteration, mirroring the neighboring slice_tight_loop_vortex.
• Environment-bound and low-signal → rebuild_naive, per the justification above: unfixable at the bench level and redundant with the vortex-array ListView rebuild suite, so removed.

Validation: A/A reruns and a stacked canary PR

• Expected one-time step changes on this PR: swapping the allocator changes the trace of every benchmark in the touched files, so this PR's report shows a few ±10-20% level shifts (including on never-flaky encode_primitives, which just shares a file). These need a one-time acknowledgment on the CodSpeed dashboard; after merge every PR compares mimalloc-vs-mimalloc.
• A/A reruns (same bench binaries measured three times, on separate runners and commits): every value reproduced exactly — 211.5µs, 137.1µs, 14.6µs, 26.3µs — with zero new flags across 1656 benchmarks.
• Canary test: canary for the flaky-benchmark fixes in #8742 (do not merge) #8743 (a Better error message in case of unknown encoding id #8681-style cold-string change stacked on this branch — the class of change that used to collect five false flags): Performance Gate Passed, ✅ 1660 untouched, zero changes reported.

No public API changes; benchmark-only.

https://claude.ai/code/session_01T6PPcdrqcNeUkfi1EGd4oC

[codspeed-hq]

Merging this PR will not alter performance

⚡ 1 improved benchmark
❌ 2 regressed benchmarks
✅ 1656 untouched benchmarks
🆕 1 new benchmark
⏩ 47 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	169.5 µs	211.5 µs	-19.86%
❌	Simulation	encode_primitives[i64, (1000, 512)]	117.3 µs	137.1 µs	-14.47%
⚡	Simulation	chunked_bool_canonical_into[(1000, 10)]	16.4 µs	14.6 µs	+12.43%
🆕	Simulation	slice_empty_tight_loop_vortex	N/A	26.3 µs	N/A

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing claude/flaky-microbenchmarks-cleanup-u68he8 (1a06040) with develop (002e40a)}

Footnotes

1. 47 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[robert3005]

let's try this