refactor: optimize block index comparisons (1.4-6.8x faster)

l0rinc commented at 1:54 am on October 16, 2025: contributor

Context

The comparator is often called either directly or for the sorting of setBlockIndexCandidates, a red-black tree-set containing valid block headers where the comparator is invoked extensively. Besided the sorted set usages, the comparator is used directly in Chainstate in FindMostWorkChain, PruneBlockIndexCandidates, ActivateBestChain and InvalidateBlock; and in ChainstateManager in LoadBlockIndex, CheckBlockIndex, ActivateSnapshot and PopulateAndValidateSnapshot.

It was profiled during the investigation of #33618 (comment).

Testing

To ensure the optimized implementations are both fast and correct, the first commit adds a dedicated benchmark to measure CBlockIndexWorkComparator performance, and the second commit adds randomized tests comparing the new implementation with the original one.

Results

GCC 15.0.1:

CBlockIndexWorkComparator: 100,772,511 cmp/s → 656,674,205 cmp/s = 6.51x faster
CheckBlockIndex: 9,091 ops/s → 14,765 ops/s = 1.62x faster

Clang 22.0.0:

CBlockIndexWorkComparator: 100,451,893 cmp/s → 683,414,234 cmp/s = 6.8x faster
CheckBlockIndex: 10,322 ops/s → 14,376 ops/s = 1.39x faster

See https://corecheck.dev/bitcoin/bitcoin/pulls/33637 for the CI’s CheckBlockIndex performance.

Compiler: gcc 15.0.1 b60450fae8 bench: add benchmark to measure CBlockIndexWorkComparator performance

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
9.92	100,772,511.62	0.0%	63.98	35.64	1.795	14.17	1.9%	5.50	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
109,996.46	9,091.20	0.2%	1,014,421.11	394,979.29	2.568	313,025.11	0.0%	5.50	`CheckBlockIndex`

e2e02177ba refactor: inline arith_uint256 comparison operator

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
6.48	154,439,491.66	0.0%	31.01	23.25	1.334	7.16	3.8%	5.50	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
105,754.86	9,455.83	0.1%	913,130.11	379,588.20	2.406	276,692.11	0.0%	5.50	`CheckBlockIndex`

85b74b01de refactor: optimize arith_uint256 comparison with spaceship operator

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
6.37	156,990,488.06	0.0%	28.85	22.87	1.261	8.61	3.2%	5.50	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
83,803.10	11,932.73	0.0%	743,565.09	300,824.84	2.472	232,646.08	0.0%	5.56	`CheckBlockIndex`

deb58eea2f refactor: optimize CBlockIndexWorkComparator with std::tie

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
1.52	656,674,205.98	0.0%	13.18	5.47	2.410	3.06	0.1%	5.50	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
67,726.84	14,765.19	0.2%	585,826.07	243,155.90	2.409	181,920.07	0.0%	5.54	`CheckBlockIndex`

Compiler: clang 22.0.0

b60450fae8 bench: add benchmark to measure CBlockIndexWorkComparator performance

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
9.96	100,451,893.46	0.0%	61.28	35.75	1.714	13.62	2.1%	5.52	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
96,878.70	10,322.19	0.1%	802,827.10	347,679.01	2.309	234,823.10	0.0%	5.50	`CheckBlockIndex`

e2e02177ba refactor: inline arith_uint256 comparison operator

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
6.13	163,183,693.10	0.0%	25.74	22.01	1.170	6.10	4.6%	5.50	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
86,307.94	11,586.42	1.2%	646,229.09	309,785.33	2.086	195,119.09	0.0%	5.54	`CheckBlockIndex`

85b74b01de refactor: optimize arith_uint256 comparison with spaceship operator

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
6.36	157,330,900.16	1.0%	26.20	22.61	1.159	6.55	4.4%	5.53	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
75,031.66	13,327.71	1.0%	650,604.15	266,934.04	2.437	149,967.14	0.0%	5.54	`CheckBlockIndex`

deb58eea2f refactor: optimize CBlockIndexWorkComparator with std::tie

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
1.46	683,414,234.14	0.0%	16.12	5.25	3.067	4.02	0.1%	5.50	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
69,559.70	14,376.14	0.1%	559,208.07	249,654.28	2.240	132,342.07	0.0%	5.51	`CheckBlockIndex`

Reproducer

Run the equivalence tests with:

0cmake -B build && cmake --build build && ./build/bin/test_bitcoin --run_test=arith_uint256_tests,blockchain_tests

Each commit shows how it changes the relevant benchmarks.

 0for compiler in gcc clang; do \
 1  if [ "$compiler" = "gcc" ]; then CC=gcc; CXX=g++; COMP_VER=$(gcc -dumpfullversion); \
 2  else CC=clang; CXX=clang++; COMP_VER=$(clang -dumpversion); fi && \
 3  echo "> Compiler: $compiler $COMP_VER" && \
 4  for commit in 70c6244b709fcf6a853ea44b91b9ed219609ac90 516e05cd7d8dbb61b05854c59f5393367e502284 d053b27c06d06cccbc6395b0aa7dc1c4193a6fc3 8ef4ce2cb3ac80cbe58b8cf76e183f9eca6fafe4 703165e6d3b9fbd58b9c31ddce0e961047fc5e71; do \
 5    git fetch origin $commit >/dev/null 2>&1 && git checkout $commit >/dev/null 2>&1 && git log -1 --pretty='%h %s' && \
 6    rm -rf build && \
 7    cmake -B build -DBUILD_BENCH=ON -DENABLE_IPC=OFF -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX >/dev/null 2>&1 && \
 8    cmake --build build -j$(nproc) >/dev/null 2>&1 && \
 9    for i in 1; do \
10      build/bin/bench_bitcoin -filter='CBlockIndexWorkComparator|CheckBlockIndex' -min-time=5000; \
11    done; \
12  done; \
13done

Note: something similar was already started in #33334, but this is a broader optimization that doesn’t use the same technique.

DrahtBot added the label Refactoring on Oct 16, 2025

DrahtBot commented at 1:54 am on October 16, 2025: contributor

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33637.

Reviews

See the guideline for information on the review process.

Type	Reviewers
Concept ACK	mzumsande
Approach ACK	Raimo33
Stale ACK	laanwj, optout21

If your review is incorrectly listed, please copy-paste <!–meta-tag:bot-skip–> into the comment that the bot should ignore.

Conflicts

Reviewers, this pull request conflicts with the following ones:

#34075 (fees: Introduce Mempool Based Fee Estimation to reduce overestimation by ismaelsadeeq)
#33854 (fix assumevalid is ignored during reindex by Eunovo)
#33324 (blocks: add -reobfuscate-blocks argument to enable (de)obfuscating existing blocks by l0rinc)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Raimo33 commented at 9:16 am on October 17, 2025: contributor

Approach ACK

I have tested the new block index comparator but I’ll refrain from acking the added benchmarks/tests

in src/bench/blockstorage.cpp:40 in dc7e70e9ac outdated

35+
36+        blocks.push_back(std::move(block));
37+    }
38+    std::ranges::shuffle(blocks, rng);
39+
40+    bench.run([&] {

sipa commented at 3:00 pm on October 17, 2025:

Mind using bench.batch(n * n).units("cmp") or so here, to put the output units in perspective?

l0rinc commented at 5:55 am on October 18, 2025:

Done, updated the commit messages and PR descriptions as well.

l0rinc force-pushed on Oct 18, 2025

l0rinc renamed this:
~~refactor: optimize block index comparisons (1.4-7.7x faster)~~
refactor: optimize block index comparisons (1.4-6.8x faster)
on Oct 18, 2025

Christewart commented at 9:34 pm on October 18, 2025: contributor

I attempted to run the script, not really sure what these results indicate. Just pasting what the results were

0Darwin Chriss-MacBook-Pro.local 24.6.0 Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:55 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6031 arm64

0Apple clang version 17.0.0 (clang-1700.3.19.1)
1Target: arm64-apple-darwin24.6.0
2Thread model: posix
3InstalledDir: /Library/Developer/CommandLineTools/usr/bin

0|              ns/cmp |               cmp/s |    err% |     total | benchmark
1|--------------------:|--------------------:|--------:|----------:|:----------
2|                4.97 |      201,237,014.91 |    0.4% |      5.49 | `CBlockIndexWorkComparator`
3
4|               ns/op |                op/s |    err% |     total | benchmark
5|--------------------:|--------------------:|--------:|----------:|:----------
6|           43,777.21 |           22,842.94 |    0.3% |      5.41 | `CheckBlockIndex`

0|              ns/cmp |               cmp/s |    err% |     total | benchmark
1|--------------------:|--------------------:|--------:|----------:|:----------
2|                1.89 |      529,694,702.31 |    0.1% |      5.50 | `CBlockIndexWorkComparator`
3
4|               ns/op |                op/s |    err% |     total | benchmark
5|--------------------:|--------------------:|--------:|----------:|:----------
6|           38,303.87 |           26,107.02 |    0.1% |      5.35 | `CheckBlockIndex`

0|              ns/cmp |               cmp/s |    err% |     total | benchmark
1|--------------------:|--------------------:|--------:|----------:|:----------
2|                2.31 |      432,334,219.11 |    0.6% |      5.45 | `CBlockIndexWorkComparator`
3
4|               ns/op |                op/s |    err% |     total | benchmark
5|--------------------:|--------------------:|--------:|----------:|:----------
6|           33,598.60 |           29,763.15 |    0.2% |      5.32 | `CheckBlockIndex`

0|              ns/cmp |               cmp/s |    err% |     total | benchmark
1|--------------------:|--------------------:|--------:|----------:|:----------
2|                0.78 |    1,287,258,159.19 |    0.1% |      5.46 | `CBlockIndexWorkComparator`
3
4|               ns/op |                op/s |    err% |     total | benchmark
5|--------------------:|--------------------:|--------:|----------:|:----------
6|           31,055.32 |           32,200.61 |    0.1% |      5.30 | `CheckBlockIndex`

l0rinc commented at 0:24 am on October 19, 2025: contributor

Thanks for the measurements @Christewart, this is how your measurements compare to mine (but most importantly how it compares to master):

laanwj approved

laanwj commented at 8:44 am on October 22, 2025: member

Code review ACK c15d839aefba017e64f14cd9cd2779655352f4b6 i did not run the benchmarks but the code changes look good, using <=> makes sense.

DrahtBot added the label Needs rebase on Oct 27, 2025

l0rinc force-pushed on Oct 28, 2025

l0rinc commented at 11:19 pm on October 28, 2025: contributor

Rebased after #29640 - only change was adjusting the comment

DrahtBot removed the label Needs rebase on Oct 28, 2025

in src/test/blockchain_tests.cpp:127 in 08d098947a outdated

122+    auto old_comparator{[](const CBlockIndex* pa, const CBlockIndex* pb) -> bool {
123+        // First sort by most total work, ...
124+        if (pa->nChainWork > pb->nChainWork) return false;
125+        if (pa->nChainWork < pb->nChainWork) return true;
126+
127+        // ... then by earliest time received, ...

mzumsande commented at 8:41 pm on October 30, 2025:

should take the latest comments from the old function after #29640

l0rinc commented at 8:37 am on October 31, 2025:

~~I have already done that, which parts do you think I’m missing?~~ Edit: ah in the tests, good point, let me update it

in src/test/arith_uint256_tests.cpp:281 in 08d098947a outdated

274@@ -275,8 +275,43 @@ BOOST_AUTO_TEST_CASE( comparison ) // <= >= < >
275         BOOST_CHECK(! (TmpL < R1L)); BOOST_CHECK(! (R1L > TmpL));
276     }
277 
278-    BOOST_CHECK_LT(ZeroL,
279-                   OneL);
280+    BOOST_CHECK_LT(ZeroL, OneL);
281+}
282+
283+BOOST_AUTO_TEST_CASE(comparison_equivalence)

mzumsande commented at 9:13 pm on October 30, 2025:

commit msg 08d098947aaeae67e3aeef262df1ecdbdbed744a: wway ->way

l0rinc commented at 10:57 am on October 31, 2025:

done

in src/test/blockchain_tests.cpp:120 in 08d098947a outdated

116@@ -117,6 +117,43 @@ BOOST_AUTO_TEST_CASE(num_chain_tx_max)
117     BOOST_CHECK_EQUAL(block_index.m_chain_tx_count, std::numeric_limits<uint64_t>::max());
118 }
119 
120+BOOST_AUTO_TEST_CASE(cblockindex_comparator_equivalence)

mzumsande commented at 9:15 pm on October 30, 2025:

this looks like a fuzz test, why not make it an actual fuzz target out of this?

mzumsande commented at 9:18 pm on October 30, 2025:

would suggest to add a bit more explanation, e.g. mentioning that old_comparator is a snapshot of an old implementation, so that people who look at this in years understand where it’s coming from.

l0rinc commented at 10:56 am on October 31, 2025:

I strongly dislike code comments, I prefer explaining with live code over dead comments - and if people disagree they can always do a blame which instantly reveals the purpose since I also over-explain in commit messages usually. Are the names old_comparator in a cblockindex_comparator_equivalence not enough to make it obvious that? I I have added a comment anyway, let me know if it helps.

l0rinc commented at 10:56 am on October 31, 2025:

I look at fuzz tests as exploratory tools that are hard to write and run and debug and do code coverage on - not trivial to maintain (see #33731), it’s definitely not my go-to way to test something. This is just a randomized property based unit test - easy to debug, easy to run and understand. But looking at the tests again, I can make it more deterministic so that it hits all important branches which allows me to reduce the iteration count - code coverage and debugging shows this regularly tests each branch now (green line on left side): Added you as coauthor.

dergoegge commented at 12:43 pm on November 6, 2025:

I look at fuzz tests as exploratory tools that are hard to write and run and debug and do code coverage … not trivial to maintain … not my go-to way to test something

There is a learning curve to writing and using fuzz testing but it is 100% worth getting in to. The maintenance aspect only relates to running them using libFuzzer on macOS.

On average, coverage guided fuzzing will be magnitudes better at finding bugs than this style of unit test. For something as small as the comparator your approach is probably good enough, although you could just have a fixed list of test cases that enumerate all possibilities (for the old comparator) instead? (maybe you do already, didn’t look)

in src/node/blockstorage.h:90 in a5584716cb

86@@ -87,12 +87,20 @@ static constexpr uint32_t UNDO_DATA_DISK_OVERHEAD{STORAGE_HEADER_BYTES + uint256
87 using BlockMap = std::unordered_map<uint256, CBlockIndex, BlockHasher>;
88 
89 struct CBlockIndexWorkComparator {
90-    bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const;
91+    // First sort by most total work (descending), then by earliest activatable time (ascending), then by pointer value (ascending).

mzumsande commented at 9:48 pm on October 30, 2025:

I find “ascending/descending” confusing here, I would have expected the opposites: “descending work” means most work first, but the strict weak ordering of C++ comparators would place the one with the lower work first.

Would suggest something like “First compare by work (less first), then by earliest activatable time (higher sequence first), then by pointer value (higher first).”

l0rinc commented at 10:56 am on October 31, 2025:

I wanted to use the SQL order-by terminology here

Ascending order puts smaller values first, where “smaller” is defined in terms of the < operator. Similarly, descending order is determined with the > operator.

But you’re right, I inverted the terminology, added a dedicated test to check for hard-coded order (first param is increasing, second is decreasing), updated the comments (this is why I find comments a lazy solution, it’s too easy to keep them up-to-date), added you as coauthor.

mzumsande commented at 10:08 pm on October 30, 2025: contributor

Concept ACK

Not 100% sure yet about 2dd0f2ced35a268bcab661c671e7c70271cdd91f, seems like inlining gives most of the speedup, whereas the gain of using the spaceship operator (which comes at the cost of readability) is marginal.

l0rinc force-pushed on Oct 31, 2025

l0rinc commented at 1:13 pm on October 31, 2025: contributor

Pushed, thanks for the review, you had a few good points, added you as coauthor.

the gain of using the spaceship operator (which comes at the cost of readability) is marginal.

the spaceship might not be very familiar to us yet, but it’s arguably simpler, having fewer moving parts, and some compilers prefer that over the manual comparisons.

in src/test/arith_uint256_tests.cpp:286 in 2d03d94535 outdated

283+BOOST_AUTO_TEST_CASE(comparison_equivalence)
284+{
285+    struct TestableArithUint256 : arith_uint256 {
286+        static int compare_original(const TestableArithUint256& a, const TestableArithUint256& b) {
287+            constexpr int WIDTH = 8; // 256 / 32
288+            for (int i = WIDTH - 1; i >= 0; i--) {

optout21 commented at 12:31 pm on November 3, 2025:

nit: --i is preferred

l0rinc commented at 12:43 pm on November 3, 2025:

Generally yes, but here I wanted to copy the original code as closely as possible

l0rinc commented at 12:48 pm on November 3, 2025:

Pushed anyway, thanks

in src/test/blockchain_tests.cpp:137 in 2d03d94535 outdated

132+
133+        // ... then by earliest activatable time, ...
134+        if (pa->nSequenceId < pb->nSequenceId)
135+            return false;
136+        if (pa->nSequenceId > pb->nSequenceId)
137+                return true;

optout21 commented at 12:35 pm on November 3, 2025:

nit: indentation seems off (extra indent)

l0rinc commented at 12:48 pm on November 3, 2025:

done, thanks

optout21 commented at 12:44 pm on November 3, 2025: contributor

utACK 5cbb9a40873203ea5a4dd0aa7127c9756da2f607

An evident, localized micro-optimization in a hot-path. I haven’t run measurements.

DrahtBot requested review from laanwj on Nov 3, 2025

DrahtBot requested review from mzumsande on Nov 3, 2025

l0rinc force-pushed on Nov 3, 2025

optout21 commented at 12:52 pm on November 3, 2025: contributor

reACK 80bfeaa240611401c1ffa41f6968b1f79b621938 Rebase.

Prev: reACK 78b43065a1746c19f74fe6380d765d5922e5c541 Rebase, no content changes. reACK 6e9061f8b7ad77df4566990121f5bebc1f9e6e15 utACK 237eec0f7ca095bc60e40ee94ba1a160a7064753 Re-acked (after 7 mins :D ), only minor/formatting changes since last review

maflcko commented at 3:34 pm on November 6, 2025: member

Profiling the performance regression in #33618 (comment) revealed that CBlockIndexWorkComparator and its underlying base_uint<256u>::CompareTo are hot paths during block validation, consuming ~4% of CPU time.

I don’t follow here. The issue is about the additional CWallet::WriteBestBlock overhead? CBlockIndexWorkComparator seems unrelated to the issue, at least I can’t see how it changed the result between 29.x and 30.x.

Instead of mentioning an unrelated issue, I think it could be better to mention what real-world effect this improvement has. I think that is -checkblockindex (default in regtest) and possibly LoadExternalBlockFile/LoadBlockIndex?

l0rinc commented at 4:22 pm on November 6, 2025: contributor

CBlockIndexWorkComparator seems unrelated to the issue

The original issue won’t be fixed since it’s not exactly a bug, but we could still speed up the regression by optimizing another bottleneck. Since you found the etymology confusing, I have removed the details from the description, only mentioned #33618 (comment) for the flame graph that show this being one of the bottlenecks. Not the biggest one, but the one we can easily optimize and clean up.

I think that is -checkblockindex (default in regtest)

Yes, this was mentioned in the benchmarking section.

possibly LoadExternalBlockFile/LoadBlockIndex

Yes, it’s used explicitly in Chainstate in FindMostWorkChain, PruneBlockIndexCandidates, ActivateBestChain and InvalidateBlock. And ChainstateManager in LoadBlockIndex, CheckBlockIndex, ActivateSnapshot and PopulateAndValidateSnapshot. But the setBlockIndexCandidates usages should already suffice as motivation, I’d say:

0git grep 'setBlockIndexCandidates.' src/validation.cpp | wc -l 
1   44

in src/node/blockstorage.h:94 in 237eec0f7c outdated

86@@ -87,12 +87,21 @@ static constexpr uint32_t UNDO_DATA_DISK_OVERHEAD{STORAGE_HEADER_BYTES + uint256
87 using BlockMap = std::unordered_map<uint256, CBlockIndex, BlockHasher>;
88 
89 struct CBlockIndexWorkComparator {
90-    bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const;
91+    // First sort by most total work (ascending), then by earliest activatable time (descending), then by pointer value (descending).
92+    // Pointer tiebreak should only happen with blocks loaded from disk, as those share the same id: 0 for blocks on the best chain, 1 for all others.
93+    bool operator()(const CBlockIndex* pa, const CBlockIndex* pb) const noexcept
94+    {
95+        return std::tie(pa->nChainWork, pb->nSequenceId, pb)

l0rinc commented at 10:50 pm on November 12, 2025:

I thought of eliminating the worst case here (when pa == pb, where it has to do the comparison for every byte) with something like:

0return pa != pb &&
1       std::tie(pa->nChainWork, pb->nSequenceId, pb)
2     < std::tie(pb->nChainWork, pa->nSequenceId, pa);

But the benchmarks indicate that it’s not really faster.

 0// Copyright (c) 2025-present The Bitcoin Core developers
 1// Distributed under the MIT software license, see the accompanying
 2// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 3
 4#include <arith_uint256.h>
 5#include <bench/bench.h>
 6#include <chain.h>
 7#include <node/blockstorage.h>
 8#include <random.h>
 9#include <uint256.h>
10
11#include <algorithm>
12#include <memory>
13#include <vector>
14
15static void CBlockIndexWorkComparator(benchmark::Bench& bench)
16{
17    FastRandomContext rng{/*fDeterministic=*/true};
18
19    constexpr size_t n{1'000};
20    std::vector<std::shared_ptr<CBlockIndex>> blocks;
21    blocks.reserve(n);
22    for (size_t i{0}; i < n; ++i) {
23        auto block{std::make_shared<CBlockIndex>()};
24        block->nChainWork = UintToArith256(rng.rand256());
25        block->nSequenceId = int32_t(rng.rand32());
26        if (i % 10 == 1) {
27            // Have some duplicates
28            if (rng.randbool()) {
29                block = blocks.back();
30            } else {
31                if (rng.randbool()) block->nChainWork = blocks.back()->nChainWork;
32                if (rng.randbool()) block->nSequenceId = blocks.back()->nSequenceId;
33            }
34        }
35        blocks.push_back(std::move(block));
36    }
37    std::ranges::shuffle(blocks, rng);
38
39    const node::CBlockIndexWorkComparator comparator;
40    bench.batch(n * n).unit("cmp").run([&] {
41        for (size_t i{0}; i < n; ++i) {
42            const auto* lhs{blocks[i].get()};
43            for (size_t j{0}; j < n; ++j) {
44                const auto* rhs{blocks[j].get()};
45                const bool result{comparator(lhs, rhs)};
46                ankerl::nanobench::doNotOptimizeAway(result);
47            }
48        }
49    });
50}
51
52BENCHMARK(CBlockIndexWorkComparator, benchmark::PriorityLevel::HIGH);

b17504b611 refactor: optimize CBlockIndexWorkComparator with std::tie

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
2.13	469,485,340.38	0.2%	15.07	5.09	2.959	4.02	0.1%	11.01	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
90,265.10	11,078.48	0.1%	637,536.00	215,946.18	2.952	178,844.00	0.0%	10.84	`CheckBlockIndex`

2b77e567d8 refactor: short-circuit CBlockIndex comparator for equality

ns/cmp	cmp/s	err%	ins/cmp	cyc/cmp	IPC	bra/cmp	miss%	total	benchmark
2.60	385,002,212.20	0.0%	18.00	6.22	2.894	5.00	0.0%	11.00	`CBlockIndexWorkComparator`

ns/op	op/s	err%	ins/op	cyc/op	IPC	bra/op	miss%	total	benchmark
94,110.53	10,625.80	0.0%	682,454.03	225,242.05	3.030	186,486.01	0.0%	11.00	`CheckBlockIndex`

DrahtBot added the label Needs rebase on Nov 25, 2025

l0rinc force-pushed on Nov 25, 2025

l0rinc commented at 2:53 pm on November 25, 2025: contributor

Rebased after https://github.com/bitcoin/bitcoin/commit/4f65a1c5db84426b9d814a18f63ac46632691ab2#diff-f14e9423bd43609969886b2e42751c55606344e0b71780564cdb23515802cec0R12, ready for review again.

DrahtBot removed the label Needs rebase on Nov 25, 2025

l0rinc referenced this in commit d42c922e92 on Nov 26, 2025

l0rinc commented at 5:13 pm on January 15, 2026: contributor

Rebased to fix silent merge conflict.

l0rinc force-pushed on Jan 15, 2026

DrahtBot added the label CI failed on Jan 15, 2026

DrahtBot commented at 7:13 pm on January 15, 2026: contributor

🚧 At least one of the CI tasks failed. Task iwyu: https://github.com/bitcoin/bitcoin/actions/runs/21039886920/job/60501264056 LLM reason (✨ experimental): IWYU reported a failure after adjusting includes in src/node/blockstorage.h, causing the CI to fail.

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.
A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.
An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

l0rinc force-pushed on Jan 15, 2026

DrahtBot removed the label CI failed on Jan 15, 2026

DrahtBot added the label Needs rebase on Feb 2, 2026

l0rinc commented at 10:44 am on February 2, 2026: contributor

Rebased after conflict with https://github.com/bitcoin/bitcoin/pull/34179/changes#diff-ed3f90693a242b38b9719af171de8f55183576957676dfa358945bea22276bd5R139, ready for review again.

l0rinc force-pushed on Feb 2, 2026

DrahtBot removed the label Needs rebase on Feb 2, 2026

sipa commented at 5:16 pm on February 2, 2026: member

You should really add a fuzz test here to compare old and new behavior as suggested before; it’s the perfect use case for it, as it avoids the need for putting presumed knowledge about potential bugs in the unit tests you create.

l0rinc force-pushed on Feb 2, 2026

l0rinc commented at 9:46 pm on February 2, 2026: contributor

You should really add a fuzz test here to compare old and new behavior

Thanks for the hint, I’ve split the differential fuzzer and kept the simple unit tests to to lock in the expected ordering of both comparators. After a while (if fuzzers can’t find any difference) we should be able to remove the fuzzers while keeping only the unit tests.

maflcko commented at 7:07 am on February 3, 2026: member

As pointed out earlier, most of the speedup comes from a move-only inline (https://github.com/bitcoin/bitcoin/pull/33637#pullrequestreview-3401558839)?

I think it would be better to just submit a trivial to review move-only change stand-alone, which wouldn’t need in-depth review and dedicated differential unit/fuzz tests. Or at least the move-only change should be the first change. This way, it is easier to see if the other changes are worth it to review/test at all.

bench: add benchmark to measure `CBlockIndexWorkComparator` performance

Profiling shows this comparator consumes a significant portion of `CheckBlockIndex()`:
... ChainstateManager::CheckBlockIndex()
    ... std::_Rb_tree<...>::find(...)
        ... node::CBlockIndexWorkComparator::operator()(...)
            ... base_uint<256u>::CompareTo(...) const

This commit introduces a benchmark that performs pairwise comparisons on 1000 randomly generated block indices (with some duplicates) to establish baseline performance metrics before further optimization.

|              ns/cmp |               cmp/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                4.35 |      229,642,637.11 |    0.2% |      1.10 | `CBlockIndexWorkComparator`

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           38,213.19 |           26,168.97 |    0.5% |      1.10 | `CheckBlockIndex`

70c6244b70

test: add sorting tests for `CBlockIndexWorkComparator` and `arith_uint256`

This test was added before other changes to create a baseline behavior.

f682c2be38

move-only: inline `CBlockIndex` comparators to header

Move `CBlockIndexWorkComparator::operator()` and `CBlockIndexHeightOnlyComparator::operator()` to the header to facilitate inlining.

|              ns/cmp |               cmp/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                2.87 |      348,283,873.30 |    0.2% |      1.10 | `CBlockIndexWorkComparator`

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           36,534.37 |           27,371.49 |    0.3% |      1.10 | `CheckBlockIndex`

516e05cd7d

fuzz: add equivalence coverage for comparison refactors

Add equivalence coverage to validate behavior while optimizing comparison operations.
The originals were kept exactly as the previous implementations were done.

The `arith_uint256` equivalence check is implemented as the `arith_uint256_comparison_equivalence` fuzz target.
It compares the refactored `operator<=>` against a snapshot of the old word-by-word comparison for all operators (<, >, <=, >=, ==, !=).

The `CBlockIndexWorkComparator` equivalence check is implemented as the `block_index_work_comparator` fuzz target.
It compares the refactored comparator against a snapshot of the old comparator logic (chain work, sequence ID, pointer tiebreak).

Add deterministic unit tests to lock in the expected ordering of both comparators.

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

70d3b99351

refactor: optimize `CBlockIndexWorkComparator` with `std::tie`

Replace manual comparison branches with a single tuple comparison, allowing the compilers to generate more efficient comparison code.

Made them explicitly `noexcept`, which can enable small optimizer wins and aligns with expectations for hot-path comparators used in ordered containers.

|              ns/cmp |               cmp/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                2.00 |      499,008,850.73 |    0.2% |      1.10 | `CBlockIndexWorkComparator`

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           32,297.54 |           30,962.11 |    0.9% |      1.10 | `CheckBlockIndex`

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

d053b27c06

refactor: inline `arith_uint256` comparison operator

Remove the `CompareTo` method and inline its logic directly into `operator<=>`, updating related comments.
This eliminates function call overhead in the hot path during block generation and chain selection.

The comparison algorithm remains unchanged, iterating from most significant to least significant byte, but now returns `std::strong_ordering` directly, removing an extra conversion and enabling better inlining.

|              ns/cmp |               cmp/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                2.18 |      459,159,880.91 |    0.2% |      1.10 | `CBlockIndexWorkComparator`

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           31,780.27 |           31,466.07 |    0.5% |      1.10 | `CheckBlockIndex`

8ef4ce2cb3

refactor: optimize `arith_uint256` comparison with spaceship operator

Replace multiple comparisons with a single C++20 spaceship operator call directly.

|              ns/cmp |               cmp/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|                0.68 |    1,477,110,677.23 |    0.2% |      1.06 | `CBlockIndexWorkComparator`

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|           27,528.11 |           36,326.51 |    0.2% |      1.10 | `CheckBlockIndex`

703165e6d3

l0rinc commented at 11:44 am on February 3, 2026: contributor

As pointed out earlier, most of the speedup comes from a move-only inline

That’s actually what enables the other ones to shine, but I don’t mind reordering the commits to make it obvious. The biggest optimization is the spaceship (with inline), it’s now the last commit.

  0for commit in 8b54cc2e71b015b651d7dcbfc3033c1f76c37e4e 9304b3b225f4dcc06b007d073035a586e1806951 0473240b4f8d29016778ebd73745c610c2b437e8 d64d9432a06879e57702f8188a764eb3fc8f2751 c2f69e78ba69ee4232c5190a41509e142b039e8b 7cb0c5f1fc70d57277689d114ab2da14af2a0696 df0153a846036f94e51ab70b7c91491b138d3569; do \
  1    git fetch origin $commit >/dev/null 2>&1 && git checkout $commit >/dev/null 2>&1 && echo "" && git log -1 --pretty='%h %s' && \
  2    rm -rfd build >/dev/null 2>&1 && cmake -B build -DBUILD_BENCH=ON -DCMAKE_BUILD_TYPE=Release >/dev/null 2>&1 && \
  3    cmake --build build -j$(nproc) >/dev/null 2>&1 && \
  4    for _ in $(seq 3); do \
  5      sleep 5; \
  6      sudo taskpolicy -t 5 -l 5 nice -n -20 ./build/bin/bench_bitcoin -filter='CBlockIndexWorkComparator|CheckBlockIndex' -min-time=1000; \
  7    done; \
  8done
  9
 10
 118b54cc2e71 bench: add benchmark to measure `CBlockIndexWorkComparator` performance
 12
 13|              ns/cmp |               cmp/s |    err% |     total | benchmark
 14|--------------------:|--------------------:|--------:|----------:|:----------
 15|                4.41 |      226,584,742.96 |    0.8% |      1.10 | `CBlockIndexWorkComparator`
 16
 17|               ns/op |                op/s |    err% |     total | benchmark
 18|--------------------:|--------------------:|--------:|----------:|:----------
 19|           38,253.37 |           26,141.49 |    0.9% |      1.09 | `CheckBlockIndex`
 20
 21|              ns/cmp |               cmp/s |    err% |     total | benchmark
 22|--------------------:|--------------------:|--------:|----------:|:----------
 23|                4.35 |      229,642,637.11 |    0.2% |      1.10 | `CBlockIndexWorkComparator`
 24
 25|               ns/op |                op/s |    err% |     total | benchmark
 26|--------------------:|--------------------:|--------:|----------:|:----------
 27|           38,213.19 |           26,168.97 |    0.5% |      1.10 | `CheckBlockIndex`
 28
 29|              ns/cmp |               cmp/s |    err% |     total | benchmark
 30|--------------------:|--------------------:|--------:|----------:|:----------
 31|                4.37 |      228,881,057.73 |    0.3% |      1.10 | `CBlockIndexWorkComparator`
 32
 33|               ns/op |                op/s |    err% |     total | benchmark
 34|--------------------:|--------------------:|--------:|----------:|:----------
 35|           38,287.78 |           26,117.99 |    0.4% |      1.10 | `CheckBlockIndex`
 36
 379304b3b225 test: add sorting tests for `CBlockIndexWorkComparator` and `arith_uint256`
 38
 39|              ns/cmp |               cmp/s |    err% |     total | benchmark
 40|--------------------:|--------------------:|--------:|----------:|:----------
 41|                4.39 |      227,961,262.13 |    0.5% |      1.10 | `CBlockIndexWorkComparator`
 42
 43|               ns/op |                op/s |    err% |     total | benchmark
 44|--------------------:|--------------------:|--------:|----------:|:----------
 45|           38,333.93 |           26,086.55 |    0.2% |      1.10 | `CheckBlockIndex`
 46
 47|              ns/cmp |               cmp/s |    err% |     total | benchmark
 48|--------------------:|--------------------:|--------:|----------:|:----------
 49|                4.39 |      227,728,690.97 |    0.7% |      1.11 | `CBlockIndexWorkComparator`
 50
 51|               ns/op |                op/s |    err% |     total | benchmark
 52|--------------------:|--------------------:|--------:|----------:|:----------
 53|           39,671.02 |           25,207.32 |    0.3% |      1.10 | `CheckBlockIndex`
 54
 55|              ns/cmp |               cmp/s |    err% |     total | benchmark
 56|--------------------:|--------------------:|--------:|----------:|:----------
 57|                4.40 |      227,294,352.21 |    0.5% |      1.09 | `CBlockIndexWorkComparator`
 58
 59|               ns/op |                op/s |    err% |     total | benchmark
 60|--------------------:|--------------------:|--------:|----------:|:----------
 61|           38,971.06 |           25,660.07 |    1.1% |      1.10 | `CheckBlockIndex`
 62
 630473240b4f move-only: inline `CBlockIndex` comparators to header
 64
 65|              ns/cmp |               cmp/s |    err% |     total | benchmark
 66|--------------------:|--------------------:|--------:|----------:|:----------
 67|                2.88 |      347,302,616.35 |    0.3% |      1.10 | `CBlockIndexWorkComparator`
 68
 69|               ns/op |                op/s |    err% |     total | benchmark
 70|--------------------:|--------------------:|--------:|----------:|:----------
 71|           36,701.33 |           27,246.97 |    0.3% |      1.10 | `CheckBlockIndex`
 72
 73|              ns/cmp |               cmp/s |    err% |     total | benchmark
 74|--------------------:|--------------------:|--------:|----------:|:----------
 75|                2.87 |      348,283,873.30 |    0.2% |      1.10 | `CBlockIndexWorkComparator`
 76
 77|               ns/op |                op/s |    err% |     total | benchmark
 78|--------------------:|--------------------:|--------:|----------:|:----------
 79|           36,790.73 |           27,180.76 |    0.6% |      1.10 | `CheckBlockIndex`
 80
 81|              ns/cmp |               cmp/s |    err% |     total | benchmark
 82|--------------------:|--------------------:|--------:|----------:|:----------
 83|                2.87 |      348,095,671.92 |    0.2% |      1.09 | `CBlockIndexWorkComparator`
 84
 85|               ns/op |                op/s |    err% |     total | benchmark
 86|--------------------:|--------------------:|--------:|----------:|:----------
 87|           36,534.37 |           27,371.49 |    0.3% |      1.10 | `CheckBlockIndex`
 88
 89d64d9432a0 test/fuzz: add equivalence coverage for comparison refactors
 90
 91|              ns/cmp |               cmp/s |    err% |     total | benchmark
 92|--------------------:|--------------------:|--------:|----------:|:----------
 93|                2.88 |      347,680,941.08 |    0.5% |      1.10 | `CBlockIndexWorkComparator`
 94
 95|               ns/op |                op/s |    err% |     total | benchmark
 96|--------------------:|--------------------:|--------:|----------:|:----------
 97|           37,044.83 |           26,994.32 |    1.1% |      1.08 | `CheckBlockIndex`
 98
 99|              ns/cmp |               cmp/s |    err% |     total | benchmark
100|--------------------:|--------------------:|--------:|----------:|:----------
101|                2.90 |      345,339,421.65 |    0.6% |      1.10 | `CBlockIndexWorkComparator`
102
103|               ns/op |                op/s |    err% |     total | benchmark
104|--------------------:|--------------------:|--------:|----------:|:----------
105|           37,174.31 |           26,900.30 |    1.4% |      1.09 | `CheckBlockIndex`
106
107|              ns/cmp |               cmp/s |    err% |     total | benchmark
108|--------------------:|--------------------:|--------:|----------:|:----------
109|                2.88 |      346,925,394.89 |    0.3% |      1.10 | `CBlockIndexWorkComparator`
110
111|               ns/op |                op/s |    err% |     total | benchmark
112|--------------------:|--------------------:|--------:|----------:|:----------
113|           37,287.05 |           26,818.96 |    1.6% |      1.09 | `CheckBlockIndex`
114
115c2f69e78ba refactor: optimize `CBlockIndexWorkComparator` with `std::tie`
116
117|              ns/cmp |               cmp/s |    err% |     total | benchmark
118|--------------------:|--------------------:|--------:|----------:|:----------
119|                2.00 |      499,008,850.73 |    0.2% |      1.10 | `CBlockIndexWorkComparator`
120
121|               ns/op |                op/s |    err% |     total | benchmark
122|--------------------:|--------------------:|--------:|----------:|:----------
123|           33,476.29 |           29,871.88 |    1.0% |      1.10 | `CheckBlockIndex`
124
125|              ns/cmp |               cmp/s |    err% |     total | benchmark
126|--------------------:|--------------------:|--------:|----------:|:----------
127|                2.03 |      493,360,414.53 |    0.5% |      1.10 | `CBlockIndexWorkComparator`
128
129|               ns/op |                op/s |    err% |     total | benchmark
130|--------------------:|--------------------:|--------:|----------:|:----------
131|           33,342.34 |           29,991.90 |    0.6% |      1.10 | `CheckBlockIndex`
132
133|              ns/cmp |               cmp/s |    err% |     total | benchmark
134|--------------------:|--------------------:|--------:|----------:|:----------
135|                2.01 |      498,514,389.19 |    0.5% |      1.10 | `CBlockIndexWorkComparator`
136
137|               ns/op |                op/s |    err% |     total | benchmark
138|--------------------:|--------------------:|--------:|----------:|:----------
139|           32,297.54 |           30,962.11 |    0.9% |      1.10 | `CheckBlockIndex`
140
1417cb0c5f1fc refactor: inline `arith_uint256` comparison operator
142
143|              ns/cmp |               cmp/s |    err% |     total | benchmark
144|--------------------:|--------------------:|--------:|----------:|:----------
145|                2.18 |      459,159,880.91 |    0.2% |      1.10 | `CBlockIndexWorkComparator`
146
147|               ns/op |                op/s |    err% |     total | benchmark
148|--------------------:|--------------------:|--------:|----------:|:----------
149|           32,006.20 |           31,243.95 |    0.7% |      1.10 | `CheckBlockIndex`
150
151|              ns/cmp |               cmp/s |    err% |     total | benchmark
152|--------------------:|--------------------:|--------:|----------:|:----------
153|                2.18 |      458,414,231.91 |    0.3% |      1.10 | `CBlockIndexWorkComparator`
154
155|               ns/op |                op/s |    err% |     total | benchmark
156|--------------------:|--------------------:|--------:|----------:|:----------
157|           31,780.27 |           31,466.07 |    0.5% |      1.10 | `CheckBlockIndex`
158
159|              ns/cmp |               cmp/s |    err% |     total | benchmark
160|--------------------:|--------------------:|--------:|----------:|:----------
161|                2.19 |      457,429,572.18 |    0.2% |      1.10 | `CBlockIndexWorkComparator`
162
163|               ns/op |                op/s |    err% |     total | benchmark
164|--------------------:|--------------------:|--------:|----------:|:----------
165|           32,536.14 |           30,735.06 |    1.2% |      1.09 | `CheckBlockIndex`
166
167df0153a846 refactor: optimize `arith_uint256` comparison with spaceship operator
168
169|              ns/cmp |               cmp/s |    err% |     total | benchmark
170|--------------------:|--------------------:|--------:|----------:|:----------
171|                0.68 |    1,473,011,136.06 |    0.5% |      1.06 | `CBlockIndexWorkComparator`
172
173|               ns/op |                op/s |    err% |     total | benchmark
174|--------------------:|--------------------:|--------:|----------:|:----------
175|           27,528.11 |           36,326.51 |    0.2% |      1.10 | `CheckBlockIndex`
176
177|              ns/cmp |               cmp/s |    err% |     total | benchmark
178|--------------------:|--------------------:|--------:|----------:|:----------
179|                0.69 |    1,451,170,702.42 |    1.0% |      1.07 | `CBlockIndexWorkComparator`
180
181|               ns/op |                op/s |    err% |     total | benchmark
182|--------------------:|--------------------:|--------:|----------:|:----------
183|           27,875.27 |           35,874.09 |    0.4% |      1.10 | `CheckBlockIndex`
184
185|              ns/cmp |               cmp/s |    err% |     total | benchmark
186|--------------------:|--------------------:|--------:|----------:|:----------
187|                0.68 |    1,477,110,677.23 |    0.2% |      1.06 | `CBlockIndexWorkComparator`
188
189|               ns/op |                op/s |    err% |     total | benchmark
190|--------------------:|--------------------:|--------:|----------:|:----------
191|           29,059.25 |           34,412.45 |    0.7% |      1.12 | `CheckBlockIndex`

Besides moving the inlines earlier, I also added noexcept to operator<=>(const base_uint and split the general unit test (which we should have regardless of the upcoming changes, they set the baseline to document current behavior) and the fuzz tests which probe and compare against the old code. Added the best measurements of 3 from above to the non-test commit messages for reference.

l0rinc force-pushed on Feb 3, 2026

refactor: optimize block index comparisons (1.4-6.8x faster) #33637

Context

Testing

Results

Reproducer

Code Coverage & Benchmarks

Reviews

Conflicts