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Add: Support level-aware prefetch in HNSW(index_gt) search #696

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Add: Support level-aware prefetch in HNSW(index_gt) search #696

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d41cef8
Add: Support level-aware prefetch in HNSW(index_gt) search
yoonseok-kim Jan 6, 2026
c5ecb5f
Fix: Enable level-aware prefetch support for C++14
yoonseok-kim Jan 6, 2026
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59 changes: 50 additions & 9 deletions include/usearch/index.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -173,6 +173,14 @@ namespace unum {
namespace usearch {

using byte_t = char;
using level_t = std::int16_t;

// C++14-compatible void_t
#ifdef USEARCH_DEFINED_CPP17
template <typename...ts> using void_t = std::void_t<ts...>;
#else
template <typename...> using void_t = void;
#endif

template <std::size_t multiple_ak> std::size_t divide_round_up(std::size_t num) noexcept {
return (num + multiple_ak - 1) / multiple_ak;
Expand Down Expand Up @@ -1521,6 +1529,8 @@ struct dummy_progress_t {
struct dummy_prefetch_t {
template <typename member_citerator_like_at>
inline void operator()(member_citerator_like_at, member_citerator_like_at) const noexcept {}
template <typename member_citerator_like_at>
inline void operator()(member_citerator_like_at, member_citerator_like_at, level_t) const noexcept {}
};

/**
Expand Down Expand Up @@ -1582,6 +1592,37 @@ template <typename object_at> static constexpr bool is_dummy() {
std::is_same<typename std::decay<object_t>::type, dummy_key_to_key_mapping_t>::value;
}

/**
* @brief Compile-time check if a prefetch functor supports the 3-argument version with level.
* This enables backward compatibility: existing 2-arg prefetch implementations still work,
* while new implementations can receive level information.
*/
template <typename prefetch_at, typename iterator_at, typename = void>
struct has_level_prefetch : std::false_type {};

template <typename prefetch_at, typename iterator_at>
struct has_level_prefetch<prefetch_at, iterator_at,
void_t<decltype(std::declval<prefetch_at>()(
std::declval<iterator_at>(),
std::declval<iterator_at>(),
std::declval<level_t>()))>> : std::true_type {};

template <typename prefetch_at, typename iterator_at>
void invoke_prefetch(prefetch_at&& prefetch, iterator_at begin, iterator_at end, level_t level, std::true_type) {
prefetch(begin, end, level);
}

template <typename prefetch_at, typename iterator_at>
void invoke_prefetch(prefetch_at&& prefetch, iterator_at begin, iterator_at end, level_t, std::false_type) {
prefetch(begin, end);
}

template <typename prefetch_at, typename iterator_at>
void invoke_prefetch(prefetch_at&& prefetch, iterator_at begin, iterator_at end, level_t level) {
invoke_prefetch(std::forward<prefetch_at>(prefetch), begin, end, level,
typename has_level_prefetch<typename std::decay<prefetch_at>::type, iterator_at>::type{});
}

template <typename, typename at> struct has_reset_gt {
static_assert(std::integral_constant<at, false>::value, "Second template parameter needs to be of function type.");
};
Expand Down Expand Up @@ -2077,7 +2118,7 @@ class index_gt {
* alignment in most common cases.
*/
using neighbors_count_t = std::uint32_t;
using level_t = std::int16_t;
using level_t = usearch::level_t;

/**
* @brief How many bytes of memory are needed to form the "head" of the node.
Expand Down Expand Up @@ -3997,7 +4038,7 @@ class index_gt {

// Optional prefetching
if (!is_dummy<prefetch_at>())
prefetch(citerator_at(closest_slot), citerator_at(closest_slot) + 1);
invoke_prefetch(prefetch, citerator_at(closest_slot), citerator_at(closest_slot) + 1, begin_level);

bool const need_lock = !is_immutable();
distance_t closest_dist = context.measure(query, citerator_at(closest_slot), metric);
Expand All @@ -4011,7 +4052,7 @@ class index_gt {
// Optional prefetching
if (!is_dummy<prefetch_at>()) {
candidates_range_t missing_candidates{*this, closest_neighbors, visits};
prefetch(missing_candidates.begin(), missing_candidates.end());
invoke_prefetch(prefetch, missing_candidates.begin(), missing_candidates.end(), level);
}

// Actual traversal
Expand Down Expand Up @@ -4054,7 +4095,7 @@ class index_gt {

// Optional prefetching
if (!is_dummy<prefetch_at>())
prefetch(citerator_at(start_slot), citerator_at(start_slot) + 1);
invoke_prefetch(prefetch, citerator_at(start_slot), citerator_at(start_slot) + 1, level);

distance_t radius = context.measure(query, citerator_at(start_slot), metric);
next.insert_reserved({-radius, start_slot});
Expand All @@ -4079,7 +4120,7 @@ class index_gt {
// Optional prefetching
if (!is_dummy<prefetch_at>()) {
candidates_range_t missing_candidates{*this, candidate_neighbors, visits};
prefetch(missing_candidates.begin(), missing_candidates.end());
invoke_prefetch(prefetch, missing_candidates.begin(), missing_candidates.end(), level);
}

// Assume the worst-case when reserving memory
Expand Down Expand Up @@ -4131,7 +4172,7 @@ class index_gt {

// Optional prefetching
if (!is_dummy<prefetch_at>())
prefetch(citerator_at(start_slot), citerator_at(start_slot) + 1);
invoke_prefetch(prefetch, citerator_at(start_slot), citerator_at(start_slot) + 1, level);

distance_t radius = context.measure(query, citerator_at(start_slot), metric);
next.insert_reserved({-radius, start_slot});
Expand Down Expand Up @@ -4167,7 +4208,7 @@ class index_gt {
// Optional prefetching
if (!is_dummy<prefetch_at>()) {
candidates_range_t missing_candidates{*this, candidate_neighbors, visits};
prefetch(missing_candidates.begin(), missing_candidates.end());
invoke_prefetch(prefetch, missing_candidates.begin(), missing_candidates.end(), level);
}

// Assume the worst-case when reserving memory
Expand Down Expand Up @@ -4219,7 +4260,7 @@ class index_gt {

// Optional prefetching
if (!is_dummy<prefetch_at>())
prefetch(citerator_at(start_slot), citerator_at(start_slot) + 1);
invoke_prefetch(prefetch, citerator_at(start_slot), citerator_at(start_slot) + 1, level_t{0});

distance_t radius = context.measure(query, citerator_at(start_slot), metric);
usearch_assert_m(next.capacity(), "The `max_heap_gt` must have been reserved in the search entry point");
Expand Down Expand Up @@ -4247,7 +4288,7 @@ class index_gt {
// Optional prefetching
if (!is_dummy<prefetch_at>()) {
candidates_range_t missing_candidates{*this, candidate_neighbors, visits};
prefetch(missing_candidates.begin(), missing_candidates.end());
invoke_prefetch(prefetch, missing_candidates.begin(), missing_candidates.end(), level_t{0});
}

// Assume the worst-case when reserving memory
Expand Down
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