From 7fd47e0e56087cd3b5fd76a532bdc3ac331e832e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C5=91rinc?= <pap.lorinc@gmail.com>
Date: Wed, 14 May 2025 14:32:08 +0200
Subject: [PATCH] bench: make `MerkleRoot` benchmark more representative

Two versions are run now, one with the mutation calculations, the other without.
To avoid unwanted compiler optimizations, we assert the expected hash, which should inhibit aggressive optimization.

To make the benchmark more similar to production `ComputeMerkleRoot` call sites, the input leaves-copying is made explicit before each run.

> ./build/bin/bench_bitcoin -filter='MerkleRoot.*' -min-time=1000

|             ns/leaf |              leaf/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|               44.18 |       22,634,858.70 |    0.0% |      1.10 | `MerkleRoot`
|               44.66 |       22,390,601.03 |    0.0% |      1.10 | `MerkleRootWithMutation`

Massif memory measurements show the excessive memory reservations:

    MB
1.332^        :
     | #      :
     | #      :
     | #      :
     | #      :
     | #    @ :
     | #    @ :
     | #    @ :
     | #    @ :
     | #    @ :
     | #    @ :
     | #    @ :
     | #    @ :
     | #::::@::::::::::::::::::::::::::::::::::::::::::::::::::::::@:::::@::::
     | #: ::@::::: :::::::: :: ::: :::::: : : :: ::: ::: : : : ::::@:::::@::::
     | #: ::@::::: :::::::: :: ::: :::::: : : :: ::: ::: : : : ::::@:::::@::::
     | #: ::@::::: :::::::: :: ::: :::::: : : :: ::: ::: : : : ::::@:::::@::::
     | #: ::@::::: :::::::: :: ::: :::::: : : :: ::: ::: : : : ::::@:::::@::::
     | #: ::@::::: :::::::: :: ::: :::::: : : :: ::: ::: : : : ::::@:::::@::::
     | #: ::@::::: :::::::: :: ::: :::::: : : :: ::: ::: : : : ::::@:::::@::::
   0 +----------------------------------------------------------------------->s
     0                                                                   226.2

showing the reallocations clearly in the stacks:
97.87% (1,366,841B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
->41.25% (576,064B) 0x969717: allocate (new_allocator.h:151)
| ->41.25% (576,064B) 0x969717: allocate (allocator.h:203)
|   ->41.25% (576,064B) 0x969717: allocate (alloc_traits.h:614)
|     ->41.25% (576,064B) 0x969717: _M_allocate (stl_vector.h:387)
|       ->41.25% (576,064B) 0x969717: _M_realloc_append<const uint256&> (vector.tcc:572)
|         ->41.25% (576,064B) 0x969717: push_back (stl_vector.h:1427)
|           ->41.25% (576,064B) 0x969717: ComputeMerkleRoot(std::vector<uint256, std::allocator<uint256> >, bool*) (merkle.cpp:55)
|             ->41.25% (576,064B) 0x2235A7: operator() (merkle_root.cpp:31)
|               ->41.25% (576,064B) 0x2235A7: ankerl::nanobench::Bench& ankerl::nanobench::Bench::run<MerkleRoot(ankerl::nanobench::Bench&)::{lambda()

Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>
---
 src/bench/merkle_root.cpp | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/src/bench/merkle_root.cpp b/src/bench/merkle_root.cpp
index 98ce197ea58..5be592707ff 100644
--- a/src/bench/merkle_root.cpp
+++ b/src/bench/merkle_root.cpp
@@ -7,21 +7,33 @@
 #include <random.h>
 #include <uint256.h>
 
+#include <cassert>
 #include <vector>
 
 static void MerkleRoot(benchmark::Bench& bench)
 {
-    FastRandomContext rng(true);
-    std::vector<uint256> leaves;
-    leaves.resize(9001);
-    for (auto& item : leaves) {
+    FastRandomContext rng{/*fDeterministic=*/true};
+
+    std::vector<uint256> hashes{};
+    hashes.resize(9001);
+    for (auto& item : hashes) {
         item = rng.rand256();
     }
-    bench.batch(leaves.size()).unit("leaf").run([&] {
-        bool mutation = false;
-        uint256 hash = ComputeMerkleRoot(std::vector<uint256>(leaves), &mutation);
-        leaves[mutation] = hash;
-    });
+
+    constexpr uint256 expected_root{"d8d4dfd014a533bc3941b8663fa6e7f3a8707af124f713164d75b0c3179ecb08"};
+    for (bool mutate : {false, true}) {
+        bench.name(mutate ? "MerkleRootWithMutation" : "MerkleRoot").batch(hashes.size()).unit("leaf").run([&] {
+            std::vector<uint256> leaves;
+            leaves.resize(hashes.size());
+            for (size_t s = 0; s < hashes.size(); s++) {
+                leaves[s] = hashes[s];
+            }
+
+            bool mutated{false};
+            const uint256 root{ComputeMerkleRoot(std::move(leaves), mutate ? &mutated : nullptr)};
+            assert(root == expected_root);
+        });
+    }
 }
 
 BENCHMARK(MerkleRoot, benchmark::PriorityLevel::HIGH);