Map

tophash only store high 8 bits of the hash result.

Optimization of bmap in Go’s map Implementation

Why Mark bmap as Pointer-Free?

In Go’s map implementation, the bmap structure represents a bucket in the hash table. If both the key and value types stored in the map are non-pointer types, the bmap can be marked as not containing pointers. This is beneficial because it allows the GC to skip scanning these buckets during its mark phase, reducing GC overhead and improving performance.

Issue: bmap.overflow is a Pointer

bmap has an overflow field, which is a pointer to additional overflow buckets. This breaks the “pointer-free” assumption.

Solution: Using hmap.extra

Instead of keeping overflow pointers inside bmap, they are stored in hmap.extra:

• hmap.extra.overflow holds overflow bucket pointers for hmap.buckets.
• hmap.extra.oldoverflow holds overflow pointers for hmap.oldbuckets (before resizing).

This keeps bmap pointer-free while ensuring overflow buckets remain reachable.

Conclusion

GC no longer scans the entire map, only extra.overflow. This optimization improves GC efficiency while maintaining correctness.