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GH-48897: [C++] Benchmark and optimize CountSetBits #48898

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Merged
pitrou merged 1 commit into from
Jan 21, 2026
Merged

GH-48897: [C++] Benchmark and optimize CountSetBits #48898

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12 changes: 12 additions & 0 deletions cpp/src/arrow/util/bit_util_benchmark.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -439,6 +439,17 @@ static void SetBitsTo(benchmark::State& state) {
state.SetBytesProcessed(state.iterations() * nbytes);
}

static void CountSetBits(benchmark::State& state) {
int64_t nbytes = state.range(0);
std::shared_ptr<Buffer> buffer = CreateRandomBuffer(nbytes);

for (auto _ : state) {
auto count = internal::CountSetBits(buffer->data(), /*bit_offset=*/0, nbytes * 8);
benchmark::DoNotOptimize(count);
}
state.SetBytesProcessed(state.iterations() * nbytes);
}

template <int64_t OffsetSrc, int64_t OffsetDest = 0>
static void CopyBitmap(benchmark::State& state) { // NOLINT non-const reference
const int64_t buffer_size = state.range(0);
Expand Down Expand Up @@ -519,6 +530,7 @@ BENCHMARK(ReverseSetBitRunReader)->Apply(SetBitRunReaderPercentageArg);
BENCHMARK(VisitBits)->Arg(kBufferSize);
BENCHMARK(VisitBitsUnrolled)->Arg(kBufferSize);
BENCHMARK(SetBitsTo)->Arg(2)->Arg(1 << 4)->Arg(1 << 10)->Arg(1 << 17);
BENCHMARK(CountSetBits)->Arg(1 << 4)->Arg(1 << 10)->Arg(1 << 17);

#ifdef ARROW_WITH_BENCHMARKS_REFERENCE
static void ReferenceNaiveBitmapWriter(benchmark::State& state) {
Expand Down
11 changes: 7 additions & 4 deletions cpp/src/arrow/util/bitmap_ops.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@

#include "arrow/util/bitmap_ops.h"

#include <array>
#include <cstdint>
#include <cstring>
#include <functional>
Expand Down Expand Up @@ -55,13 +56,15 @@ int64_t CountSetBits(const uint8_t* data, int64_t bit_offset, int64_t length) {
constexpr int64_t kCountUnrollFactor = 4;
const int64_t words_rounded =
bit_util::RoundDown(p.aligned_words, kCountUnrollFactor);
int64_t count_unroll[kCountUnrollFactor] = {0};
std::array<int64_t, kCountUnrollFactor> count_unroll{};

// Unroll the loop for better performance
for (int64_t i = 0; i < words_rounded; i += kCountUnrollFactor) {
for (int64_t k = 0; k < kCountUnrollFactor; k++) {
count_unroll[k] += bit_util::PopCount(u64_data[k]);
}
// (hand-unrolled as some gcc versions would unnest a nested `for` loop)
count_unroll[0] += bit_util::PopCount(u64_data[0]);
count_unroll[1] += bit_util::PopCount(u64_data[1]);
count_unroll[2] += bit_util::PopCount(u64_data[2]);
count_unroll[3] += bit_util::PopCount(u64_data[3]);
u64_data += kCountUnrollFactor;
}
for (int64_t k = 0; k < kCountUnrollFactor; k++) {
Expand Down
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