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Replace custom ctz() with C++20 std::countr_zero #813

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Merged
laurynas-biveinis merged 1 commit into from
Dec 2, 2025
Merged

Replace custom ctz() with C++20 std::countr_zero #813

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2 changes: 1 addition & 1 deletion art_common.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -187,7 +187,7 @@ template <typename T>
const T t = v | (v - 1u); // t gets v's least significant 0 bits set to 1
// Next set to 1 the most significant bit to change, set to 0 the
// least significant ones, and add the necessary 1 bits.
const T w = (t + 1) | (((~t & -~t) - 1) >> (ctz(v) + 1));
const T w = (t + 1) | (((~t & -~t) - 1) >> (std::countr_zero(v) + 1));
return w;
}

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36 changes: 20 additions & 16 deletions art_internal_impl.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@

#include <algorithm>
#include <array>
#include <bit>
#include <cstddef>
#include <cstdint>
#include <iostream>
Expand Down Expand Up @@ -578,7 +579,7 @@ union [[nodiscard]] key_prefix_snapshot {
/// Return the number of bytes in common between this key_prefix and a view of
/// the next 64-bits (max) of some the shifted_key from which any leading
/// bytes already matched by the traversal path have been discarded.
[[nodiscard]] UNODB_DETAIL_CONSTEXPR_NOT_MSVC auto get_shared_length(
[[nodiscard]] constexpr auto get_shared_length(
std::uint64_t shifted_key_u64) const noexcept {
return shared_len(shifted_key_u64, u64, length());
}
Expand All @@ -591,14 +592,13 @@ union [[nodiscard]] key_prefix_snapshot {

private:
// from unodb::detail::key_prefix
[[nodiscard, gnu::const]] static UNODB_DETAIL_CONSTEXPR_NOT_MSVC unsigned
shared_len(std::uint64_t k1, std::uint64_t k2,
unsigned clamp_byte_pos) noexcept {
[[nodiscard, gnu::const]] static constexpr unsigned shared_len(
std::uint64_t k1, std::uint64_t k2, unsigned clamp_byte_pos) noexcept {
UNODB_DETAIL_ASSERT(clamp_byte_pos < 8);

const auto diff = k1 ^ k2;
const auto clamped = diff | (1ULL << (clamp_byte_pos * 8U));
return static_cast<unsigned>(detail::ctz(clamped) >> 3U);
return static_cast<unsigned>(std::countr_zero(clamped) >> 3U);
}
}; // class key_prefix_snapshot
static_assert(sizeof(key_prefix_snapshot) == sizeof(std::uint64_t));
Expand Down Expand Up @@ -733,7 +733,7 @@ union [[nodiscard]] key_prefix {

const auto diff = k1 ^ k2;
const auto clamped = diff | (1ULL << (clamp_byte_pos * 8U));
return static_cast<unsigned>(detail::ctz(clamped) >> 3U);
return static_cast<unsigned>(std::countr_zero(clamped) >> 3U);
}

[[nodiscard, gnu::const]] static constexpr std::uint64_t make_u64(
Expand Down Expand Up @@ -1480,7 +1480,7 @@ class basic_inode_4 : public basic_inode_4_parent<ArtPolicy> {
const auto bit_field =
static_cast<unsigned>(_mm_movemask_epi8(matching_key_positions)) & mask;
if (bit_field != 0) {
const auto i = detail::ctz(bit_field);
const auto i = static_cast<std::uint8_t>(std::countr_zero(bit_field));
return std::make_pair(
i, static_cast<critical_section_policy<node_ptr> *>(&children[i]));
}
Expand All @@ -1502,7 +1502,7 @@ class basic_inode_4 : public basic_inode_4_parent<ArtPolicy> {

if (masked_pos == 0) return parent_class::child_not_found;

const auto i = static_cast<unsigned>(detail::ctz(masked_pos) >> 3U);
const auto i = static_cast<unsigned>(std::countr_zero(masked_pos) >> 3U);
return std::make_pair(
i, static_cast<critical_section_policy<node_ptr> *>(&children[i]));
#else // #ifdef UNODB_DETAIL_X86_64
Expand Down Expand Up @@ -1902,7 +1902,7 @@ class basic_inode_16 : public basic_inode_16_parent<ArtPolicy> {
const auto bit_field =
static_cast<unsigned>(_mm_movemask_epi8(matching_key_positions)) & mask;
if (bit_field != 0) {
const auto i = detail::ctz(bit_field);
const auto i = static_cast<std::uint8_t>(std::countr_zero(bit_field));
return std::make_pair(
i, static_cast<critical_section_policy<node_ptr> *>(&children[i]));
}
Expand All @@ -1924,7 +1924,7 @@ class basic_inode_16 : public basic_inode_16_parent<ArtPolicy> {

if (masked_pos == 0) return parent_class::child_not_found;

const auto i = static_cast<unsigned>(detail::ctz(masked_pos) >> 2U);
const auto i = static_cast<unsigned>(std::countr_zero(masked_pos) >> 2U);
return std::make_pair(
i, static_cast<critical_section_policy<node_ptr> *>(&children[i]));
#else
Expand Down Expand Up @@ -2053,9 +2053,10 @@ class basic_inode_16 : public basic_inode_16_parent<ArtPolicy> {
const auto mask = (1U << children_count_) - 1;
const auto bit_field =
static_cast<unsigned>(_mm_movemask_epi8(lesser_key_positions)) & mask;
const auto result = (bit_field != 0)
? detail::ctz(bit_field)
: static_cast<std::uint8_t>(children_count_);
const auto result =
(bit_field != 0)
? static_cast<std::uint8_t>(std::countr_zero(bit_field))
: static_cast<std::uint8_t>(children_count_);
#else
// This is also the best current ARM implementation
const auto result = static_cast<std::uint8_t>(
Expand Down Expand Up @@ -2233,7 +2234,8 @@ class basic_inode_48 : public basic_inode_48_parent<ArtPolicy> {
const auto cmp_mask =
static_cast<std::uint64_t>(_mm_movemask_epi8(vec_cmp));
if (cmp_mask != 0) {
i = (i << 1U) + (((detail::ctz(cmp_mask)) + 1U) >> 1U);
i = (i << 1U) +
((static_cast<unsigned>(std::countr_zero(cmp_mask)) + 1U) >> 1U);
break;
}
i += 4;
Expand Down Expand Up @@ -2265,7 +2267,8 @@ class basic_inode_48 : public basic_inode_48_parent<ArtPolicy> {
_mm256_permute4x64_epi64(interleaved_vec_cmp, 0b11'01'10'00);
const auto cmp_mask =
static_cast<std::uint64_t>(_mm256_movemask_epi8(vec_cmp));
i = (i << 2U) + (detail::ctz(cmp_mask) >> 1U);
i = (i << 2U) +
(static_cast<unsigned>(std::countr_zero(cmp_mask)) >> 1U);
break;
}
i += 4;
Expand Down Expand Up @@ -2298,7 +2301,8 @@ class basic_inode_48 : public basic_inode_48_parent<ArtPolicy> {
// NOLINTNEXTLINE(misc-const-correctness)
vget_lane_u64(vreinterpret_u64_u8(narrowed_cmp), 0);
if (scalar_pos != 0) {
i = (i << 1U) + static_cast<unsigned>(detail::ctz(scalar_pos) >> 3U);
i = (i << 1U) +
static_cast<unsigned>(std::countr_zero(scalar_pos) >> 3U);
break;
}
i += 4;
Expand Down
42 changes: 0 additions & 42 deletions portability_builtins.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -54,48 +54,6 @@ template <typename T>
#endif // UNODB_DETAIL_MSVC
}

/// Return the number of trailing zero bits in \a x.
/// \pre Argument may not be zero
template <typename T>
[[nodiscard, gnu::const]] UNODB_DETAIL_CONSTEXPR_NOT_MSVC std::uint8_t ctz(
T x) noexcept {
static_assert(std::is_same_v<unsigned, T> ||
// NOLINTNEXTLINE(google-runtime-int)
std::is_same_v<unsigned long, T> || // NOLINT(runtime/int)
// NOLINTNEXTLINE(google-runtime-int)
std::is_same_v<unsigned long long, T>); // NOLINT(runtime/int)

if constexpr (std::is_same_v<unsigned, T>) {
#ifndef UNODB_DETAIL_MSVC
return static_cast<std::uint8_t>(__builtin_ctz(x));
#else
unsigned long result; // NOLINT(runtime/int)
_BitScanForward(&result, x);
return static_cast<std::uint8_t>(result);
#endif
}
// NOLINTNEXTLINE(google-runtime-int)
if constexpr (std::is_same_v<unsigned long, T>) { // NOLINT(runtime/int)
#ifndef UNODB_DETAIL_MSVC
return static_cast<std::uint8_t>(__builtin_ctzl(x));
#else
unsigned long result; // NOLINT(runtime/int)
_BitScanForward(&result, x);
return static_cast<std::uint8_t>(result);
#endif
}
// NOLINTNEXTLINE(google-runtime-int)
if constexpr (std::is_same_v<unsigned long long, T>) { // NOLINT(runtime/int)
#ifndef UNODB_DETAIL_MSVC
return static_cast<std::uint8_t>(__builtin_ctzll(x));
#else
unsigned long result; // NOLINT(runtime/int)
_BitScanForward64(&result, x);
return static_cast<std::uint8_t>(result);
#endif
} // cppcheck-suppress missingReturn
}

/// Return the number of one bits in \a x.
[[nodiscard, gnu::const]] UNODB_DETAIL_CONSTEXPR_NOT_MSVC unsigned popcount(
unsigned x) noexcept {
Expand Down
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