reflect: add support for struct types

compiler/interface.go

@@ -53,21 +53,28 @@ func (c *Compiler) getTypeCode(typ types.Type) llvm.Value {
 		// Some type classes contain more information for underlying types or
 		// element types. Store it directly in the typecode global to make
 		// reflect lowering simpler.
-		var elementType types.Type
+		var references llvm.Value
 		switch typ := typ.(type) {
 		case *types.Named:
-			elementType = typ.Underlying()
+			references = c.getTypeCode(typ.Underlying())
 		case *types.Chan:
-			elementType = typ.Elem()
+			references = c.getTypeCode(typ.Elem())
 		case *types.Pointer:
-			elementType = typ.Elem()
+			references = c.getTypeCode(typ.Elem())
 		case *types.Slice:
-			elementType = typ.Elem()
+			references = c.getTypeCode(typ.Elem())
+		case *types.Struct:
+			// Take a pointer to the typecodeID of the first field (if it exists).
+			structGlobal := c.makeStructTypeFields(typ)
+			references = llvm.ConstGEP(structGlobal, []llvm.Value{
+				llvm.ConstInt(llvm.Int32Type(), 0, false),
+				llvm.ConstInt(llvm.Int32Type(), 0, false),
+			})
 		}
-		if elementType != nil {
+		if !references.IsNil() {
 			// Set the 'references' field of the runtime.typecodeID struct.
 			globalValue := c.getZeroValue(global.Type().ElementType())
-			globalValue = llvm.ConstInsertValue(globalValue, c.getTypeCode(elementType), []uint32{0})
+			globalValue = llvm.ConstInsertValue(globalValue, references, []uint32{0})
 			global.SetInitializer(globalValue)
 			global.SetLinkage(llvm.PrivateLinkage)
 		}
@@ -76,6 +83,48 @@ func (c *Compiler) getTypeCode(typ types.Type) llvm.Value {
 	return global
 }
 
+// makeStructTypeFields creates a new global that stores all type information
+// related to this struct type, and returns the resulting global. This global is
+// actually an array of all the fields in the structs.
+func (c *Compiler) makeStructTypeFields(typ *types.Struct) llvm.Value {
+	// The global is an array of runtime.structField structs.
+	runtimeStructField := c.getLLVMRuntimeType("structField")
+	structGlobalType := llvm.ArrayType(runtimeStructField, typ.NumFields())
+	structGlobal := llvm.AddGlobal(c.mod, structGlobalType, "reflect/types.structFields")
+	structGlobalValue := c.getZeroValue(structGlobalType)
+	for i := 0; i < typ.NumFields(); i++ {
+		fieldGlobalValue := c.getZeroValue(runtimeStructField)
+		fieldGlobalValue = llvm.ConstInsertValue(fieldGlobalValue, c.getTypeCode(typ.Field(i).Type()), []uint32{0})
+		fieldName := c.makeGlobalBytes([]byte(typ.Field(i).Name()), "reflect/types.structFieldName")
+		fieldName = llvm.ConstGEP(fieldName, []llvm.Value{
+			llvm.ConstInt(llvm.Int32Type(), 0, false),
+			llvm.ConstInt(llvm.Int32Type(), 0, false),
+		})
+		fieldName.SetLinkage(llvm.PrivateLinkage)
+		fieldName.SetUnnamedAddr(true)
+		fieldGlobalValue = llvm.ConstInsertValue(fieldGlobalValue, fieldName, []uint32{1})
+		if typ.Tag(i) != "" {
+			fieldTag := c.makeGlobalBytes([]byte(typ.Tag(i)), "reflect/types.structFieldTag")
+			fieldTag = llvm.ConstGEP(fieldTag, []llvm.Value{
+				llvm.ConstInt(llvm.Int32Type(), 0, false),
+				llvm.ConstInt(llvm.Int32Type(), 0, false),
+			})
+			fieldTag.SetLinkage(llvm.PrivateLinkage)
+			fieldTag.SetUnnamedAddr(true)
+			fieldGlobalValue = llvm.ConstInsertValue(fieldGlobalValue, fieldTag, []uint32{2})
+		}
+		if typ.Field(i).Embedded() {
+			fieldEmbedded := llvm.ConstInt(c.ctx.Int1Type(), 1, false)
+			fieldGlobalValue = llvm.ConstInsertValue(fieldGlobalValue, fieldEmbedded, []uint32{3})
+		}
+		structGlobalValue = llvm.ConstInsertValue(structGlobalValue, fieldGlobalValue, []uint32{uint32(i)})
+	}
+	structGlobal.SetInitializer(structGlobalValue)
+	structGlobal.SetUnnamedAddr(true)
+	structGlobal.SetLinkage(llvm.PrivateLinkage)
+	return structGlobal
+}
+
 // getTypeCodeName returns a name for this type that can be used in the
 // interface lowering pass to assign type codes as expected by the reflect
 // package. See getTypeCodeNum.

compiler/llvm.go

@@ -152,3 +152,46 @@ func (c *Compiler) splitBasicBlock(afterInst llvm.Value, insertAfter llvm.BasicB
 
 	return newBlock
 }
+
+// makeGlobalBytes creates a new LLVM global with the given name and bytes as
+// contents, and returns the global.
+// Note that it is left with the default linkage etc., you should set
+// linkage/constant/etc properties yourself.
+func (c *Compiler) makeGlobalBytes(buf []byte, name string) llvm.Value {
+	globalType := llvm.ArrayType(c.ctx.Int8Type(), len(buf))
+	global := llvm.AddGlobal(c.mod, globalType, name)
+	value := llvm.Undef(globalType)
+	for i, ch := range buf {
+		value = llvm.ConstInsertValue(value, llvm.ConstInt(c.ctx.Int8Type(), uint64(ch), false), []uint32{uint32(i)})
+	}
+	global.SetInitializer(value)
+	return global
+}
+
+// getGlobalBytes returns the byte slice contained in the i8 array of the
+// provided global. It can recover the bytes originally created using
+// makeGlobalBytes.
+func getGlobalBytes(global llvm.Value) []byte {
+	value := global.Initializer()
+	buf := make([]byte, value.Type().ArrayLength())
+	for i := range buf {
+		buf[i] = byte(llvm.ConstExtractValue(value, []uint32{uint32(i)}).ZExtValue())
+	}
+	return buf
+}
+
+// replaceGlobalByteWithArray replaces a global i8 in the module with a byte
+// array, using a GEP to make the types match. It is a convenience function used
+// for creating reflection sidetables, for example.
+func (c *Compiler) replaceGlobalByteWithArray(name string, buf []byte) llvm.Value {
+	global := c.makeGlobalBytes(buf, name+".tmp")
+	oldGlobal := c.mod.NamedGlobal(name)
+	gep := llvm.ConstGEP(global, []llvm.Value{
+		llvm.ConstInt(c.ctx.Int32Type(), 0, false),
+		llvm.ConstInt(c.ctx.Int32Type(), 0, false),
+	})
+	oldGlobal.ReplaceAllUsesWith(gep)
+	oldGlobal.EraseFromParentAsGlobal()
+	global.SetName(name)
+	return global
+}

compiler/reflect.go

@@ -28,6 +28,8 @@ package compiler
 // non-basic types have their underlying type stored in a sidetable.
 
 import (
+	"encoding/binary"
+	"go/ast"
 	"math/big"
 	"strings"
 
@@ -65,11 +67,25 @@ type typeCodeAssignmentState struct {
 	// package (or are simply unused in the compiled program).
 	fallbackIndex int
 
+	// This is the length of an uintptr. Only used occasionally to know whether
+	// a given number can be encoded as a varint.
+	uintptrLen int
+
 	// Map of named types to their type code. It is important that named types
 	// get unique IDs for each type.
 	namedBasicTypes    map[string]int
 	namedNonBasicTypes map[string]int
 
+	// Map of struct types to their type code.
+	structTypes               map[string]int
+	structTypesSidetable      []byte
+	needsStructNamesSidetable bool
+
+	// Map of struct names and tags to their name string.
+	structNames               map[string]int
+	structNamesSidetable      []byte
+	needsStructTypesSidetable bool
+
 	// This byte array is stored in reflect.namedNonBasicTypesSidetable and is
 	// used at runtime to get details about a named non-basic type.
 	// Entries are varints (see makeVarint below and readVarint in
@@ -82,10 +98,6 @@ type typeCodeAssignmentState struct {
 	// needsNamedTypesSidetable.
 	namedNonBasicTypesSidetable []byte
 
-	// This is the length of an uintptr. Only used occasionally to know whether
-	// a given number can be encoded as a varint.
-	uintptrLen int
-
 	// This indicates whether namedNonBasicTypesSidetable needs to be created at
 	// all. If it is false, namedNonBasicTypesSidetable will contain simple
 	// monotonically increasing numbers.
@@ -109,13 +121,17 @@ func (c *Compiler) assignTypeCodes(typeSlice typeInfoSlice) {
 	// Assign typecodes the way the reflect package expects.
 	state := typeCodeAssignmentState{
 		fallbackIndex:                    1,
+		uintptrLen:                       c.uintptrType.IntTypeWidth(),
 		namedBasicTypes:                  make(map[string]int),
 		namedNonBasicTypes:               make(map[string]int),
-		uintptrLen:                       c.uintptrType.IntTypeWidth(),
+		structTypes:                      make(map[string]int),
+		structNames:                      make(map[string]int),
 		needsNamedNonBasicTypesSidetable: len(getUses(c.mod.NamedGlobal("reflect.namedNonBasicTypesSidetable"))) != 0,
+		needsStructTypesSidetable:        len(getUses(c.mod.NamedGlobal("reflect.structTypesSidetable"))) != 0,
+		needsStructNamesSidetable:        len(getUses(c.mod.NamedGlobal("reflect.structNamesSidetable"))) != 0,
 	}
 	for _, t := range typeSlice {
-		num := c.getTypeCodeNum(t.typecode, &state)
+		num := state.getTypeCodeNum(t.typecode)
 		if num.BitLen() > c.uintptrType.IntTypeWidth() || !num.IsUint64() {
 			// TODO: support this in some way, using a side table for example.
 			// That's less efficient but better than not working at all.
@@ -128,29 +144,26 @@ func (c *Compiler) assignTypeCodes(typeSlice typeInfoSlice) {
 
 	// Only create this sidetable when it is necessary.
 	if state.needsNamedNonBasicTypesSidetable {
-		// Create the sidetable and replace the old dummy global with this value.
-		globalType := llvm.ArrayType(c.ctx.Int8Type(), len(state.namedNonBasicTypesSidetable))
-		global := llvm.AddGlobal(c.mod, globalType, "reflect.namedNonBasicTypesSidetable.tmp")
-		value := llvm.Undef(globalType)
-		for i, ch := range state.namedNonBasicTypesSidetable {
-			value = llvm.ConstInsertValue(value, llvm.ConstInt(c.ctx.Int8Type(), uint64(ch), false), []uint32{uint32(i)})
+		global := c.replaceGlobalByteWithArray("reflect.namedNonBasicTypesSidetable", state.namedNonBasicTypesSidetable)
+		global.SetLinkage(llvm.InternalLinkage)
+		global.SetUnnamedAddr(true)
 	}
-		global.SetInitializer(value)
-		oldGlobal := c.mod.NamedGlobal("reflect.namedNonBasicTypesSidetable")
-		gep := llvm.ConstGEP(global, []llvm.Value{
-			llvm.ConstInt(c.ctx.Int32Type(), 0, false),
-			llvm.ConstInt(c.ctx.Int32Type(), 0, false),
-		})
-		oldGlobal.ReplaceAllUsesWith(gep)
-		oldGlobal.EraseFromParentAsGlobal()
-		global.SetName("reflect.namedNonBasicTypesSidetable")
+	if state.needsStructTypesSidetable {
+		global := c.replaceGlobalByteWithArray("reflect.structTypesSidetable", state.structTypesSidetable)
+		global.SetLinkage(llvm.InternalLinkage)
+		global.SetUnnamedAddr(true)
+	}
+	if state.needsStructNamesSidetable {
+		global := c.replaceGlobalByteWithArray("reflect.structNamesSidetable", state.structNamesSidetable)
+		global.SetLinkage(llvm.InternalLinkage)
+		global.SetUnnamedAddr(true)
 	}
 }
 
 // getTypeCodeNum returns the typecode for a given type as expected by the
 // reflect package. Also see getTypeCodeName, which serializes types to a string
 // based on a types.Type value for this function.
-func (c *Compiler) getTypeCodeNum(typecode llvm.Value, state *typeCodeAssignmentState) *big.Int {
+func (state *typeCodeAssignmentState) getTypeCodeNum(typecode llvm.Value) *big.Int {
 	// Note: see src/reflect/type.go for bit allocations.
 	class, value := getClassAndValueFromTypeCode(typecode)
 	name := ""
@@ -186,7 +199,7 @@ func (c *Compiler) getTypeCodeNum(typecode llvm.Value, state *typeCodeAssignment
 		switch class {
 		case "chan":
 			sub := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
-			num = c.getTypeCodeNum(sub, state)
+			num = state.getTypeCodeNum(sub)
 			classNumber = 0
 		case "interface":
 			num = big.NewInt(int64(state.fallbackIndex))
@@ -194,11 +207,11 @@ func (c *Compiler) getTypeCodeNum(typecode llvm.Value, state *typeCodeAssignment
 			classNumber = 1
 		case "pointer":
 			sub := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
-			num = c.getTypeCodeNum(sub, state)
+			num = state.getTypeCodeNum(sub)
 			classNumber = 2
 		case "slice":
 			sub := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0})
-			num = c.getTypeCodeNum(sub, state)
+			num = state.getTypeCodeNum(sub)
 			classNumber = 3
 		case "array":
 			num = big.NewInt(int64(state.fallbackIndex))
@@ -213,8 +226,7 @@ func (c *Compiler) getTypeCodeNum(typecode llvm.Value, state *typeCodeAssignment
 			state.fallbackIndex++
 			classNumber = 6
 		case "struct":
-			num = big.NewInt(int64(state.fallbackIndex))
-			state.fallbackIndex++
+			num = big.NewInt(int64(state.getStructTypeNum(typecode)))
 			classNumber = 7
 		default:
 			panic("unknown type kind: " + class)
@@ -283,36 +295,125 @@ func (state *typeCodeAssignmentState) getNonBasicNamedTypeNum(name string, value
 	return num
 }
 
-// makeVarint encodes a varint in a way that should be easy to decode.
-// It may need to be decoded very quickly at runtime at low-powered processors
-// so should be efficient to decode.
-// The current algorithm is probably not even close to efficient, but it is easy
-// to change as the format is only used inside the same program.
-func makeVarint(n uint64) []byte {
-	// This is the reverse of what src/runtime/sidetables.go does.
-	buf := make([]byte, 0, 8)
-	for {
-		c := byte(n & 0x7f << 1)
-		n >>= 7
-		if n != 0 {
-			c |= 1
-		}
-		buf = append(buf, c)
-		if n == 0 {
-			break
-		}
-	}
-	reverseBytes(buf)
-	return buf
+// getStructTypeNum returns the struct type number, which is an index into
+// reflect.structTypesSidetable or an unique number for every struct if this
+// sidetable is not needed in the to-be-compiled program.
+func (state *typeCodeAssignmentState) getStructTypeNum(typecode llvm.Value) int {
+	name := typecode.Name()
+	if num, ok := state.structTypes[name]; ok {
+		// This struct already has an assigned type code.
+		return num
 	}
 
-func reverseBytes(s []byte) {
-	// Actually copied from https://blog.golang.org/why-generics
-	first := 0
-	last := len(s) - 1
-	for first < last {
-		s[first], s[last] = s[last], s[first]
-		first++
-		last--
+	if !state.needsStructTypesSidetable {
+		// We don't need struct sidetables, so we can just assign monotonically
+		// increasing numbers to each struct type.
+		num := len(state.structTypes)
+		state.structTypes[name] = num
+		return num
+	}
+
+	// Get the fields this struct type contains.
+	// The struct number will be the start index of
+	structTypeGlobal := llvm.ConstExtractValue(typecode.Initializer(), []uint32{0}).Operand(0).Initializer()
+	numFields := structTypeGlobal.Type().ArrayLength()
+
+	// The first data that is stored in the struct sidetable is the number of
+	// fields this struct contains. This is usually just a single byte because
+	// most structs don't contain that many fields, but make it a varint just
+	// to be sure.
+	buf := makeVarint(uint64(numFields))
+
+	// Iterate over every field in the struct.
+	// Every field is stored sequentially in the struct sidetable. Fields can
+	// be retrieved from this list of fields at runtime by iterating over all
+	// of them until the right field has been found.
+	// Perhaps adding some index would speed things up, but it would also make
+	// the sidetable bigger.
+	for i := 0; i < numFields; i++ {
+		// Collect some information about this field.
+		field := llvm.ConstExtractValue(structTypeGlobal, []uint32{uint32(i)})
+
+		nameGlobal := llvm.ConstExtractValue(field, []uint32{1})
+		if nameGlobal == llvm.ConstPointerNull(nameGlobal.Type()) {
+			panic("compiler: no name for this struct field")
+		}
+		fieldNameBytes := getGlobalBytes(nameGlobal.Operand(0))
+		fieldNameNumber := state.getStructNameNumber(fieldNameBytes)
+
+		// See whether this struct field has an associated tag, and if so,
+		// store that tag in the tags sidetable.
+		tagGlobal := llvm.ConstExtractValue(field, []uint32{2})
+		hasTag := false
+		tagNumber := 0
+		if tagGlobal != llvm.ConstPointerNull(tagGlobal.Type()) {
+			hasTag = true
+			tagBytes := getGlobalBytes(tagGlobal.Operand(0))
+			tagNumber = state.getStructNameNumber(tagBytes)
+		}
+
+		// The 'embedded' or 'anonymous' flag for this field.
+		embedded := llvm.ConstExtractValue(field, []uint32{3}).ZExtValue() != 0
+
+		// The first byte in the struct types sidetable is a flags byte with
+		// two bits in it.
+		flagsByte := byte(0)
+		if embedded {
+			flagsByte |= 1
+		}
+		if hasTag {
+			flagsByte |= 2
+		}
+		if ast.IsExported(string(fieldNameBytes)) {
+			flagsByte |= 4
+		}
+		buf = append(buf, flagsByte)
+
+		// Get the type number and add it to the buffer.
+		// All fields have a type, so include it directly here.
+		typeNum := state.getTypeCodeNum(llvm.ConstExtractValue(field, []uint32{0}))
+		if typeNum.BitLen() > state.uintptrLen || !typeNum.IsUint64() {
+			// TODO: make this a regular error
+			panic("struct field has a type code that is too big")
+		}
+		buf = append(buf, makeVarint(typeNum.Uint64())...)
+
+		// Add the name.
+		buf = append(buf, makeVarint(uint64(fieldNameNumber))...)
+
+		// Add the tag, if there is one.
+		if hasTag {
+			buf = append(buf, makeVarint(uint64(tagNumber))...)
 		}
 	}
+
+	num := len(state.structTypesSidetable)
+	state.structTypes[name] = num
+	state.structTypesSidetable = append(state.structTypesSidetable, buf...)
+	return num
+}
+
+// getStructNameNumber stores this string (name or tag) onto the struct names
+// sidetable. The format is a varint of the length of the struct, followed by
+// the raw bytes of the name. Multiple identical strings are stored under the
+// same name for space efficiency.
+func (state *typeCodeAssignmentState) getStructNameNumber(nameBytes []byte) int {
+	name := string(nameBytes)
+	if n, ok := state.structNames[name]; ok {
+		// This name was used before, re-use it now (for space efficiency).
+		return n
+	}
+	// This name is not yet in the names sidetable. Add it now.
+	n := len(state.structNamesSidetable)
+	state.structNames[name] = n
+	state.structNamesSidetable = append(state.structNamesSidetable, makeVarint(uint64(len(nameBytes)))...)
+	state.structNamesSidetable = append(state.structNamesSidetable, nameBytes...)
+	return n
+}
+
+// makeVarint is a small helper function that returns the bytes of the number in
+// varint encoding.
+func makeVarint(n uint64) []byte {
+	buf := make([]byte, binary.MaxVarintLen64)
+	return buf[:binary.PutUvarint(buf, n)]
+}

src/reflect/sidetables.go

@@ -10,22 +10,48 @@ import (
 //go:extern reflect.namedNonBasicTypesSidetable
 var namedNonBasicTypesSidetable byte
 
-func readVarint(buf unsafe.Pointer) Type {
-	var t Type
+//go:extern reflect.structTypesSidetable
+var structTypesSidetable byte
+
+//go:extern reflect.structNamesSidetable
+var structNamesSidetable byte
+
+// readStringSidetable reads a string from the given table (like
+// structNamesSidetable) and returns this string. No heap allocation is
+// necessary because it makes the string point directly to the raw bytes of the
+// table.
+func readStringSidetable(table unsafe.Pointer, index uintptr) string {
+	nameLen, namePtr := readVarint(unsafe.Pointer(uintptr(table) + index))
+	return *(*string)(unsafe.Pointer(&StringHeader{
+		Data: uintptr(namePtr),
+		Len:  nameLen,
+	}))
+}
+
+// readVarint decodes a varint as used in the encoding/binary package.
+// It has an input pointer and returns the read varint and the pointer
+// incremented to the next field in the data structure, just after the varint.
+//
+// Details:
+// https://github.com/golang/go/blob/e37a1b1c/src/encoding/binary/varint.go#L7-L25
+func readVarint(buf unsafe.Pointer) (uintptr, unsafe.Pointer) {
+	var n uintptr
+	shift := uintptr(0)
 	for {
-		// Read the next byte.
+		// Read the next byte in the buffer.
 		c := *(*byte)(buf)
 
-		// Add this byte to the type code. The upper 7 bits are the value.
-		t = t<<7 | Type(c>>1)
-
-		// Check whether this is the last byte of this varint. The lower bit
-		// indicates whether any bytes follow.
-		if c%1 == 0 {
-			return t
-		}
+		// Decode the bits from this byte and add them to the output number.
+		n |= uintptr(c&0x7f) << shift
+		shift += 7
 
 		// Increment the buf pointer (pointer arithmetic!).
 		buf = unsafe.Pointer(uintptr(buf) + 1)
+
+		// Check whether this is the last byte of this varint. The upper bit
+		// (msb) indicates whether any bytes follow.
+		if c>>7 == 0 {
+			return n, buf
+		}
 	}
 }

src/reflect/type.go

@@ -145,23 +145,102 @@ func (t Type) Kind() Kind {
 func (t Type) Elem() Type {
 	switch t.Kind() {
 	case Chan, Ptr, Slice:
-		// Look at the 'n' bit in the type code (see the top of this file) to
-		// see whether this is a named type.
-		if (t>>4)%2 != 0 {
-			// This is a named type. The element type is stored in a sidetable.
-			namedTypeNum := t >> 5
-			return readVarint(unsafe.Pointer(uintptr(unsafe.Pointer(&namedNonBasicTypesSidetable)) + uintptr(namedTypeNum)))
-		}
-		// Not a named type, so the element type is stored directly in the type
-		// code.
-		return t >> 5
+		return t.stripPrefix()
 	default: // not implemented: Array, Map
 		panic("unimplemented: (reflect.Type).Elem()")
 	}
 }
 
+// stripPrefix removes the "prefix" (the first 5 bytes of the type code) from
+// the type code. If this is a named type, it will resolve the underlying type
+// (which is the data for this named type). If it is not, the lower bits are
+// simply shifted off.
+//
+// The behavior is only defined for non-basic types.
+func (t Type) stripPrefix() Type {
+	// Look at the 'n' bit in the type code (see the top of this file) to see
+	// whether this is a named type.
+	if (t>>4)%2 != 0 {
+		// This is a named type. The data is stored in a sidetable.
+		namedTypeNum := t >> 5
+		n, _ := readVarint(unsafe.Pointer(uintptr(unsafe.Pointer(&namedNonBasicTypesSidetable)) + uintptr(namedTypeNum)))
+		return Type(n)
+	}
+	// Not a named type, so the value is stored directly in the type code.
+	return t >> 5
+}
+
+// Field returns the type of the i'th field of this struct type. It panics if t
+// is not a struct type.
 func (t Type) Field(i int) StructField {
-	panic("unimplemented: (reflect.Type).Field()")
+	if t.Kind() != Struct {
+		panic(&TypeError{"Field"})
+	}
+	structIdentifier := t.stripPrefix()
+
+	numField, p := readVarint(unsafe.Pointer(uintptr(unsafe.Pointer(&structTypesSidetable)) + uintptr(structIdentifier)))
+	if uint(i) >= uint(numField) {
+		panic("reflect: field index out of range")
+	}
+
+	// Iterate over every field in the struct and update the StructField each
+	// time, until the target field has been reached. This is very much not
+	// efficient, but it is easy to implement.
+	// Adding a jump table at the start to jump to the field directly would
+	// make this much faster, but that would also impact code size.
+	field := StructField{}
+	offset := uintptr(0)
+	for fieldNum := 0; fieldNum <= i; fieldNum++ {
+		// Read some flags of this field, like whether the field is an
+		// embedded field.
+		flagsByte := *(*uint8)(p)
+		p = unsafe.Pointer(uintptr(p) + 1)
+
+		// Read the type of this struct field.
+		var fieldType uintptr
+		fieldType, p = readVarint(p)
+		field.Type = Type(fieldType)
+
+		// Move Offset forward to align it to this field's alignment.
+		// Assume alignment is a power of two.
+		offset = align(offset, uintptr(field.Type.Align()))
+		field.Offset = offset
+		offset += field.Type.Size() // starting (unaligned) offset for next field
+
+		// Read the field name.
+		var nameNum uintptr
+		nameNum, p = readVarint(p)
+		field.Name = readStringSidetable(unsafe.Pointer(&structNamesSidetable), nameNum)
+
+		// The first bit in the flagsByte indicates whether this is an embedded
+		// field.
+		field.Anonymous = flagsByte&1 != 0
+
+		// The second bit indicates whether there is a tag.
+		if flagsByte&2 != 0 {
+			// There is a tag.
+			var tagNum uintptr
+			tagNum, p = readVarint(p)
+			field.Tag = readStringSidetable(unsafe.Pointer(&structNamesSidetable), tagNum)
+		} else {
+			// There is no tag.
+			field.Tag = ""
+		}
+
+		// The third bit indicates whether this field is exported.
+		if flagsByte&4 != 0 {
+			// This field is exported.
+			field.PkgPath = ""
+		} else {
+			// This field is unexported.
+			// TODO: list the real package path here. Storing it should not
+			// significantly impact binary size as there is only a limited
+			// number of packages in any program.
+			field.PkgPath = "<unimplemented>"
+		}
+	}
+
+	return field
 }
 
 // Bits returns the number of bits that this type uses. It is only valid for
@@ -179,10 +258,19 @@ func (t Type) Len() int {
 	panic("unimplemented: (reflect.Type).Len()")
 }
 
+// NumField returns the number of fields of a struct type. It panics for other
+// type kinds.
 func (t Type) NumField() int {
-	panic("unimplemented: (reflect.Type).NumField()")
+	if t.Kind() != Struct {
+		panic(&TypeError{"NumField"})
+	}
+	structIdentifier := t.stripPrefix()
+	n, _ := readVarint(unsafe.Pointer(uintptr(unsafe.Pointer(&structTypesSidetable)) + uintptr(structIdentifier)))
+	return int(n)
 }
 
+// Size returns the size in bytes of a given type. It is similar to
+// unsafe.Sizeof.
 func (t Type) Size() uintptr {
 	switch t.Kind() {
 	case Bool, Int8, Uint8:
@@ -211,6 +299,15 @@ func (t Type) Size() uintptr {
 		return unsafe.Sizeof(uintptr(0))
 	case Slice:
 		return unsafe.Sizeof(SliceHeader{})
+	case Interface:
+		return unsafe.Sizeof(interfaceHeader{})
+	case Struct:
+		numField := t.NumField()
+		if numField == 0 {
+			return 0
+		}
+		lastField := t.Field(numField - 1)
+		return lastField.Offset + lastField.Type.Size()
 	default:
 		panic("unimplemented: size of type")
 	}
@@ -246,6 +343,18 @@ func (t Type) Align() int {
 		return int(unsafe.Alignof(uintptr(0)))
 	case Slice:
 		return int(unsafe.Alignof(SliceHeader{}))
+	case Interface:
+		return int(unsafe.Alignof(interfaceHeader{}))
+	case Struct:
+		numField := t.NumField()
+		alignment := 1
+		for i := 0; i < numField; i++ {
+			fieldAlignment := t.Field(i).Type.Align()
+			if fieldAlignment > alignment {
+				alignment = fieldAlignment
+			}
+		}
+		return alignment
 	default:
 		panic("unimplemented: alignment of type")
 	}
@@ -269,9 +378,19 @@ func (t Type) AssignableTo(u Type) bool {
 	return false
 }
 
+// A StructField describes a single field in a struct.
 type StructField struct {
+	// Name indicates the field name.
 	Name string
+
+	// PkgPath is the package path where the struct containing this field is
+	// declared for unexported fields, or the empty string for exported fields.
+	PkgPath string
+
 	Type      Type
+	Tag       string
+	Anonymous bool
+	Offset    uintptr
 }
 
 // TypeError is the error that is used in a panic when invoking a method on a

src/reflect/value.go

@@ -4,10 +4,28 @@ import (
 	"unsafe"
 )
 
+type valueFlags uint8
+
+// Flags list some useful flags that contain some extra information not
+// contained in an interface{} directly, like whether this value was exported at
+// all (it is possible to read unexported fields using reflection, but it is not
+// possible to modify them).
+const (
+	valueFlagIndirect valueFlags = 1 << iota
+	valueFlagExported
+)
+
 type Value struct {
 	typecode Type
 	value    unsafe.Pointer
-	indirect bool
+	flags    valueFlags
+}
+
+// isIndirect returns whether the value pointer in this Value is always a
+// pointer to the value. If it is false, it is only a pointer to the value if
+// the value is bigger than a pointer.
+func (v Value) isIndirect() bool {
+	return v.flags&valueFlagIndirect != 0
 }
 
 func Indirect(v Value) Value {
@@ -22,6 +40,7 @@ func ValueOf(i interface{}) Value {
 	return Value{
 		typecode: v.typecode,
 		value:    v.value,
+		flags:    valueFlagExported,
 	}
 }
 
@@ -30,7 +49,7 @@ func (v Value) Interface() interface{} {
 		typecode: v.typecode,
 		value:    v.value,
 	}
-	if v.indirect && v.Type().Size() <= unsafe.Sizeof(uintptr(0)) {
+	if v.isIndirect() && v.Type().Size() <= unsafe.Sizeof(uintptr(0)) {
 		// Value was indirect but must be put back directly in the interface
 		// value.
 		var value uintptr
@@ -109,13 +128,13 @@ func (v Value) Addr() Value {
 }
 
 func (v Value) CanSet() bool {
-	return v.indirect
+	return v.flags&(valueFlagExported|valueFlagIndirect) == valueFlagExported|valueFlagIndirect
 }
 
 func (v Value) Bool() bool {
 	switch v.Kind() {
 	case Bool:
-		if v.indirect {
+		if v.isIndirect() {
 			return *((*bool)(v.value))
 		} else {
 			return uintptr(v.value) != 0
@@ -128,31 +147,31 @@ func (v Value) Bool() bool {
 func (v Value) Int() int64 {
 	switch v.Kind() {
 	case Int:
-		if v.indirect || unsafe.Sizeof(int(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(int(0)) > unsafe.Sizeof(uintptr(0)) {
 			return int64(*(*int)(v.value))
 		} else {
 			return int64(int(uintptr(v.value)))
 		}
 	case Int8:
-		if v.indirect {
+		if v.isIndirect() {
 			return int64(*(*int8)(v.value))
 		} else {
 			return int64(int8(uintptr(v.value)))
 		}
 	case Int16:
-		if v.indirect {
+		if v.isIndirect() {
 			return int64(*(*int16)(v.value))
 		} else {
 			return int64(int16(uintptr(v.value)))
 		}
 	case Int32:
-		if v.indirect || unsafe.Sizeof(int32(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(int32(0)) > unsafe.Sizeof(uintptr(0)) {
 			return int64(*(*int32)(v.value))
 		} else {
 			return int64(int32(uintptr(v.value)))
 		}
 	case Int64:
-		if v.indirect || unsafe.Sizeof(int64(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(int64(0)) > unsafe.Sizeof(uintptr(0)) {
 			return int64(*(*int64)(v.value))
 		} else {
 			return int64(int64(uintptr(v.value)))
@@ -165,37 +184,37 @@ func (v Value) Int() int64 {
 func (v Value) Uint() uint64 {
 	switch v.Kind() {
 	case Uintptr:
-		if v.indirect {
+		if v.isIndirect() {
 			return uint64(*(*uintptr)(v.value))
 		} else {
 			return uint64(uintptr(v.value))
 		}
 	case Uint8:
-		if v.indirect {
+		if v.isIndirect() {
 			return uint64(*(*uint8)(v.value))
 		} else {
 			return uint64(uintptr(v.value))
 		}
 	case Uint16:
-		if v.indirect {
+		if v.isIndirect() {
 			return uint64(*(*uint16)(v.value))
 		} else {
 			return uint64(uintptr(v.value))
 		}
 	case Uint:
-		if v.indirect || unsafe.Sizeof(uint(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(uint(0)) > unsafe.Sizeof(uintptr(0)) {
 			return uint64(*(*uint)(v.value))
 		} else {
 			return uint64(uintptr(v.value))
 		}
 	case Uint32:
-		if v.indirect || unsafe.Sizeof(uint32(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(uint32(0)) > unsafe.Sizeof(uintptr(0)) {
 			return uint64(*(*uint32)(v.value))
 		} else {
 			return uint64(uintptr(v.value))
 		}
 	case Uint64:
-		if v.indirect || unsafe.Sizeof(uint64(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(uint64(0)) > unsafe.Sizeof(uintptr(0)) {
 			return uint64(*(*uint64)(v.value))
 		} else {
 			return uint64(uintptr(v.value))
@@ -208,7 +227,7 @@ func (v Value) Uint() uint64 {
 func (v Value) Float() float64 {
 	switch v.Kind() {
 	case Float32:
-		if v.indirect || unsafe.Sizeof(float32(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(float32(0)) > unsafe.Sizeof(uintptr(0)) {
 			// The float is stored as an external value on systems with 16-bit
 			// pointers.
 			return float64(*(*float32)(v.value))
@@ -218,7 +237,7 @@ func (v Value) Float() float64 {
 			return float64(*(*float32)(unsafe.Pointer(&v.value)))
 		}
 	case Float64:
-		if v.indirect || unsafe.Sizeof(float64(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(float64(0)) > unsafe.Sizeof(uintptr(0)) {
 			// For systems with 16-bit and 32-bit pointers.
 			return *(*float64)(v.value)
 		} else {
@@ -234,7 +253,7 @@ func (v Value) Float() float64 {
 func (v Value) Complex() complex128 {
 	switch v.Kind() {
 	case Complex64:
-		if v.indirect || unsafe.Sizeof(complex64(0)) > unsafe.Sizeof(uintptr(0)) {
+		if v.isIndirect() || unsafe.Sizeof(complex64(0)) > unsafe.Sizeof(uintptr(0)) {
 			// The complex number is stored as an external value on systems with
 			// 16-bit and 32-bit pointers.
 			return complex128(*(*complex64)(v.value))
@@ -295,15 +314,17 @@ func (v Value) Cap() int {
 	}
 }
 
+// NumField returns the number of fields of this struct. It panics for other
+// value types.
 func (v Value) NumField() int {
-	panic("unimplemented: (reflect.Value).NumField()")
+	return v.Type().NumField()
 }
 
 func (v Value) Elem() Value {
 	switch v.Kind() {
 	case Ptr:
 		ptr := v.value
-		if v.indirect {
+		if v.isIndirect() {
 			ptr = *(*unsafe.Pointer)(ptr)
 		}
 		if ptr == nil {
@@ -312,15 +333,77 @@ func (v Value) Elem() Value {
 		return Value{
 			typecode: v.Type().Elem(),
 			value:    ptr,
-			indirect: true,
+			flags:    v.flags | valueFlagIndirect,
 		}
 	default: // not implemented: Interface
 		panic(&ValueError{"Elem"})
 	}
 }
 
+// Field returns the value of the i'th field of this struct.
 func (v Value) Field(i int) Value {
-	panic("unimplemented: (reflect.Value).Field()")
+	structField := v.Type().Field(i)
+	flags := v.flags
+	if structField.PkgPath != "" {
+		// The fact that PkgPath is present means that this field is not
+		// exported.
+		flags &^= valueFlagExported
+	}
+
+	size := v.Type().Size()
+	fieldSize := structField.Type.Size()
+	if v.isIndirect() || fieldSize > unsafe.Sizeof(uintptr(0)) {
+		// v.value was already a pointer to the value and it should stay that
+		// way.
+		return Value{
+			flags:    flags,
+			typecode: structField.Type,
+			value:    unsafe.Pointer(uintptr(v.value) + structField.Offset),
+		}
+	}
+
+	// The fieldSize is smaller than uintptr, which means that the value will
+	// have to be stored directly in the interface value.
+
+	if fieldSize == 0 {
+		// The struct field is zero sized.
+		// This is a rare situation, but because it's undefined behavior
+		// to shift the size of the value (zeroing the value), handle this
+		// situation explicitly.
+		return Value{
+			flags:    flags,
+			typecode: structField.Type,
+			value:    unsafe.Pointer(uintptr(0)),
+		}
+	}
+
+	if size > unsafe.Sizeof(uintptr(0)) {
+		// The value was not stored in the interface before but will be
+		// afterwards, so load the value (from the correct offset) and return
+		// it.
+		ptr := unsafe.Pointer(uintptr(v.value) + structField.Offset)
+		loadedValue := uintptr(0)
+		shift := uintptr(0)
+		for i := uintptr(0); i < fieldSize; i++ {
+			loadedValue |= uintptr(*(*byte)(ptr)) << shift
+			shift += 8
+			ptr = unsafe.Pointer(uintptr(ptr) + 1)
+		}
+		return Value{
+			flags:    0,
+			typecode: structField.Type,
+			value:    unsafe.Pointer(loadedValue),
+		}
+	}
+
+	// The value was already stored directly in the interface and it still
+	// is. Cut out the part of the value that we need.
+	mask := ^uintptr(0) >> ((unsafe.Sizeof(uintptr(0)) - fieldSize) * 8)
+	return Value{
+		flags:    flags,
+		typecode: structField.Type,
+		value:    unsafe.Pointer((uintptr(v.value) >> (structField.Offset * 8)) & mask),
+	}
 }
 
 func (v Value) Index(i int) Value {
@@ -333,7 +416,7 @@ func (v Value) Index(i int) Value {
 		}
 		elem := Value{
 			typecode: v.Type().Elem(),
-			indirect: true,
+			flags:    v.flags | valueFlagIndirect,
 		}
 		addr := uintptr(slice.Data) + elem.Type().Size()*uintptr(i) // pointer to new value
 		elem.value = unsafe.Pointer(addr)
@@ -385,15 +468,13 @@ func (it *MapIter) Next() bool {
 }
 
 func (v Value) Set(x Value) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	if !v.Type().AssignableTo(x.Type()) {
 		panic("reflect: cannot set")
 	}
 	size := v.Type().Size()
 	xptr := x.value
-	if size <= unsafe.Sizeof(uintptr(0)) && !x.indirect {
+	if size <= unsafe.Sizeof(uintptr(0)) && !x.isIndirect() {
 		value := x.value
 		xptr = unsafe.Pointer(&value)
 	}
@@ -401,9 +482,7 @@ func (v Value) Set(x Value) {
 }
 
 func (v Value) SetBool(x bool) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	switch v.Kind() {
 	case Bool:
 		*(*bool)(v.value) = x
@@ -413,9 +492,7 @@ func (v Value) SetBool(x bool) {
 }
 
 func (v Value) SetInt(x int64) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	switch v.Kind() {
 	case Int:
 		*(*int)(v.value) = int(x)
@@ -433,9 +510,7 @@ func (v Value) SetInt(x int64) {
 }
 
 func (v Value) SetUint(x uint64) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	switch v.Kind() {
 	case Uint:
 		*(*uint)(v.value) = uint(x)
@@ -455,9 +530,7 @@ func (v Value) SetUint(x uint64) {
 }
 
 func (v Value) SetFloat(x float64) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	switch v.Kind() {
 	case Float32:
 		*(*float32)(v.value) = float32(x)
@@ -469,9 +542,7 @@ func (v Value) SetFloat(x float64) {
 }
 
 func (v Value) SetComplex(x complex128) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	switch v.Kind() {
 	case Complex64:
 		*(*complex64)(v.value) = complex64(x)
@@ -483,9 +554,7 @@ func (v Value) SetComplex(x complex128) {
 }
 
 func (v Value) SetString(x string) {
-	if !v.indirect {
-		panic("reflect: value is not addressable")
-	}
+	v.checkAddressable()
 	switch v.Kind() {
 	case String:
 		*(*string)(v.value) = x
@@ -494,6 +563,12 @@ func (v Value) SetString(x string) {
 	}
 }
 
+func (v Value) checkAddressable() {
+	if !v.isIndirect() {
+		panic("reflect: value is not addressable")
+	}
+}
+
 func MakeSlice(typ Type, len, cap int) Value {
 	panic("unimplemented: reflect.MakeSlice()")
 }

src/runtime/interface.go

@@ -50,10 +50,20 @@ type typecodeID struct {
 	// * named type: the underlying type
 	// * interface: null
 	// * chan/pointer/slice: the element type
-	// * array/func/map/struct: TODO
+	// * struct: GEP of structField array (to typecode field)
+	// * array/func/map: TODO
 	references *typecodeID
 }
 
+// structField is used by the compiler to pass information to the interface
+// lowering pass. It is not used in the final binary.
+type structField struct {
+	typecode *typecodeID // type of this struct field
+	name     *uint8      // pointer to char array
+	tag      *uint8      // pointer to char array, or nil
+	embedded bool
+}
+
 // Pseudo type used before interface lowering. By using a struct instead of a
 // function call, this is simpler to reason about during init interpretation
 // than a function call. Also, by keeping the method set around it is easier to

testdata/reflect.go

@@ -11,6 +11,17 @@ type (
 	myslice2 []myint
 	mychan   chan int
 	myptr    *int
+	point    struct {
+		X int16
+		Y int16
+	}
+	mystruct struct {
+		n    int `foo:"bar"`
+		some point
+		zero struct{}
+		buf  []byte
+		Buf  []byte
+	}
 )
 
 func main() {
@@ -86,6 +97,12 @@ func main() {
 		// structs
 		struct{}{},
 		struct{ error }{},
+		struct {
+			a uint8
+			b int16
+			c int8
+		}{42, 321, 123},
+		mystruct{5, point{-5, 3}, struct{}{}, []byte{'G', 'o'}, []byte{'X'}},
 	} {
 		showValue(reflect.ValueOf(v), "")
 	}
@@ -291,7 +308,14 @@ func showValue(rv reflect.Value, indent string) {
 			showValue(rv.Index(i), indent+"  ")
 		}
 	case reflect.Struct:
-		println(indent + "  struct")
+		println(indent+"  struct:", rt.NumField())
+		for i := 0; i < rv.NumField(); i++ {
+			field := rt.Field(i)
+			println(indent+"  field:", i, field.Name)
+			println(indent+"  tag:", field.Tag)
+			println(indent+"  embedded:", field.Anonymous)
+			showValue(rv.Field(i), indent+"  ")
+		}
 	default:
 		println(indent + "  unknown type kind!")
 	}

testdata/reflect.txt

@@ -217,9 +217,82 @@ reflect type: map
   map
   nil: false
 reflect type: struct
-  struct
+  struct: 0
 reflect type: struct
-  struct
+  struct: 1
+  field: 0 error
+  tag: 
+  embedded: true
+  reflect type: interface
+    interface
+    nil: true
+reflect type: struct
+  struct: 3
+  field: 0 a
+  tag: 
+  embedded: false
+  reflect type: uint8
+    uint: 42
+  field: 1 b
+  tag: 
+  embedded: false
+  reflect type: int16
+    int: 321
+  field: 2 c
+  tag: 
+  embedded: false
+  reflect type: int8
+    int: 123
+reflect type: struct
+  struct: 5
+  field: 0 n
+  tag: foo:"bar"
+  embedded: false
+  reflect type: int
+    int: 5
+  field: 1 some
+  tag: 
+  embedded: false
+  reflect type: struct
+    struct: 2
+    field: 0 X
+    tag: 
+    embedded: false
+    reflect type: int16
+      int: -5
+    field: 1 Y
+    tag: 
+    embedded: false
+    reflect type: int16
+      int: 3
+  field: 2 zero
+  tag: 
+  embedded: false
+  reflect type: struct
+    struct: 0
+  field: 3 buf
+  tag: 
+  embedded: false
+  reflect type: slice
+    slice: uint8 2 2
+    pointer: true
+    nil: false
+    indexing: 0
+    reflect type: uint8
+      uint: 71
+    indexing: 1
+    reflect type: uint8
+      uint: 111
+  field: 4 Buf
+  tag: 
+  embedded: false
+  reflect type: slice
+    slice: uint8 1 1
+    pointer: true
+    nil: false
+    indexing: 0
+    reflect type: uint8 settable=true
+      uint: 88
 
 sizes:
 int8 1 8