From a936dfe8a5fda03dc25da1d247c792eee8f48fe2 Mon Sep 17 00:00:00 2001
From: Roman Gershman <roman@dragonflydb.io>
Date: Sun, 19 Jan 2025 20:07:22 +0200
Subject: [PATCH] chore: add Dash::Prefetch function (#4476)

* chore: add Dash::Prefetch function

It's not being used at this time.

* chore: fixes
---
 src/core/dash.h          | 11 +++++++++++
 src/core/dash_internal.h | 10 ++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/core/dash.h b/src/core/dash.h
index 0c67fd6b2..05a99560f 100644
--- a/src/core/dash.h
+++ b/src/core/dash.h
@@ -149,6 +149,9 @@ class DashTable : public detail::DashTableBase {
   template <typename U> const_iterator Find(U&& key) const;
   template <typename U> iterator Find(U&& key);
 
+  // Prefetches the memory where the key would resize into the cache.
+  template <typename U> void Prefetch(U&& key) const;
+
   // Find first entry with given key hash that evaulates to true on pred.
   // Pred accepts either (const key&) or (const key&, const value&)
   template <typename Pred> iterator FindFirst(uint64_t key_hash, Pred&& pred);
@@ -699,6 +702,14 @@ auto DashTable<_Key, _Value, Policy>::Find(U&& key) -> iterator {
   return FindFirst(DoHash(key), EqPred(key));
 }
 
+template <typename _Key, typename _Value, typename Policy>
+template <typename U>
+void DashTable<_Key, _Value, Policy>::Prefetch(U&& key) const {
+  uint64_t key_hash = DoHash(key);
+  uint32_t seg_id = SegmentId(key_hash);
+  segment_[seg_id]->Prefetch(key_hash);
+}
+
 template <typename _Key, typename _Value, typename Policy>
 template <typename Pred>
 auto DashTable<_Key, _Value, Policy>::FindFirst(uint64_t key_hash, Pred&& pred) -> iterator {
diff --git a/src/core/dash_internal.h b/src/core/dash_internal.h
index 25981140d..418ec4963 100644
--- a/src/core/dash_internal.h
+++ b/src/core/dash_internal.h
@@ -502,6 +502,7 @@ template <typename _Key, typename _Value, typename Policy = DefaultSegmentPolicy
 
   // Find item with given key hash and truthy predicate
   template <typename Pred> Iterator FindIt(Hash_t key_hash, Pred&& pred) const;
+  void Prefetch(Hash_t key_hash) const;
 
   // Returns valid iterator if succeeded or invalid if not (it's full).
   // Requires: key should be not present in the segment.
@@ -1188,6 +1189,15 @@ auto Segment<Key, Value, Policy>::FindIt(Hash_t key_hash, Pred&& pred) const ->
   return Iterator{};
 }
 
+template <typename Key, typename Value, typename Policy>
+void Segment<Key, Value, Policy>::Prefetch(Hash_t key_hash) const {
+  uint8_t bidx = BucketIndex(key_hash);
+  const Bucket& target = bucket_[bidx];
+
+  // Prefetch the home bucket that might hold the key with high probability.
+  __builtin_prefetch(&target, 0, 1);
+}
+
 template <typename Key, typename Value, typename Policy>
 template <typename Cb>
 void Segment<Key, Value, Policy>::TraverseAll(Cb&& cb) const {