util/deephash: expand fast-path capabilities (#5404)

Add support for maps and interfaces to the fast path.
Add cycle-detection to the pointer handling logic.
This logic is mostly copied from the slow path.

A future commit will delete the slow path once
the fast path never falls back to the slow path.

Performance:

	name                 old time/op    new time/op    delta
	Hash-24                18.5µs ± 1%    14.9µs ± 2%  -19.52%  (p=0.000 n=10+10)
	HashPacketFilter-24    2.54µs ± 1%    2.60µs ± 1%   +2.19%  (p=0.000 n=10+10)
	HashMapAcyclic-24      31.6µs ± 1%    30.5µs ± 1%   -3.42%  (p=0.000 n=9+8)
	TailcfgNode-24         1.44µs ± 2%    1.43µs ± 1%     ~     (p=0.171 n=10+10)
	HashArray-24            324ns ± 1%     324ns ± 2%     ~     (p=0.425 n=9+9)

The additional cycle detection logic doesn't incur much slow down
since it only activates if a type is recursive, which does not apply
for any of the types that we care about.

There is a notable performance boost since we switch from the fath path
to the slow path less often. Most notably, a struct with a field that
could not be handled by the fast path would previously cause
the entire struct to go through the slow path.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/deephash/deephash.go

@@ -14,7 +14,7 @@
 //   - time.Time are compared based on whether they are the same instant in time
 //     and also in the same zone offset. Monotonic measurements and zone names
 //     are ignored as part of the hash.
-//   - netip.Addr are compared based on a shallow comparison of the struct. 
+//   - netip.Addr are compared based on a shallow comparison of the struct.
 //
 // WARNING: This package, like most of the tailscale.com Go module,
 // should be considered Tailscale-internal; we make no API promises.
@@ -25,7 +25,6 @@ import (
 	"encoding/binary"
 	"encoding/hex"
 	"fmt"
-	"log"
 	"math"
 	"net/netip"
 	"reflect"
@@ -246,7 +245,7 @@ func (ti *typeInfo) hasher() typeHasherFunc {
 }
 
 func (ti *typeInfo) buildHashFuncOnce() {
-	ti.hashFuncLazy = genTypeHasher(ti.rtype)
+	ti.hashFuncLazy = genTypeHasher(ti)
 }
 
 func (h *hasher) hashBoolv(v addressableValue) bool {
@@ -380,13 +379,8 @@ func genHashPtrToMemoryRange(eleType reflect.Type) typeHasherFunc {
 	}
 }
 
-const debug = false
-
-func genTypeHasher(t reflect.Type) typeHasherFunc {
-	if debug {
-		log.Printf("generating func for %v", t)
-	}
-
+func genTypeHasher(ti *typeInfo) typeHasherFunc {
+	t := ti.rtype
 	switch t.Kind() {
 	case reflect.Bool:
 		return (*hasher).hashBoolv
@@ -436,30 +430,67 @@ func genTypeHasher(t reflect.Type) typeHasherFunc {
 		default:
 			return genHashStructFields(t)
 		}
+	case reflect.Map:
+		return func(h *hasher, v addressableValue) bool {
+			if v.IsNil() {
+				h.HashUint8(0) // indicates nil
+				return true
+			}
+			if ti.isRecursive {
+				ptr := pointerOf(v)
+				if idx, ok := h.visitStack.seen(ptr); ok {
+					h.HashUint8(2) // indicates cycle
+					h.HashUint64(uint64(idx))
+					return true
+				}
+				h.visitStack.push(ptr)
+				defer h.visitStack.pop(ptr)
+			}
+			h.HashUint8(1) // indicates visiting a map
+			h.hashMap(v, ti, ti.isRecursive)
+			return true
+		}
 	case reflect.Pointer:
 		et := t.Elem()
 		if typeIsMemHashable(et) {
 			return genHashPtrToMemoryRange(et)
 		}
-		if !typeIsRecursive(t) {
-			eti := getTypeInfo(et)
-			return func(h *hasher, v addressableValue) bool {
-				if v.IsNil() {
-					h.HashUint8(0) // indicates nil
+		eti := getTypeInfo(et)
+		return func(h *hasher, v addressableValue) bool {
+			if v.IsNil() {
+				h.HashUint8(0) // indicates nil
+				return true
+			}
+			if ti.isRecursive {
+				ptr := pointerOf(v)
+				if idx, ok := h.visitStack.seen(ptr); ok {
+					h.HashUint8(2) // indicates cycle
+					h.HashUint64(uint64(idx))
 					return true
 				}
-				h.HashUint8(1)                   // indicates visiting a pointer
-				va := addressableValue{v.Elem()} // dereferenced pointer is always addressable
-				return eti.hasher()(h, va)
+				h.visitStack.push(ptr)
+				defer h.visitStack.pop(ptr)
 			}
+			h.HashUint8(1)                   // indicates visiting a pointer
+			va := addressableValue{v.Elem()} // dereferenced pointer is always addressable
+			return eti.hasher()(h, va)
 		}
-	}
+	case reflect.Interface:
+		return func(h *hasher, v addressableValue) bool {
+			if v.IsNil() {
+				h.HashUint8(0) // indicates nil
+				return true
+			}
+			va := newAddressableValue(v.Elem().Type())
+			va.Set(v.Elem())
 
-	return func(h *hasher, v addressableValue) bool {
-		if debug {
-			log.Printf("unhandled type %v", v.Type())
+			h.HashUint8(1) // indicates visiting interface value
+			h.hashType(va.Type())
+			h.hashValue(va, true)
+			return true
 		}
-		return false
+	default: // Func, Chan, UnsafePointer
+		return noopHasherFunc
 	}
 }
 
@@ -646,11 +677,8 @@ func (h *hasher) hashValue(v addressableValue, forceCycleChecking bool) {
 func (h *hasher) hashValueWithType(v addressableValue, ti *typeInfo, forceCycleChecking bool) {
 	doCheckCycles := forceCycleChecking || ti.isRecursive
 
-	if !doCheckCycles {
-		hf := ti.hasher()
-		if hf(h, v) {
-			return
-		}
+	if ti.hasher()(h, v) {
+		return
 	}
 
 	// Generic handling.