Speed up built-in string hash function (#106)

.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

.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

.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

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()
 }

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

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
+}