From c3b502018e15a1ef54a4e2ced12a1c392746a5bc Mon Sep 17 00:00:00 2001
From: Andrei Pechkurov <37772591+puzpuzpuz@users.noreply.github.com>
Date: Tue, 26 Sep 2023 21:44:06 +0300
Subject: [PATCH] Speed up built-in string hash function (#106)

---
 .github/workflows/build-32-bit.yml |  2 +-
 .github/workflows/build.yml        |  2 +-
 .github/workflows/coverage.yml     |  2 +-
 export_test.go                     |  6 ++++++
 util.go                            | 16 ++++++++++++----
 util_test.go                       | 25 ++++++++++++++++++++++++-
 6 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/.github/workflows/build-32-bit.yml b/.github/workflows/build-32-bit.yml
index 3b99130..bc24367 100644
--- a/.github/workflows/build-32-bit.yml
+++ b/.github/workflows/build-32-bit.yml
@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: [1.20.x]
+        go-version: [1.21.x]
     name: Build with Go ${{ matrix.go-version }} 32-bit
     steps:
       - uses: actions/checkout@v3
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index f8ad21e..c5ad8bb 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: [1.20.x]
+        go-version: [1.21.x]
     name: Build with Go ${{ matrix.go-version }}
     steps:
       - uses: actions/checkout@v3
diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index da65143..30f8f55 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: [1.20.x]
+        go-version: [1.21.x]
     name: Build with Go ${{ matrix.go-version }}
     steps:
       - uses: actions/checkout@v3
diff --git a/export_test.go b/export_test.go
index 7d82818..16d373c 100644
--- a/export_test.go
+++ b/export_test.go
@@ -1,5 +1,7 @@
 package xsync
 
+import "hash/maphash"
+
 const (
 	EntriesPerMapBucket = entriesPerMapBucket
 	MapLoadFactor       = mapLoadFactor
@@ -48,6 +50,10 @@ func DisableAssertions() {
 	assertionsEnabled = false
 }
 
+func HashString(seed maphash.Seed, s string) uint64 {
+	return hashString(seed, s)
+}
+
 func Fastrand() uint32 {
 	return fastrand()
 }
diff --git a/util.go b/util.go
index 81a596a..d8a64f8 100644
--- a/util.go
+++ b/util.go
@@ -2,7 +2,9 @@ package xsync
 
 import (
 	"hash/maphash"
+	"reflect"
 	"runtime"
+	"unsafe"
 	_ "unsafe"
 )
 
@@ -44,12 +46,18 @@ func parallelism() uint32 {
 
 // hashString calculates a hash of s with the given seed.
 func hashString(seed maphash.Seed, s string) uint64 {
-	var h maphash.Hash
-	h.SetSeed(seed)
-	h.WriteString(s)
-	return h.Sum64()
+	seed64 := *(*uint64)(unsafe.Pointer(&seed))
+	if s == "" {
+		return seed64
+	}
+	strh := (*reflect.StringHeader)(unsafe.Pointer(&s))
+	return uint64(memhash(unsafe.Pointer(strh.Data), uintptr(seed64), uintptr(strh.Len)))
 }
 
+//go:noescape
+//go:linkname memhash runtime.memhash
+func memhash(p unsafe.Pointer, h, s uintptr) uintptr
+
 //go:noescape
 //go:linkname fastrand runtime.fastrand
 func fastrand() uint32
diff --git a/util_test.go b/util_test.go
index c4bbf22..32b3a3d 100644
--- a/util_test.go
+++ b/util_test.go
@@ -1,6 +1,7 @@
 package xsync_test
 
 import (
+	"hash/maphash"
 	"math/rand"
 	"testing"
 
@@ -25,7 +26,7 @@ func TestNextPowOf2(t *testing.T) {
 // This test is here to catch potential problems
 // with fastrand-related changes.
 func TestFastrand(t *testing.T) {
-	count := 10000
+	count := 100
 	set := make(map[uint32]struct{}, count)
 
 	for i := 0; i < count; i++ {
@@ -51,3 +52,25 @@ func BenchmarkRand(b *testing.B) {
 	}
 	// about 12 ns/op on x86-64
 }
+
+func BenchmarkMapHashString(b *testing.B) {
+	fn := func(seed maphash.Seed, s string) uint64 {
+		var h maphash.Hash
+		h.SetSeed(seed)
+		h.WriteString(s)
+		return h.Sum64()
+	}
+	seed := maphash.MakeSeed()
+	for i := 0; i < b.N; i++ {
+		_ = fn(seed, benchmarkKeyPrefix)
+	}
+	// about 13ns/op on x86-64
+}
+
+func BenchmarkHashString(b *testing.B) {
+	seed := maphash.MakeSeed()
+	for i := 0; i < b.N; i++ {
+		_ = HashString(seed, benchmarkKeyPrefix)
+	}
+	// about 4ns/op on x86-64
+}