[puzpuzpuz]

bucketPadded is exactly 64 bytes (one cache line) and contains GC-relevant pointers, so make([]bucketPadded, n) is allocated via Go's small-object path. For size classes that fall into the [16, 512) bucket range, the Go allocator prefixes the allocation with an 8-byte malloc header, which makes the slice base sit at offset 8 (mod 64) rather than 0. Each bucket
therefore occupies two cache lines.

The fix reorder the fields in bucket so that mu precedes next:

type bucket struct {
    meta    uint64
    entries [entriesPerMapBucket]unsafe.Pointer // *entry
    mu      sync.Mutex
    next    unsafe.Pointer // *bucketPadded
}

Benchmarks

buckets	before (old order)	after (fixed)
64	~22 ns	~17 ns (≈ −23%)
128	~15.4 ns	~13.2 ns (≈ −14%)
256	~13.3 ns	~11.2 ns (≈ −16%)
512	~10.7 ns	~10.6 ns
1024	~10.8 ns	~10.7 ns

The existing BenchmarkMap_WarmUp / BenchmarkMapInt_WarmUp benchmarks use WithPresize(1000), which produces a 512-bucket table - already in the aligned range. They should show no change.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.85%. Comparing base (1d45188) to head (f7479ec).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #206      +/-   ##
==========================================
- Coverage   99.54%   98.85%   -0.69%     
==========================================
  Files           7        7              
  Lines         875      875              
==========================================
- Hits          871      865       -6     
- Misses          4        9       +5     
- Partials        0        1       +1     

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