remove Elem() optimization

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

@@ -415,14 +415,6 @@ var kinds = []abi.Kind{
 	types.TUNSAFEPTR:  abi.UnsafePointer,
 }
 
-var elemKinds = []abi.Kind{
-	abi.Array,
-	abi.Chan,
-	abi.Map,
-	abi.Pointer,
-	abi.Slice,
-}
-
 var (
 	memhashvarlen  *obj.LSym
 	memequalvarlen *obj.LSym
@@ -481,14 +473,6 @@ func dcommontype(c rttype.Cursor, t *types.Type) {
 		tflag |= abi.TFlagGCMaskOnDemand
 	}
 
-	kind := kinds[t.Kind()]
-	if slices.Contains(elemKinds, kind) {
-		tflag |= abi.TFlagHasElem
-		if kind == abi.Map {
-			tflag |= abi.TFlagHasElemSecond
-		}
-	}
-
 	exported := false
 	p := t.NameString()
 	// If we're writing out type T,
@@ -526,6 +510,7 @@ func dcommontype(c rttype.Cursor, t *types.Type) {
 	c.Field("Align_").WriteUint8(uint8(t.Alignment()))
 	c.Field("FieldAlign_").WriteUint8(uint8(t.Alignment()))
 
+	kind := kinds[t.Kind()]
 	if types.IsDirectIface(t) {
 		kind |= abi.KindDirectIface
 	}

src/internal/abi/type.go

@@ -5,7 +5,6 @@
 package abi
 
 import (
-	"internal/goarch"
 	"unsafe"
 )
 
@@ -126,12 +125,6 @@ const (
 	// has type **byte instead of *byte. The runtime will store a
 	// pointer to the GC pointer bitmask in *GCData.
 	TFlagGCMaskOnDemand TFlag = 1 << 4
-
-	// TFlagHasElem signals that the Type has an Elem.
-	TFlagHasElem TFlag = 1 << 5
-
-	// TFlagHasElem signals that the Type has an Elem, but at an PtrBytes higher offset.
-	TFlagHasElemSecond TFlag = 1 << 6
 )
 
 // NameOff is the offset to a name from moduledata.types.  See resolveNameOff in runtime.
@@ -383,25 +376,24 @@ func (t *Type) Uncommon() *UncommonType {
 	}
 }
 
-// Compile time asserts for (*Type).Elem().
-func _() {
-	const _, _ = unsafe.Offsetof(ChanType{}.Elem) - baseElemOffset, baseElemOffset - unsafe.Offsetof(ChanType{}.Elem)
-	const _, _ = unsafe.Offsetof(mapType{}.Elem) - baseElemOffset + goarch.PtrSize, baseElemOffset + goarch.PtrSize - unsafe.Offsetof(mapType{}.Elem)
-	const _, _ = unsafe.Offsetof(PtrType{}.Elem) - baseElemOffset, baseElemOffset - unsafe.Offsetof(PtrType{}.Elem)
-	const _, _ = unsafe.Offsetof(SliceType{}.Elem) - baseElemOffset, baseElemOffset - unsafe.Offsetof(SliceType{}.Elem)
-	var _, _, _, _, _ *Type = ArrayType{}.Elem, ChanType{}.Elem, mapType{}.Elem, PtrType{}.Elem, SliceType{}.Elem
-}
-
-const baseElemOffset = unsafe.Offsetof(ArrayType{}.Elem)
-
 // Elem returns the element type for t if t is an array, channel, map, pointer, or slice, otherwise nil.
 func (t *Type) Elem() *Type {
-	if t.TFlag&TFlagHasElem != 0 {
-		// Unfortunately, the mapType does not have the Elem field at the same offset as other
-		// types do, it is PtrBytes ahead. So we have to calculate the offset based on TFlagHasElemSecond flag.
-		// Currently we can't really change the definition of mapType to reorder these fields,
-		// because mapType is referenced with linknames from other modules (non-std).
-		return *(**Type)(unsafe.Add(unsafe.Pointer(t), baseElemOffset+(goarch.PtrSize*uintptr((t.TFlag&TFlagHasElemSecond)>>6))))
+	switch t.Kind() {
+	case Array:
+		tt := (*ArrayType)(unsafe.Pointer(t))
+		return tt.Elem
+	case Chan:
+		tt := (*ChanType)(unsafe.Pointer(t))
+		return tt.Elem
+	case Map:
+		tt := (*mapType)(unsafe.Pointer(t))
+		return tt.Elem
+	case Pointer:
+		tt := (*PtrType)(unsafe.Pointer(t))
+		return tt.Elem
+	case Slice:
+		tt := (*SliceType)(unsafe.Pointer(t))
+		return tt.Elem
 	}
 	return nil
 }

src/reflect/map_noswiss.go

@@ -69,7 +69,7 @@ func MapOf(key, elem Type) Type {
 	var imap any = (map[unsafe.Pointer]unsafe.Pointer)(nil)
 	mt := **(**mapType)(unsafe.Pointer(&imap))
 	mt.Str = resolveReflectName(newName(s, "", false, false))
-	mt.TFlag = abi.TFlagHasElem | abi.TFlagHasElemSecond
+	mt.TFlag = 0
 	mt.Hash = fnv1(etyp.Hash, 'm', byte(ktyp.Hash>>24), byte(ktyp.Hash>>16), byte(ktyp.Hash>>8), byte(ktyp.Hash))
 	mt.Key = ktyp
 	mt.Elem = etyp

src/reflect/map_swiss.go

@@ -65,7 +65,7 @@ func MapOf(key, elem Type) Type {
 	var imap any = (map[unsafe.Pointer]unsafe.Pointer)(nil)
 	mt := **(**mapType)(unsafe.Pointer(&imap))
 	mt.Str = resolveReflectName(newName(s, "", false, false))
-	mt.TFlag = abi.TFlagHasElem | abi.TFlagHasElemSecond
+	mt.TFlag = 0
 	mt.Hash = fnv1(etyp.Hash, 'm', byte(ktyp.Hash>>24), byte(ktyp.Hash>>16), byte(ktyp.Hash>>8), byte(ktyp.Hash))
 	mt.Key = ktyp
 	mt.Elem = etyp

src/reflect/type.go

@@ -1361,7 +1361,6 @@ func (t *rtype) ptrTo() *abi.Type {
 
 	pp.Str = resolveReflectName(newName(s, "", false, false))
 	pp.PtrToThis = 0
-	pp.TFlag = abi.TFlagHasElem
 
 	// For the type structures linked into the binary, the
 	// compiler provides a good hash of the string.
@@ -2086,7 +2085,7 @@ func SliceOf(t Type) Type {
 	var islice any = ([]unsafe.Pointer)(nil)
 	prototype := *(**sliceType)(unsafe.Pointer(&islice))
 	slice := *prototype
-	slice.TFlag = abi.TFlagHasElem
+	slice.TFlag = 0
 	slice.Str = resolveReflectName(newName(s, "", false, false))
 	slice.Hash = fnv1(typ.Hash, '[')
 	slice.Elem = typ
@@ -2472,9 +2471,9 @@ func StructOf(fields []StructField) Type {
 
 	typ.Str = resolveReflectName(newName(str, "", false, false))
 	if isRegularMemory(toType(&typ.Type)) {
-		typ.TFlag = abi.TFlagRegularMemory & abi.TFlagHasElem
+		typ.TFlag = abi.TFlagRegularMemory
 	} else {
-		typ.TFlag = abi.TFlagHasElem
+		typ.TFlag = 0
 	}
 	typ.Hash = hash
 	typ.Size_ = size
@@ -2612,7 +2611,7 @@ func ArrayOf(length int, elem Type) Type {
 	var iarray any = [1]unsafe.Pointer{}
 	prototype := *(**arrayType)(unsafe.Pointer(&iarray))
 	array := *prototype
-	array.TFlag = (typ.TFlag & abi.TFlagRegularMemory) | abi.TFlagHasElem
+	array.TFlag = typ.TFlag & abi.TFlagRegularMemory
 	array.Str = resolveReflectName(newName(s, "", false, false))
 	array.Hash = fnv1(typ.Hash, '[')
 	for n := uint32(length); n > 0; n >>= 8 {

src/reflect/type_test.go

@@ -118,13 +118,6 @@ func BenchmarkTypeForError(b *testing.B) {
 	}
 }
 
-func BenchmarkSliceTypeElem(b *testing.B) {
-	slice := reflect.TypeFor[[]int]()
-	for i := 0; i < b.N; i++ {
-		sinkType = slice.Elem()
-	}
-}
-
 func TestType_CanSeq(t *testing.T) {
 	tests := []struct {
 		name string