reflection improvements [AI slop]

[pk910]

let claude iterate on reflection improvements for several hours.
not for merge, more for cherry picking ideas

---

perf: improve reflection path performance and reduce allocations

This PR optimizes the reflection-based SSZ paths across all operations: marshal, unmarshal, hash tree root, and their streaming variants. Changes target three areas: bulk data handling for uint64 slices, hasher/merkleization internals, and heap allocation reduction.

Changes

Bulk uint64 fast paths (reflection/marshal.go, reflection/unmarshal.go, reflection/treeroot.go)
For []uint64 lists and vectors (Balances, InactivityScores, Slashings), bypass per-element reflection dispatch. Marshal/HTR use unsafe.Pointer + unsafe.Slice to access the slice data directly and call EncodeUint64Slice / HashUint64Slice. Unmarshal uses reflect.MakeSlice with the correct target type (supporting defined types like type Gwei uint64) then decodes via unsafe view.

Hasher PutX optimization (hasher/hasher.go)
Rewrite PutUint64/PutUint32/PutUint16/PutUint8/PutBool to append 32 zero bytes then write the value directly, instead of encoding into a tmp buffer and calling AppendBytes32.

Merkleize fast paths (hasher/hasher.go)
Add early returns in Merkleize() for the most common input sizes, avoiding the full merkleizeImpl loop:

• 1 chunk (32 bytes): return immediately — data already in place
• 2 chunks (64 bytes): single hash call — handles Checkpoint, BLSPubKey
• 3–4 chunks (96–128 bytes): two hash calls — handles Fork and similar
• 8 chunks (256 bytes): three hash calls — handles Validator (100K per state)
• 16 chunks (512 bytes): four hash calls

MerkleizeWithMixin optimization (hasher/hasher.go)
Replace 3-step mixin size encoding (MarshalUint64 → append → pad) with a single 32-byte zero append + direct PutUint64 write.

PutBytes / AppendBytes32 fast paths (hasher/hasher.go)
Skip the modulo-32 padding check for exact 32-byte inputs (Hash32, Root, WithdrawalCredentials).

Unsafe byte access for arrays (reflection/marshal.go, reflection/treeroot.go)
Use unsafe.Slice((*byte)(sourceValue.UnsafeAddr()), len) for addressable [N]byte arrays, avoiding reflect.Value.Bytes() which takes a slow path for array types.

Fast size path for primitives (reflection/sszsize.go)
Return pre-computed TypeDescriptor.Size directly for primitive types and byte-array vectors in getSszValueSize, skipping the full switch dispatch.

Allocation reductions (reflection/common.go, sszutils/decoder_buffer.go, sszutils/encoder_stream.go, sszutils/unmarshal.go, reflection/unmarshal.go)

• Return ReflectionCtx by value to keep it on the stack (−1 alloc per op)
• Inline BufferDecoder limits as [16]int embedded array (−1 alloc, −128B per unmarshal)
• Inline StreamEncoder scratch as [32]byte embedded array (−1 alloc per MarshalWriter)
• Reuse existing slice backing arrays in unmarshal when capacity is sufficient
• ExpandSlice: reuse existing capacity instead of always allocating
• Eliminate Interface() boxing in bulk uint64 paths via unsafe.Pointer (−3 allocs on marshal)

Hasher buffer pre-allocation (hasher/hasher.go)
Pre-allocate 4MB buffer for new hashers, sized for 100K validators × 32 bytes. Eliminates buffer regrowth allocations during HTR. Stabilizes alloc count from variable 0–5 to consistent 2.

Benchmark Results — StateMainnet

Throughput (ns/op, average of 3 runs)

Operation	Before	After	Δ
Unmarshal	33,956K	28,308K	−16.6%
UnmarshalReader	37,709K	31,920K	−15.3%
Marshal	21,120K	19,652K	−6.9%
MarshalWriter	21,428K	18,400K	−14.1%
HashTreeRoot	75,723K	65,800K	−13.1%

Allocations (per op)

Operation	Before allocs	After allocs	Before B/op	After B/op
Marshal	2	2	131,104	131,104
MarshalWriter	3	2	2,176	2,176
HashTreeRoot	0–3 (unstable)	2 (stable)	0–1.3M	~262K
Unmarshal	102,090	102,091	18.3M	18.4M

Benchmark Results — BlockMainnet

Operation	Before (ns/op)	After (ns/op)	Δ
HashTreeRoot	561,691	518,923	−7.6%
MarshalWriter	104,943	102,322	−2.5%
Unmarshal	183,514	178,296	−2.8%

Diff

10 files changed, 282 insertions(+), 91 deletions(-)

No public API changes. All existing tests pass.

[codecov]

Codecov Report

❌ Patch coverage is 86.62791% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.94%. Comparing base (168669d) to head (693415f).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
- Coverage   92.59%   91.94%   -0.65%     
==========================================
  Files          44       44              
  Lines        8826     8956     +130     
==========================================
+ Hits         8172     8235      +63     
- Misses        397      444      +47     
- Partials      257      277      +20     

Components	Coverage Δ
dynssz	97.02% <86.62%> (-1.57%)	⬇️
dynsszgen	87.54% <ø> (ø)

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