From b8ebbc2ecadb64239dc0767b55296ab9c4510b0f Mon Sep 17 00:00:00 2001
From: Abseil Team <absl-team@google.com>
Date: Thu, 13 Jul 2023 10:51:10 -0700
Subject: Adding sw prefetchers to absl::hash.

PiperOrigin-RevId: 547850162
Change-Id: I43208c7fa1eaa2a7acfad5891b80c150ee58c65f
---
 absl/hash/BUILD.bazel                | 1 +
 absl/hash/CMakeLists.txt             | 1 +
 absl/hash/internal/low_level_hash.cc | 6 ++++++
 3 files changed, 8 insertions(+)

(limited to 'absl')

diff --git a/absl/hash/BUILD.bazel b/absl/hash/BUILD.bazel
index 7f964ae7..4346fc49 100644
--- a/absl/hash/BUILD.bazel
+++ b/absl/hash/BUILD.bazel
@@ -183,6 +183,7 @@ cc_library(
     deps = [
         "//absl/base:config",
         "//absl/base:endian",
+        "//absl/base:prefetch",
         "//absl/numeric:int128",
     ],
 )
diff --git a/absl/hash/CMakeLists.txt b/absl/hash/CMakeLists.txt
index 1adce617..65fd2a5f 100644
--- a/absl/hash/CMakeLists.txt
+++ b/absl/hash/CMakeLists.txt
@@ -165,6 +165,7 @@ absl_cc_library(
     absl::config
     absl::endian
     absl::int128
+    absl::prefetch
 )
 
 absl_cc_test(
diff --git a/absl/hash/internal/low_level_hash.cc b/absl/hash/internal/low_level_hash.cc
index c917457a..b5db0b89 100644
--- a/absl/hash/internal/low_level_hash.cc
+++ b/absl/hash/internal/low_level_hash.cc
@@ -15,6 +15,7 @@
 #include "absl/hash/internal/low_level_hash.h"
 
 #include "absl/base/internal/unaligned_access.h"
+#include "absl/base/prefetch.h"
 #include "absl/numeric/int128.h"
 
 namespace absl {
@@ -29,6 +30,8 @@ static uint64_t Mix(uint64_t v0, uint64_t v1) {
 
 uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed,
                       const uint64_t salt[5]) {
+  // Prefetch the cacheline that data resides in.
+  PrefetchToLocalCache(data);
   const uint8_t* ptr = static_cast<const uint8_t*>(data);
   uint64_t starting_length = static_cast<uint64_t>(len);
   uint64_t current_state = seed ^ salt[0];
@@ -40,6 +43,9 @@ uint64_t LowLevelHash(const void* data, size_t len, uint64_t seed,
     uint64_t duplicated_state = current_state;
 
     do {
+      // Always prefetch the next cacheline.
+      PrefetchToLocalCache(ptr + ABSL_CACHELINE_SIZE);
+
       uint64_t a = absl::base_internal::UnalignedLoad64(ptr);
       uint64_t b = absl::base_internal::UnalignedLoad64(ptr + 8);
       uint64_t c = absl::base_internal::UnalignedLoad64(ptr + 16);
-- 
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