8c07800b19
3dd815f048c80c9a35f01972e0537eb42531aec7 validation: pre-reserve leaves to prevent reallocs with odd vtx count (Lőrinc)
7fd47e0e56087cd3b5fd76a532bdc3ac331e832e bench: make `MerkleRoot` benchmark more representative (Lőrinc)
f0a21831087410687c4ca31ac00e44f380b859be test: adjust `ComputeMerkleRoot` tests (Lőrinc)
Pull request description:
#### Summary
`ComputeMerkleRoot` [duplicates the last hash](39b6c139bd/src/consensus/merkle.cpp (L54-L56)) when the input size is odd. If the caller provides a `std::vector` whose capacity equals its size, that extra `push_back` forces a reallocation, doubling its capacity (causing peak memory usage of 3x the necessary size).
This affects roughly half of the created blocks (those with odd transaction counts), causing unnecessary memory fragmentation during every block validation.
#### Fix
* Pre-reserves vector capacity to account for the odd-count duplication using `(size + 1) & ~1ULL`.
* This syntax produces [optimal assembly](https://github.com/bitcoin/bitcoin/pull/32497#discussion_r2553107836) across x86/ARM and 32/64-bit platforms for GCC & Clang.
* Eliminates default construction of `uint256` objects that are immediately overwritten by switching from `resize` to `reserve` + `push_back`.
#### Memory Impact
[Memory profiling](https://github.com/bitcoin/bitcoin/pull/32497#issuecomment-3563724551) shows **50% reduction in peak allocation** (576KB → 288KB) and elimination of reallocation overhead.
#### Validation
The benchmark was updated to use an odd leaf count to demonstrate the real-world scenario where the reallocation occurs.
A full `-reindex-chainstate` up to block **896 408** ran without triggering the asserts.
<details>
<summary>Validation asserts</summary>
Temporary asserts (not included in this PR) confirm that `push_back` never reallocates and that the coinbase witness hash remains null:
```cpp
if (hashes.size() & 1) {
assert(hashes.size() < hashes.capacity()); // TODO remove
hashes.push_back(hashes.back());
}
leaves.reserve((block.vtx.size() + 1) & ~1ULL); // capacity rounded up to even
leaves.emplace_back();
assert(leaves.back().IsNull()); // TODO remove
```
</details>
#### Benchmark Performance
While the main purpose is to improve predictability, the reduced memory operations also improve hashing throughput slightly.
ACKs for top commit:
achow101:
ACK 3dd815f048c80c9a35f01972e0537eb42531aec7
optout21:
reACK 3dd815f048c80c9a35f01972e0537eb42531aec7
hodlinator:
re-ACK 3dd815f048c80c9a35f01972e0537eb42531aec7
vasild:
ACK 3dd815f048c80c9a35f01972e0537eb42531aec7
w0xlt:
ACK 3dd815f048c80c9a35f01972e0537eb42531aec7 with minor nits.
danielabrozzoni:
Code review ACK 3dd815f048
Tree-SHA512: e7b578f9deadc0de7d61c062c7f65c5e1d347548ead4a4bb74b056396ad7df3f1c564327edc219670e6e2b2cb51f4e1ccfd4f58dd414aeadf2008d427065c11f
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