cmd/compile, etc: bring back ptrToThis

This was removed in CL 19695 but it slows down reflect.New, which ends
up on the hot path of things like JSON decoding.

There is no immediate cost in binary size, but it will make it harder to
further shrink run time type information in Go 1.8.

Before

	BenchmarkNew-40         30000000                36.3 ns/op

After

	BenchmarkNew-40         50000000                29.5 ns/op

Fixes #16161
Updates #16117

Change-Id: If7cb7f3e745d44678f3f5cf3a5338c59847529d2
Reviewed-on: https://go-review.googlesource.com/24400Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -820,14 +820,10 @@ func dcommontype(s *Sym, ot int, t *Type) int {
 		algsym = dalgsym(t)
 	}
 
+	var sptr *Sym
 	tptr := Ptrto(t)
 	if !t.IsPtr() && (t.Sym != nil || methods(tptr) != nil) {
-		sptr := dtypesym(tptr)
-		r := obj.Addrel(Linksym(s))
-		r.Off = 0
-		r.Siz = 0
-		r.Sym = sptr.Lsym
-		r.Type = obj.R_USETYPE
+		sptr = dtypesym(tptr)
 	}
 
 	gcsym, useGCProg, ptrdata := dgcsym(t)
@@ -845,7 +841,7 @@ func dcommontype(s *Sym, ot int, t *Type) int {
 	//		alg           *typeAlg
 	//		gcdata        *byte
 	//		str           nameOff
-	//		_             int32
+	//		ptrToThis     typeOff
 	//	}
 	ot = duintptr(s, ot, uint64(t.Width))
 	ot = duintptr(s, ot, uint64(ptrdata))
@@ -909,8 +905,12 @@ func dcommontype(s *Sym, ot int, t *Type) int {
 	ot = dsymptr(s, ot, gcsym, 0) // gcdata
 
 	nsym := dname(p, "", nil, exported)
-	ot = dsymptrOffLSym(Linksym(s), ot, nsym, 0)
+	ot = dsymptrOffLSym(Linksym(s), ot, nsym, 0) // str
+	if sptr == nil {
 		ot = duint32(s, ot, 0)
+	} else {
+		ot = dsymptrOffLSym(Linksym(s), ot, Linksym(sptr), 0) // ptrToThis
+	}
 
 	return ot
 }

src/reflect/all_test.go

@@ -5741,3 +5741,10 @@ func TestOffsetLock(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+func BenchmarkNew(b *testing.B) {
+	v := TypeOf(XM{})
+	for i := 0; i < b.N; i++ {
+		New(v)
+	}
+}

src/reflect/type.go

@@ -285,7 +285,7 @@ type rtype struct {
 	alg        *typeAlg // algorithm table
 	gcdata     *byte    // garbage collection data
 	str        nameOff  // string form
-	_          int32    // unused; keeps rtype always a multiple of ptrSize
+	ptrToThis  typeOff  // type for pointer to this type, may be zero
 }
 
 // a copy of runtime.typeAlg
@@ -1430,6 +1430,10 @@ func PtrTo(t Type) Type {
 }
 
 func (t *rtype) ptrTo() *rtype {
+	if t.ptrToThis != 0 {
+		return t.typeOff(t.ptrToThis)
+	}
+
 	// Check the cache.
 	ptrMap.RLock()
 	if m := ptrMap.m; m != nil {
@@ -1927,6 +1931,7 @@ func MapOf(key, elem Type) Type {
 	mt.bucketsize = uint16(mt.bucket.size)
 	mt.reflexivekey = isReflexive(ktyp)
 	mt.needkeyupdate = needKeyUpdate(ktyp)
+	mt.ptrToThis = 0
 
 	return cachePut(ckey, &mt.rtype)
 }
@@ -2065,6 +2070,7 @@ func FuncOf(in, out []Type, variadic bool) Type {
 
 	// Populate the remaining fields of ft and store in cache.
 	ft.str = resolveReflectName(newName(str, "", "", false))
+	ft.ptrToThis = 0
 	funcLookupCache.m[hash] = append(funcLookupCache.m[hash], &ft.rtype)
 
 	return &ft.rtype
@@ -2295,6 +2301,7 @@ func SliceOf(t Type) Type {
 	slice.str = resolveReflectName(newName(s, "", "", false))
 	slice.hash = fnv1(typ.hash, '[')
 	slice.elem = typ
+	slice.ptrToThis = 0
 
 	return cachePut(ckey, &slice.rtype)
 }
@@ -2842,6 +2849,7 @@ func ArrayOf(count int, elem Type) Type {
 	}
 	array.hash = fnv1(array.hash, ']')
 	array.elem = typ
+	array.ptrToThis = 0
 	max := ^uintptr(0) / typ.size
 	if uintptr(count) > max {
 		panic("reflect.ArrayOf: array size would exceed virtual address space")

src/runtime/type.go

@@ -38,7 +38,7 @@ type _type struct {
 	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
 	gcdata    *byte
 	str       nameOff
-	_      int32
+	ptrToThis typeOff
 }
 
 func (t *_type) string() string {