## Summary
Replace the `BinaryNode` interface with `NodeRef uint32` indices into
typed arena pools, eliminating GC-scanned pointers from binary trie
nodes.
Inspired by [fjl's
observation](https://github.com/ethereum/go-ethereum/pull/34034#issuecomment-4075176446):
> *"if the binary trie produces such a large graph, it should probably
be changed so that the trie node type does not contain pointers. The
runtime does not scan objects that do not contain pointers, so it can
really help with the performance to build it this way."*
### The problem
CPU profiling of the binary trie (EIP-7864) showed **44% of CPU time in
garbage collection**. Each `InternalNode` held two `BinaryNode`
interface values (2 pointer-words each), and the GC scanned every one.
With ~25K `InternalNode`s in memory during block processing, this
created enormous GC pressure.
### The solution
`NodeRef` is a compact `uint32` (2-bit kind tag + 30-bit pool index).
`NodeStore` manages chunked typed pools per node kind:
- **InternalNode pool**: ZERO Go pointers (children are `NodeRef`, hash
is `[32]byte`) → noscan spans
- **HashedNode pool**: ZERO Go pointers → noscan spans
- **StemNode pool**: retains `Values [][]byte` (matching existing
format)
The serialization format is unchanged — flat InternalNode
`[type][leftHash][rightHash]` = 65 bytes.
## Benchmark: Apple M4 Pro (`--benchtime=10s --count=3`, on top of
#34021)
| Metric | Baseline | Arena | Delta |
|--------|----------|-------|-------|
| Approve (Mgas/s) | 374 | 382 | **+2.1%** |
| BalanceOf (Mgas/s) | 885 | 901 | **+1.8%** |
| Approve allocs/op | 775K | **607K** | **-21.7%** |
| BalanceOf allocs/op | 265K | **228K** | **-14.0%** |
## Benchmark: AMD EPYC 48-core (50GB state, execution-specs ERC-20, on
top of #34021 + #34032)
| Benchmark | Baseline | Arena | Delta |
|-----------|----------|-------|-------|
| erc20_approve (write) | 22.4 Mgas/s | **27.0 Mgas/s** | **+20.5%** |
| mixed_sload_sstore | 62.9 Mgas/s | **97.3 Mgas/s** | **+54.7%** |
| erc20_balanceof (read) | 180.8 Mgas/s | 167.6 Mgas/s | -7.3% (cold
cache variance) |
The arena benefit scales with heap size — the EPYC (larger heap, more GC
pressure) shows much larger gains than the M4 Pro (efficient unified
memory). The mixed workload baseline was unstable (62.9 vs 16.3 Mgas/s
between runs due to GC-induced throughput collapse); the arena
eliminates this entirely (95-97 Mgas/s, stable).
## Dependencies
Benchmarked with #34021 (H01 N+1 fix) + #34032 (R14 parallel hashing).
No code dependency — applies independently to master.
All test suites pass (`trie/bintrie` with `-race`, `core/state`,
`triedb/pathdb`, `cmd/geth`).
---------
Co-authored-by: Guillaume Ballet <3272758+gballet@users.noreply.github.com>
