util/deephash: generate type-specific hasher funcs

name                old time/op    new time/op    delta
Hash-8                71.1µs ± 2%    71.5µs ± 1%     ~     (p=0.114 n=9+8)
HashPacketFilter-8    8.39µs ± 1%    4.83µs ± 2%  -42.38%  (p=0.000 n=8+9)
HashMapAcyclic-8      56.2µs ± 1%    56.9µs ± 2%   +1.17%  (p=0.035 n=10+9)
TailcfgNode-8         6.49µs ± 2%    3.54µs ± 1%  -45.37%  (p=0.000 n=9+9)
HashArray-8            729ns ± 2%     566ns ± 3%  -22.30%  (p=0.000 n=10+10)

name                old alloc/op   new alloc/op   delta
Hash-8                 24.0B ± 0%     24.0B ± 0%     ~     (all equal)
HashPacketFilter-8     24.0B ± 0%     24.0B ± 0%     ~     (all equal)
HashMapAcyclic-8       0.00B          0.00B          ~     (all equal)
TailcfgNode-8          0.00B          0.00B          ~     (all equal)
HashArray-8            0.00B          0.00B          ~     (all equal)

name                old allocs/op  new allocs/op  delta
Hash-8                  1.00 ± 0%      1.00 ± 0%     ~     (all equal)
HashPacketFilter-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)
HashMapAcyclic-8        0.00           0.00          ~     (all equal)
TailcfgNode-8           0.00           0.00          ~     (all equal)
HashArray-8             0.00           0.00          ~     (all equal)

Change-Id: I34c4e786e748fe60280646d40cc63a2adb2ea6fe
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/deephash/deephash.go

@@ -26,6 +26,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"hash"
+	"log"
 	"math"
 	"reflect"
 	"sync"
@@ -142,6 +143,35 @@ func Hash(v any) (s Sum) {
 	return h.sum()
 }
 
+// HasherForType is like Hash, but it returns a Hash func that's specialized for
+// the provided reflect type, avoiding a map lookup per value.
+func HasherForType[T any]() func(T) Sum {
+	var zeroT T
+	ti := getTypeInfo(reflect.TypeOf(zeroT))
+	seedOnce.Do(initSeed)
+
+	return func(v T) Sum {
+		h := hasherPool.Get().(*hasher)
+		defer hasherPool.Put(h)
+		h.reset()
+		h.hashUint64(seed)
+
+		rv := reflect.ValueOf(v)
+
+		if rv.IsValid() {
+			// Always treat the Hash input as an interface (it is), including hashing
+			// its type, otherwise two Hash calls of different types could hash to the
+			// same bytes off the different types and get equivalent Sum values. This is
+			// the same thing that we do for reflect.Kind Interface in hashValue, but
+			// the initial reflect.ValueOf from an interface value effectively strips
+			// the interface box off so we have to do it at the top level by hand.
+			h.hashType(rv.Type())
+			h.hashValueWithType(rv, ti, false)
+		}
+		return h.sum()
+	}
+}
+
 // Update sets last to the hash of v and reports whether its value changed.
 func Update(last *Sum, v ...any) (changed bool) {
 	sum := Hash(v)
@@ -170,14 +200,26 @@ func (h *hasher) hashUint32(i uint32) {
 	binary.LittleEndian.PutUint32(h.scratch[:4], i)
 	h.bw.Write(h.scratch[:4])
 }
+func (h *hasher) hashLen(n int) {
+	binary.LittleEndian.PutUint64(h.scratch[:8], uint64(n))
+	h.bw.Write(h.scratch[:8])
+}
 func (h *hasher) hashUint64(i uint64) {
 	binary.LittleEndian.PutUint64(h.scratch[:8], i)
 	h.bw.Write(h.scratch[:8])
 }
 
-var uint8Type = reflect.TypeOf(byte(0))
+var (
+	uint8Type    = reflect.TypeOf(byte(0))
+	timeTimeType = reflect.TypeOf(time.Time{})
+)
 
 // typeInfo describes properties of a type.
+//
+// A non-nil typeInfo is populated into the typeHasher map
+// when its type is first requested, before its func is created.
+// Its func field fn is only populated once the type has been created.
+// This is used for recursive types.
 type typeInfo struct {
 	rtype       reflect.Type
 	canMemHash  bool
@@ -190,11 +232,394 @@ type typeInfo struct {
 	// keyTypeInfo is the map key type's typeInfo.
 	// It's set when rtype is of Kind Map.
 	keyTypeInfo *typeInfo
+
+	hashFuncOnce sync.Once
+	hashFuncLazy typeHasherFunc // nil until created
 }
 
+// returns ok if it was handled; else slow path runs
+type typeHasherFunc func(h *hasher, v reflect.Value) (ok bool)
+
 var typeInfoMap sync.Map           // map[reflect.Type]*typeInfo
 var typeInfoMapPopulate sync.Mutex // just for adding to typeInfoMap
 
+func (ti *typeInfo) hasher() typeHasherFunc {
+	ti.hashFuncOnce.Do(ti.buildHashFuncOnce)
+	return ti.hashFuncLazy
+}
+
+func (ti *typeInfo) buildHashFuncOnce() {
+	ti.hashFuncLazy = genTypeHasher(ti.rtype)
+}
+
+func (h *hasher) hashBoolv(v reflect.Value) bool {
+	var b byte
+	if v.Bool() {
+		b = 1
+	}
+	h.hashUint8(b)
+	return true
+}
+
+func (h *hasher) hashUint8v(v reflect.Value) bool {
+	h.hashUint8(uint8(v.Uint()))
+	return true
+}
+
+func (h *hasher) hashInt8v(v reflect.Value) bool {
+	h.hashUint8(uint8(v.Int()))
+	return true
+}
+
+func (h *hasher) hashUint16v(v reflect.Value) bool {
+	h.hashUint16(uint16(v.Uint()))
+	return true
+}
+
+func (h *hasher) hashInt16v(v reflect.Value) bool {
+	h.hashUint16(uint16(v.Int()))
+	return true
+}
+
+func (h *hasher) hashUint32v(v reflect.Value) bool {
+	h.hashUint32(uint32(v.Uint()))
+	return true
+}
+
+func (h *hasher) hashInt32v(v reflect.Value) bool {
+	h.hashUint32(uint32(v.Int()))
+	return true
+}
+
+func (h *hasher) hashUint64v(v reflect.Value) bool {
+	h.hashUint64(v.Uint())
+	return true
+}
+
+func (h *hasher) hashInt64v(v reflect.Value) bool {
+	h.hashUint64(uint64(v.Int()))
+	return true
+}
+
+func hashStructAppenderTo(h *hasher, v reflect.Value) bool {
+	if !v.CanInterface() {
+		return false // slow path
+	}
+	var a appenderTo
+	if v.CanAddr() {
+		a = v.Addr().Interface().(appenderTo)
+	} else {
+		a = v.Interface().(appenderTo)
+	}
+	size := h.scratch[:8]
+	record := a.AppendTo(size)
+	binary.LittleEndian.PutUint64(record, uint64(len(record)-len(size)))
+	h.bw.Write(record)
+	return true
+}
+
+// hashPointerAppenderTo hashes v, a reflect.Ptr, that implements appenderTo.
+func hashPointerAppenderTo(h *hasher, v reflect.Value) bool {
+	if !v.CanInterface() {
+		return false // slow path
+	}
+	if v.IsNil() {
+		h.hashUint8(0) // indicates nil
+		return true
+	}
+	h.hashUint8(1) // indicates visiting a pointer
+	a := v.Interface().(appenderTo)
+	size := h.scratch[:8]
+	record := a.AppendTo(size)
+	binary.LittleEndian.PutUint64(record, uint64(len(record)-len(size)))
+	h.bw.Write(record)
+	return true
+}
+
+// fieldInfo describes a struct field.
+type fieldInfo struct {
+	index      int // index of field for reflect.Value.Field(n)
+	typeInfo   *typeInfo
+	canMemHash bool
+	offset     uintptr // when we can memhash the field
+	size       uintptr // when we can memhash the field
+}
+
+// mergeContiguousFieldsCopy returns a copy of f with contiguous memhashable fields
+// merged together. Such fields get a bogus index and fu value.
+func mergeContiguousFieldsCopy(in []fieldInfo) []fieldInfo {
+	ret := make([]fieldInfo, 0, len(in))
+	var last *fieldInfo
+	for _, f := range in {
+		// Combine two fields if they're both contiguous & memhash-able.
+		if f.canMemHash && last != nil && last.canMemHash && last.offset+last.size == f.offset {
+			last.size += f.size
+			last.index = -1
+			last.typeInfo = nil
+		} else {
+			ret = append(ret, f)
+			last = &ret[len(ret)-1]
+		}
+	}
+	return ret
+}
+
+// genHashStructFields generates a typeHasherFunc for t, which must be of kind Struct.
+func genHashStructFields(t reflect.Type) typeHasherFunc {
+	fields := make([]fieldInfo, 0, t.NumField())
+	for i, n := 0, t.NumField(); i < n; i++ {
+		sf := t.Field(i)
+		if sf.Type.Size() == 0 {
+			continue
+		}
+		fields = append(fields, fieldInfo{
+			index:      i,
+			typeInfo:   getTypeInfo(sf.Type),
+			canMemHash: canMemHash(sf.Type),
+			offset:     sf.Offset,
+			size:       sf.Type.Size(),
+		})
+	}
+	fieldsIfCanAddr := mergeContiguousFieldsCopy(fields)
+	return structHasher{fields, fieldsIfCanAddr}.hash
+}
+
+type structHasher struct {
+	fields, fieldsIfCanAddr []fieldInfo
+}
+
+func (sh structHasher) hash(h *hasher, v reflect.Value) bool {
+	var base unsafe.Pointer
+	if v.CanAddr() {
+		base = v.Addr().UnsafePointer()
+		for _, f := range sh.fieldsIfCanAddr {
+			if f.canMemHash {
+				h.bw.Write(unsafe.Slice((*byte)(unsafe.Pointer(uintptr(base)+f.offset)), f.size))
+			} else if !f.typeInfo.hasher()(h, v.Field(f.index)) {
+				return false
+			}
+		}
+	} else {
+		for _, f := range sh.fields {
+			if !f.typeInfo.hasher()(h, v.Field(f.index)) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// genHashPtrToMemoryRange returns a hasher where the reflect.Value is a Ptr to
+// the provided eleType.
+func genHashPtrToMemoryRange(eleType reflect.Type) typeHasherFunc {
+	size := eleType.Size()
+	return func(h *hasher, v reflect.Value) bool {
+		if v.IsNil() {
+			h.hashUint8(0) // indicates nil
+		} else {
+			h.hashUint8(1) // indicates visiting a pointer
+			h.bw.Write(unsafe.Slice((*byte)(v.UnsafePointer()), size))
+		}
+		return true
+	}
+}
+
+const debug = false
+
+func genTypeHasher(t reflect.Type) typeHasherFunc {
+	if debug {
+		log.Printf("generating func for %v", t)
+	}
+
+	switch t.Kind() {
+	case reflect.Bool:
+		return (*hasher).hashBoolv
+	case reflect.Int8:
+		return (*hasher).hashInt8v
+	case reflect.Int16:
+		return (*hasher).hashInt16v
+	case reflect.Int32:
+		return (*hasher).hashInt32v
+	case reflect.Int, reflect.Int64:
+		return (*hasher).hashInt64v
+	case reflect.Uint8:
+		return (*hasher).hashUint8v
+	case reflect.Uint16:
+		return (*hasher).hashUint16v
+	case reflect.Uint32:
+		return (*hasher).hashUint32v
+	case reflect.Uint, reflect.Uintptr, reflect.Uint64:
+		return (*hasher).hashUint64v
+	case reflect.Float32:
+		return (*hasher).hashFloat32v
+	case reflect.Float64:
+		return (*hasher).hashFloat64v
+	case reflect.Complex64:
+		return (*hasher).hashComplex64v
+	case reflect.Complex128:
+		return (*hasher).hashComplex128v
+	case reflect.String:
+		return (*hasher).hashString
+	case reflect.Slice:
+		et := t.Elem()
+		if canMemHash(et) {
+			return (*hasher).hashSliceMem
+		}
+		eti := getTypeInfo(et)
+		return genHashSliceElements(eti)
+	case reflect.Array:
+		et := t.Elem()
+		eti := getTypeInfo(et)
+		return genHashArray(t, eti)
+	case reflect.Struct:
+		if t == timeTimeType {
+			return (*hasher).hashTimev
+		}
+		if t.Implements(appenderToType) {
+			return hashStructAppenderTo
+		}
+		return genHashStructFields(t)
+	case reflect.Pointer:
+		et := t.Elem()
+		if canMemHash(et) {
+			return genHashPtrToMemoryRange(et)
+		}
+		if t.Implements(appenderToType) {
+			return hashPointerAppenderTo
+		}
+		if !typeIsRecursive(t) {
+			eti := getTypeInfo(et)
+			return func(h *hasher, v reflect.Value) bool {
+				if v.IsNil() {
+					h.hashUint8(0) // indicates nil
+					return true
+				}
+				h.hashUint8(1) // indicates visiting a pointer
+				return eti.hasher()(h, v.Elem())
+			}
+		}
+	}
+
+	return func(h *hasher, v reflect.Value) bool {
+		if debug {
+			log.Printf("unhandled type %v", v.Type())
+		}
+		return false
+	}
+}
+
+// hashString hashes v, of kind String.
+func (h *hasher) hashString(v reflect.Value) bool {
+	s := v.String()
+	h.hashLen(len(s))
+	h.bw.WriteString(s)
+	return true
+}
+
+func (h *hasher) hashFloat32v(v reflect.Value) bool {
+	h.hashUint32(math.Float32bits(float32(v.Float())))
+	return true
+}
+
+func (h *hasher) hashFloat64v(v reflect.Value) bool {
+	h.hashUint64(math.Float64bits(v.Float()))
+	return true
+}
+
+func (h *hasher) hashComplex64v(v reflect.Value) bool {
+	c := complex64(v.Complex())
+	h.hashUint32(math.Float32bits(real(c)))
+	h.hashUint32(math.Float32bits(imag(c)))
+	return true
+}
+
+func (h *hasher) hashComplex128v(v reflect.Value) bool {
+	c := v.Complex()
+	h.hashUint64(math.Float64bits(real(c)))
+	h.hashUint64(math.Float64bits(imag(c)))
+	return true
+}
+
+// hashString hashes v, of kind time.Time.
+func (h *hasher) hashTimev(v reflect.Value) bool {
+	var t time.Time
+	if v.CanAddr() {
+		t = *(v.Addr().Interface().(*time.Time))
+	} else {
+		t = v.Interface().(time.Time)
+	}
+	b := t.AppendFormat(h.scratch[:1], time.RFC3339Nano)
+	b[0] = byte(len(b) - 1) // more than sufficient width; if not, good enough.
+	h.bw.Write(b)
+	return true
+}
+
+// hashSliceMem hashes v, of kind Slice, with a memhash-able element type.
+func (h *hasher) hashSliceMem(v reflect.Value) bool {
+	vLen := v.Len()
+	h.hashUint64(uint64(vLen))
+	if vLen == 0 {
+		return true
+	}
+	h.bw.Write(unsafe.Slice((*byte)(v.UnsafePointer()), v.Type().Elem().Size()*uintptr(vLen)))
+	return true
+}
+
+func genHashArrayMem(n int, arraySize uintptr, efu *typeInfo) typeHasherFunc {
+	byElement := genHashArrayElements(n, efu)
+	return func(h *hasher, v reflect.Value) bool {
+		if v.CanAddr() {
+			h.bw.Write(unsafe.Slice((*byte)(v.Addr().UnsafePointer()), arraySize))
+			return true
+		}
+		return byElement(h, v)
+	}
+}
+
+func genHashArrayElements(n int, eti *typeInfo) typeHasherFunc {
+	return func(h *hasher, v reflect.Value) bool {
+		for i := 0; i < n; i++ {
+			if !eti.hasher()(h, v.Index(i)) {
+				return false
+			}
+		}
+		return true
+	}
+}
+
+func noopHasherFunc(h *hasher, v reflect.Value) bool { return true }
+
+func genHashArray(t reflect.Type, eti *typeInfo) typeHasherFunc {
+	if t.Size() == 0 {
+		return noopHasherFunc
+	}
+	et := t.Elem()
+	if canMemHash(et) {
+		return genHashArrayMem(t.Len(), t.Size(), eti)
+	}
+	n := t.Len()
+	return genHashArrayElements(n, eti)
+}
+
+func genHashSliceElements(eti *typeInfo) typeHasherFunc {
+	return sliceElementHasher{eti}.hash
+}
+
+type sliceElementHasher struct {
+	eti *typeInfo
+}
+
+func (seh sliceElementHasher) hash(h *hasher, v reflect.Value) bool {
+	vLen := v.Len()
+	h.hashUint64(uint64(vLen))
+	for i := 0; i < vLen; i++ {
+		if !seh.eti.hasher()(h, v.Index(i)) {
+			return false
+		}
+	}
+	return true
+}
+
 func getTypeInfo(t reflect.Type) *typeInfo {
 	if f, ok := typeInfoMap.Load(t); ok {
 		return f.(*typeInfo)
@@ -353,6 +778,13 @@ func (h *hasher) hashValueWithType(v reflect.Value, ti *typeInfo, forceCycleChec
 	w := h.bw
 	doCheckCycles := forceCycleChecking || ti.isRecursive
 
+	if !doCheckCycles {
+		hf := ti.hasher()
+		if hf(h, v) {
+			return
+		}
+	}
+
 	// Generic handling.
 	switch v.Kind() {
 	default: