libgo: Merge from revision 18783:00cce3a34d7e of master library.

[official-gcc.git] / libgo / go / reflect / value.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package reflect

import (
        "math"
        "runtime"
        "strconv"
        "unsafe"
)

const bigEndian = false // can be smarter if we find a big-endian machine
const ptrSize = unsafe.Sizeof((*byte)(nil))
const cannotSet = "cannot set value obtained from unexported struct field"

// TODO: This will have to go away when
// the new gc goes in.
func memmove(adst, asrc unsafe.Pointer, n uintptr) {
        dst := uintptr(adst)
        src := uintptr(asrc)
        switch {
        case src < dst && src+n > dst:
                // byte copy backward
                // careful: i is unsigned
                for i := n; i > 0; {
                        i--
                        *(*byte)(unsafe.Pointer(dst + i)) = *(*byte)(unsafe.Pointer(src + i))
                }
        case (n|src|dst)&(ptrSize-1) != 0:
                // byte copy forward
                for i := uintptr(0); i < n; i++ {
                        *(*byte)(unsafe.Pointer(dst + i)) = *(*byte)(unsafe.Pointer(src + i))
                }
        default:
                // word copy forward
                for i := uintptr(0); i < n; i += ptrSize {
                        *(*uintptr)(unsafe.Pointer(dst + i)) = *(*uintptr)(unsafe.Pointer(src + i))
                }
        }
}

// Value is the reflection interface to a Go value.
//
// Not all methods apply to all kinds of values.  Restrictions,
// if any, are noted in the documentation for each method.
// Use the Kind method to find out the kind of value before
// calling kind-specific methods.  Calling a method
// inappropriate to the kind of type causes a run time panic.
//
// The zero Value represents no value.
// Its IsValid method returns false, its Kind method returns Invalid,
// its String method returns "<invalid Value>", and all other methods panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
//
// A Value can be used concurrently by multiple goroutines provided that
// the underlying Go value can be used concurrently for the equivalent
// direct operations.
type Value struct {
        // typ holds the type of the value represented by a Value.
        typ *rtype

        // Pointer-valued data or, if flagIndir is set, pointer to data.
        // Valid when either flagIndir is set or typ.pointers() is true.
        // Gccgo always uses this field.
        ptr unsafe.Pointer

        // Non-pointer-valued data.  When the data is smaller
        // than a word, it begins at the first byte (in the memory
        // address sense) of this field.
        // Valid when flagIndir is not set and typ.pointers() is false.
        // Gccgo never uses this field.
        // scalar uintptr

        // flag holds metadata about the value.
        // The lowest bits are flag bits:
        //      - flagRO: obtained via unexported field, so read-only
        //      - flagIndir: val holds a pointer to the data
        //      - flagAddr: v.CanAddr is true (implies flagIndir)
        //      - flagMethod: v is a method value.
        // The next five bits give the Kind of the value.
        // This repeats typ.Kind() except for method values.
        // The remaining 23+ bits give a method number for method values.
        // If flag.kind() != Func, code can assume that flagMethod is unset.
        // If typ.size > ptrSize, code can assume that flagIndir is set.
        flag

        // A method value represents a curried method invocation
        // like r.Read for some receiver r.  The typ+val+flag bits describe
        // the receiver r, but the flag's Kind bits say Func (methods are
        // functions), and the top bits of the flag give the method number
        // in r's type's method table.
}

type flag uintptr

const (
        flagRO flag = 1 << iota
        flagIndir
        flagAddr
        flagMethod
        flagMethodFn         // gccgo: first fn parameter is always pointer
        flagKindShift        = iota
        flagKindWidth        = 5 // there are 27 kinds
        flagKindMask    flag = 1<<flagKindWidth - 1
        flagMethodShift      = flagKindShift + flagKindWidth
)

func (f flag) kind() Kind {
        return Kind((f >> flagKindShift) & flagKindMask)
}

// pointer returns the underlying pointer represented by v.
// v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
func (v Value) pointer() unsafe.Pointer {
        if v.typ.size != ptrSize || !v.typ.pointers() {
                panic("can't call pointer on a non-pointer Value")
        }
        if v.flag&flagIndir != 0 {
                return *(*unsafe.Pointer)(v.ptr)
        }
        return v.ptr
}

// packEface converts v to the empty interface.
func packEface(v Value) interface{} {
        t := v.typ
        var i interface{}
        e := (*emptyInterface)(unsafe.Pointer(&i))
        // First, fill in the data portion of the interface.
        switch {
        case v.Kind() != Ptr && v.Kind() != UnsafePointer:
                // Value is indirect, and so is the interface we're making.
                if v.flag&flagIndir == 0 {
                        panic("reflect: missing flagIndir")
                }
                ptr := v.ptr
                if v.flag&flagAddr != 0 {
                        // TODO: pass safe boolean from valueInterface so
                        // we don't need to copy if safe==true?
                        c := unsafe_New(t)
                        memmove(c, ptr, t.size)
                        ptr = c
                }
                e.word = iword(ptr)
        case v.flag&flagIndir != 0:
                // Value is indirect, but interface is direct.  We need
                // to load the data at v.ptr into the interface data word.
                if t.pointers() {
                        e.word = iword(*(*unsafe.Pointer)(v.ptr))
                } else {
                        e.word = iword(loadScalar(v.ptr, t.size))
                }
        default:
                // Value is direct, and so is the interface.
                if t.pointers() {
                        e.word = iword(v.ptr)
                } else {
                        // e.word = iword(v.scalar)
                        panic("reflect: missing flagIndir")
                }
        }
        // Now, fill in the type portion.  We're very careful here not
        // to have any operation between the e.word and e.typ assignments
        // that would let the garbage collector observe the partially-built
        // interface value.
        e.typ = t
        return i
}

// unpackEface converts the empty interface i to a Value.
func unpackEface(i interface{}) Value {
        e := (*emptyInterface)(unsafe.Pointer(&i))
        // NOTE: don't read e.word until we know whether it is really a pointer or not.
        t := e.typ
        if t == nil {
                return Value{}
        }
        f := flag(t.Kind()) << flagKindShift
        if t.Kind() != Ptr && t.Kind() != UnsafePointer {
                f |= flagIndir
        }
        return Value{t, unsafe.Pointer(e.word), f}
}

// A ValueError occurs when a Value method is invoked on
// a Value that does not support it.  Such cases are documented
// in the description of each method.
type ValueError struct {
        Method string
        Kind   Kind
}

func (e *ValueError) Error() string {
        if e.Kind == 0 {
                return "reflect: call of " + e.Method + " on zero Value"
        }
        return "reflect: call of " + e.Method + " on " + e.Kind.String() + " Value"
}

// methodName returns the name of the calling method,
// assumed to be two stack frames above.
func methodName() string {
        pc, _, _, _ := runtime.Caller(2)
        f := runtime.FuncForPC(pc)
        if f == nil {
                return "unknown method"
        }
        return f.Name()
}

// An iword is the word that would be stored in an
// interface to represent a given value v.  Specifically, if v is
// bigger than a pointer, its word is a pointer to v's data.
// Otherwise, its word holds the data stored
// in its leading bytes (so is not a pointer).
// Because the value sometimes holds a pointer, we use
// unsafe.Pointer to represent it, so that if iword appears
// in a struct, the garbage collector knows that might be
// a pointer.
// TODO: get rid of all occurrences of iword (except in the interface decls below?)
// We want to get rid of the "feature" that an unsafe.Pointer is sometimes a pointer
// and sometimes a uintptr.
type iword unsafe.Pointer

// Get an iword that represents this value.
// TODO: this function goes away at some point
func (v Value) iword() iword {
        t := v.typ
        if t == nil {
                return iword(nil)
        }
        if v.flag&flagIndir != 0 {
                if v.kind() != Ptr && v.kind() != UnsafePointer {
                        return iword(v.ptr)
                }
                // Have indirect but want direct word.
                if t.pointers() {
                        return iword(*(*unsafe.Pointer)(v.ptr))
                }
                return iword(loadScalar(v.ptr, v.typ.size))
        }
        if t.pointers() {
                return iword(v.ptr)
        }
        // return iword(v.scalar)
        panic("reflect: missing flagIndir")
}

// Build a Value from a type/iword pair, plus any extra flags.
// TODO: this function goes away at some point
func fromIword(t *rtype, w iword, fl flag) Value {
        fl |= flag(t.Kind()) << flagKindShift
        if t.Kind() != Ptr && t.Kind() != UnsafePointer {
                return Value{t, unsafe.Pointer(w) /* 0, */, fl | flagIndir}
        } else if t.pointers() {
                return Value{t, unsafe.Pointer(w) /* 0, */, fl}
        } else {
                panic("reflect: can't reach")
        }
}

// loadScalar loads n bytes at p from memory into a uintptr
// that forms the second word of an interface.  The data
// must be non-pointer in nature.
func loadScalar(p unsafe.Pointer, n uintptr) uintptr {
        // Run the copy ourselves instead of calling memmove
        // to avoid moving w to the heap.
        var w uintptr
        switch n {
        default:
                panic("reflect: internal error: loadScalar of " + strconv.Itoa(int(n)) + "-byte value")
        case 0:
        case 1:
                *(*uint8)(unsafe.Pointer(&w)) = *(*uint8)(p)
        case 2:
                *(*uint16)(unsafe.Pointer(&w)) = *(*uint16)(p)
        case 3:
                *(*[3]byte)(unsafe.Pointer(&w)) = *(*[3]byte)(p)
        case 4:
                *(*uint32)(unsafe.Pointer(&w)) = *(*uint32)(p)
        case 5:
                *(*[5]byte)(unsafe.Pointer(&w)) = *(*[5]byte)(p)
        case 6:
                *(*[6]byte)(unsafe.Pointer(&w)) = *(*[6]byte)(p)
        case 7:
                *(*[7]byte)(unsafe.Pointer(&w)) = *(*[7]byte)(p)
        case 8:
                *(*uint64)(unsafe.Pointer(&w)) = *(*uint64)(p)
        }
        return w
}

// storeScalar stores n bytes from w into p.
func storeScalar(p unsafe.Pointer, w uintptr, n uintptr) {
        // Run the copy ourselves instead of calling memmove
        // to avoid moving w to the heap.
        switch n {
        default:
                panic("reflect: internal error: storeScalar of " + strconv.Itoa(int(n)) + "-byte value")
        case 0:
        case 1:
                *(*uint8)(p) = *(*uint8)(unsafe.Pointer(&w))
        case 2:
                *(*uint16)(p) = *(*uint16)(unsafe.Pointer(&w))
        case 3:
                *(*[3]byte)(p) = *(*[3]byte)(unsafe.Pointer(&w))
        case 4:
                *(*uint32)(p) = *(*uint32)(unsafe.Pointer(&w))
        case 5:
                *(*[5]byte)(p) = *(*[5]byte)(unsafe.Pointer(&w))
        case 6:
                *(*[6]byte)(p) = *(*[6]byte)(unsafe.Pointer(&w))
        case 7:
                *(*[7]byte)(p) = *(*[7]byte)(unsafe.Pointer(&w))
        case 8:
                *(*uint64)(p) = *(*uint64)(unsafe.Pointer(&w))
        }
}

// emptyInterface is the header for an interface{} value.
type emptyInterface struct {
        typ  *rtype
        word iword
}

// nonEmptyInterface is the header for a interface value with methods.
type nonEmptyInterface struct {
        // see ../runtime/iface.c:/Itab
        itab *struct {
                typ *rtype                 // dynamic concrete type
                fun [100000]unsafe.Pointer // method table
        }
        word iword
}

// mustBe panics if f's kind is not expected.
// Making this a method on flag instead of on Value
// (and embedding flag in Value) means that we can write
// the very clear v.mustBe(Bool) and have it compile into
// v.flag.mustBe(Bool), which will only bother to copy the
// single important word for the receiver.
func (f flag) mustBe(expected Kind) {
        k := f.kind()
        if k != expected {
                panic(&ValueError{methodName(), k})
        }
}

// mustBeExported panics if f records that the value was obtained using
// an unexported field.
func (f flag) mustBeExported() {
        if f == 0 {
                panic(&ValueError{methodName(), 0})
        }
        if f&flagRO != 0 {
                panic("reflect: " + methodName() + " using value obtained using unexported field")
        }
}

// mustBeAssignable panics if f records that the value is not assignable,
// which is to say that either it was obtained using an unexported field
// or it is not addressable.
func (f flag) mustBeAssignable() {
        if f == 0 {
                panic(&ValueError{methodName(), Invalid})
        }
        // Assignable if addressable and not read-only.
        if f&flagRO != 0 {
                panic("reflect: " + methodName() + " using value obtained using unexported field")
        }
        if f&flagAddr == 0 {
                panic("reflect: " + methodName() + " using unaddressable value")
        }
}

// Addr returns a pointer value representing the address of v.
// It panics if CanAddr() returns false.
// Addr is typically used to obtain a pointer to a struct field
// or slice element in order to call a method that requires a
// pointer receiver.
func (v Value) Addr() Value {
        if v.flag&flagAddr == 0 {
                panic("reflect.Value.Addr of unaddressable value")
        }
        return Value{v.typ.ptrTo(), v.ptr /* 0, */, (v.flag & flagRO) | flag(Ptr)<<flagKindShift}
}

// Bool returns v's underlying value.
// It panics if v's kind is not Bool.
func (v Value) Bool() bool {
        v.mustBe(Bool)
        if v.flag&flagIndir != 0 {
                return *(*bool)(v.ptr)
        }
        // return *(*bool)(unsafe.Pointer(&v.scalar))
        panic("reflect: missing flagIndir")
}

// Bytes returns v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) Bytes() []byte {
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Uint8 {
                panic("reflect.Value.Bytes of non-byte slice")
        }
        // Slice is always bigger than a word; assume flagIndir.
        return *(*[]byte)(v.ptr)
}

// runes returns v's underlying value.
// It panics if v's underlying value is not a slice of runes (int32s).
func (v Value) runes() []rune {
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Int32 {
                panic("reflect.Value.Bytes of non-rune slice")
        }
        // Slice is always bigger than a word; assume flagIndir.
        return *(*[]rune)(v.ptr)
}

// CanAddr returns true if the value's address can be obtained with Addr.
// Such values are called addressable.  A value is addressable if it is
// an element of a slice, an element of an addressable array,
// a field of an addressable struct, or the result of dereferencing a pointer.
// If CanAddr returns false, calling Addr will panic.
func (v Value) CanAddr() bool {
        return v.flag&flagAddr != 0
}

// CanSet returns true if the value of v can be changed.
// A Value can be changed only if it is addressable and was not
// obtained by the use of unexported struct fields.
// If CanSet returns false, calling Set or any type-specific
// setter (e.g., SetBool, SetInt64) will panic.
func (v Value) CanSet() bool {
        return v.flag&(flagAddr|flagRO) == flagAddr
}

// Call calls the function v with the input arguments in.
// For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]).
// Call panics if v's Kind is not Func.
// It returns the output results as Values.
// As in Go, each input argument must be assignable to the
// type of the function's corresponding input parameter.
// If v is a variadic function, Call creates the variadic slice parameter
// itself, copying in the corresponding values.
func (v Value) Call(in []Value) []Value {
        v.mustBe(Func)
        v.mustBeExported()
        return v.call("Call", in)
}

// CallSlice calls the variadic function v with the input arguments in,
// assigning the slice in[len(in)-1] to v's final variadic argument.
// For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]...).
// Call panics if v's Kind is not Func or if v is not variadic.
// It returns the output results as Values.
// As in Go, each input argument must be assignable to the
// type of the function's corresponding input parameter.
func (v Value) CallSlice(in []Value) []Value {
        v.mustBe(Func)
        v.mustBeExported()
        return v.call("CallSlice", in)
}

var makeFuncStubFn = makeFuncStub
var makeFuncStubCode = **(**uintptr)(unsafe.Pointer(&makeFuncStubFn))

func (v Value) call(op string, in []Value) []Value {
        // Get function pointer, type.
        t := v.typ
        var (
                fn   unsafe.Pointer
                rcvr iword
        )
        if v.flag&flagMethod != 0 {
                t, fn, rcvr = methodReceiver(op, v, int(v.flag)>>flagMethodShift)
        } else if v.flag&flagIndir != 0 {
                fn = *(*unsafe.Pointer)(v.ptr)
        } else {
                fn = v.ptr
        }

        if fn == nil {
                panic("reflect.Value.Call: call of nil function")
        }

        // If target is makeFuncStub, short circuit the unpack onto stack /
        // pack back into []Value for the args and return values.  Just do the
        // call directly.
        // We need to do this here because otherwise we have a situation where
        // reflect.callXX calls makeFuncStub, neither of which knows the
        // layout of the args.  That's bad for precise gc & stack copying.
        x := (*makeFuncImpl)(fn)
        if x.code == makeFuncStubCode {
                return x.call(in)
        }

        isSlice := op == "CallSlice"
        n := t.NumIn()
        if isSlice {
                if !t.IsVariadic() {
                        panic("reflect: CallSlice of non-variadic function")
                }
                if len(in) < n {
                        panic("reflect: CallSlice with too few input arguments")
                }
                if len(in) > n {
                        panic("reflect: CallSlice with too many input arguments")
                }
        } else {
                if t.IsVariadic() {
                        n--
                }
                if len(in) < n {
                        panic("reflect: Call with too few input arguments")
                }
                if !t.IsVariadic() && len(in) > n {
                        panic("reflect: Call with too many input arguments")
                }
        }
        for _, x := range in {
                if x.Kind() == Invalid {
                        panic("reflect: " + op + " using zero Value argument")
                }
        }
        for i := 0; i < n; i++ {
                if xt, targ := in[i].Type(), t.In(i); !xt.AssignableTo(targ) {
                        panic("reflect: " + op + " using " + xt.String() + " as type " + targ.String())
                }
        }
        if !isSlice && t.IsVariadic() {
                // prepare slice for remaining values
                m := len(in) - n
                slice := MakeSlice(t.In(n), m, m)
                elem := t.In(n).Elem()
                for i := 0; i < m; i++ {
                        x := in[n+i]
                        if xt := x.Type(); !xt.AssignableTo(elem) {
                                panic("reflect: cannot use " + xt.String() + " as type " + elem.String() + " in " + op)
                        }
                        slice.Index(i).Set(x)
                }
                origIn := in
                in = make([]Value, n+1)
                copy(in[:n], origIn)
                in[n] = slice
        }

        nin := len(in)
        if nin != t.NumIn() {
                panic("reflect.Value.Call: wrong argument count")
        }
        nout := t.NumOut()

        if v.flag&flagMethod != 0 {
                nin++
        }
        firstPointer := len(in) > 0 && t.In(0).Kind() != Ptr && v.flag&flagMethodFn != 0
        params := make([]unsafe.Pointer, nin)
        off := 0
        if v.flag&flagMethod != 0 {
                // Hard-wired first argument.
                p := new(iword)
                *p = rcvr
                params[0] = unsafe.Pointer(p)
                off = 1
        }
        for i, pv := range in {
                pv.mustBeExported()
                targ := t.In(i).(*rtype)
                pv = pv.assignTo("reflect.Value.Call", targ, nil)
                if pv.flag&flagIndir == 0 {
                        p := new(unsafe.Pointer)
                        *p = pv.ptr
                        params[off] = unsafe.Pointer(p)
                } else {
                        params[off] = pv.ptr
                }
                if i == 0 && firstPointer {
                        p := new(unsafe.Pointer)
                        *p = params[off]
                        params[off] = unsafe.Pointer(p)
                }
                off++
        }

        ret := make([]Value, nout)
        results := make([]unsafe.Pointer, nout)
        for i := 0; i < nout; i++ {
                v := New(t.Out(i))
                results[i] = unsafe.Pointer(v.Pointer())
                ret[i] = Indirect(v)
        }

        var pp *unsafe.Pointer
        if len(params) > 0 {
                pp = &params[0]
        }
        var pr *unsafe.Pointer
        if len(results) > 0 {
                pr = &results[0]
        }

        call(t, fn, v.flag&flagMethod != 0, firstPointer, pp, pr)

        return ret
}

// methodReceiver returns information about the receiver
// described by v. The Value v may or may not have the
// flagMethod bit set, so the kind cached in v.flag should
// not be used.
func methodReceiver(op string, v Value, methodIndex int) (t *rtype, fn unsafe.Pointer, rcvr iword) {
        i := methodIndex
        if v.typ.Kind() == Interface {
                tt := (*interfaceType)(unsafe.Pointer(v.typ))
                if i < 0 || i >= len(tt.methods) {
                        panic("reflect: internal error: invalid method index")
                }
                m := &tt.methods[i]
                if m.pkgPath != nil {
                        panic("reflect: " + op + " of unexported method")
                }
                t = m.typ
                iface := (*nonEmptyInterface)(v.ptr)
                if iface.itab == nil {
                        panic("reflect: " + op + " of method on nil interface value")
                }
                fn = unsafe.Pointer(&iface.itab.fun[i])
                rcvr = iface.word
        } else {
                ut := v.typ.uncommon()
                if ut == nil || i < 0 || i >= len(ut.methods) {
                        panic("reflect: internal error: invalid method index")
                }
                m := &ut.methods[i]
                if m.pkgPath != nil {
                        panic("reflect: " + op + " of unexported method")
                }
                fn = unsafe.Pointer(&m.tfn)
                t = m.mtyp
                // Can't call iword here, because it checks v.kind,
                // and that is always Func.
                if v.flag&flagIndir != 0 && (v.typ.Kind() == Ptr || v.typ.Kind() == UnsafePointer) {
                        rcvr = iword(loadScalar(v.ptr, v.typ.size))
                } else {
                        rcvr = iword(v.ptr)
                }
        }
        return
}

// align returns the result of rounding x up to a multiple of n.
// n must be a power of two.
func align(x, n uintptr) uintptr {
        return (x + n - 1) &^ (n - 1)
}

// funcName returns the name of f, for use in error messages.
func funcName(f func([]Value) []Value) string {
        pc := *(*uintptr)(unsafe.Pointer(&f))
        rf := runtime.FuncForPC(pc)
        if rf != nil {
                return rf.Name()
        }
        return "closure"
}

// Cap returns v's capacity.
// It panics if v's Kind is not Array, Chan, or Slice.
func (v Value) Cap() int {
        k := v.kind()
        switch k {
        case Array:
                return v.typ.Len()
        case Chan:
                return int(chancap(v.pointer()))
        case Slice:
                // Slice is always bigger than a word; assume flagIndir.
                return (*sliceHeader)(v.ptr).Cap
        }
        panic(&ValueError{"reflect.Value.Cap", k})
}

// Close closes the channel v.
// It panics if v's Kind is not Chan.
func (v Value) Close() {
        v.mustBe(Chan)
        v.mustBeExported()
        chanclose(v.pointer())
}

// Complex returns v's underlying value, as a complex128.
// It panics if v's Kind is not Complex64 or Complex128
func (v Value) Complex() complex128 {
        k := v.kind()
        switch k {
        case Complex64:
                if v.flag&flagIndir != 0 {
                        return complex128(*(*complex64)(v.ptr))
                }
                // return complex128(*(*complex64)(unsafe.Pointer(&v.scalar)))
                panic("reflect: missing flagIndir")
        case Complex128:
                // complex128 is always bigger than a word; assume flagIndir.
                return *(*complex128)(v.ptr)
        }
        panic(&ValueError{"reflect.Value.Complex", k})
}

// Elem returns the value that the interface v contains
// or that the pointer v points to.
// It panics if v's Kind is not Interface or Ptr.
// It returns the zero Value if v is nil.
func (v Value) Elem() Value {
        k := v.kind()
        switch k {
        case Interface:
                var eface interface{}
                if v.typ.NumMethod() == 0 {
                        eface = *(*interface{})(v.ptr)
                } else {
                        eface = (interface{})(*(*interface {
                                M()
                        })(v.ptr))
                }
                x := unpackEface(eface)
                x.flag |= v.flag & flagRO
                return x
        case Ptr:
                ptr := v.ptr
                if v.flag&flagIndir != 0 {
                        ptr = *(*unsafe.Pointer)(ptr)
                }
                // The returned value's address is v's value.
                if ptr == nil {
                        return Value{}
                }
                tt := (*ptrType)(unsafe.Pointer(v.typ))
                typ := tt.elem
                fl := v.flag&flagRO | flagIndir | flagAddr
                fl |= flag(typ.Kind() << flagKindShift)
                return Value{typ, ptr /* 0, */, fl}
        }
        panic(&ValueError{"reflect.Value.Elem", k})
}

// Field returns the i'th field of the struct v.
// It panics if v's Kind is not Struct or i is out of range.
func (v Value) Field(i int) Value {
        v.mustBe(Struct)
        tt := (*structType)(unsafe.Pointer(v.typ))
        if i < 0 || i >= len(tt.fields) {
                panic("reflect: Field index out of range")
        }
        field := &tt.fields[i]
        typ := field.typ

        // Inherit permission bits from v.
        fl := v.flag & (flagRO | flagIndir | flagAddr)
        // Using an unexported field forces flagRO.
        if field.pkgPath != nil {
                fl |= flagRO
        }
        fl |= flag(typ.Kind()) << flagKindShift

        var ptr unsafe.Pointer
        // var scalar uintptr
        switch {
        case fl&flagIndir != 0:
                // Indirect.  Just bump pointer.
                ptr = unsafe.Pointer(uintptr(v.ptr) + field.offset)
        case typ.pointers():
                if field.offset != 0 {
                        panic("field access of ptr value isn't at offset 0")
                }
                ptr = v.ptr
        case bigEndian:
                // Must be scalar.  Discard leading bytes.
                // scalar = v.scalar << (field.offset * 8)
                panic("reflect: missing flagIndir")
        default:
                // Must be scalar.  Discard leading bytes.
                // scalar = v.scalar >> (field.offset * 8)
                panic("reflect: missing flagIndir")
        }

        return Value{typ, ptr /* scalar, */, fl}
}

// FieldByIndex returns the nested field corresponding to index.
// It panics if v's Kind is not struct.
func (v Value) FieldByIndex(index []int) Value {
        v.mustBe(Struct)
        for i, x := range index {
                if i > 0 {
                        if v.Kind() == Ptr && v.Elem().Kind() == Struct {
                                v = v.Elem()
                        }
                }
                v = v.Field(x)
        }
        return v
}

// FieldByName returns the struct field with the given name.
// It returns the zero Value if no field was found.
// It panics if v's Kind is not struct.
func (v Value) FieldByName(name string) Value {
        v.mustBe(Struct)
        if f, ok := v.typ.FieldByName(name); ok {
                return v.FieldByIndex(f.Index)
        }
        return Value{}
}

// FieldByNameFunc returns the struct field with a name
// that satisfies the match function.
// It panics if v's Kind is not struct.
// It returns the zero Value if no field was found.
func (v Value) FieldByNameFunc(match func(string) bool) Value {
        v.mustBe(Struct)
        if f, ok := v.typ.FieldByNameFunc(match); ok {
                return v.FieldByIndex(f.Index)
        }
        return Value{}
}

// Float returns v's underlying value, as a float64.
// It panics if v's Kind is not Float32 or Float64
func (v Value) Float() float64 {
        k := v.kind()
        switch k {
        case Float32:
                if v.flag&flagIndir != 0 {
                        return float64(*(*float32)(v.ptr))
                }
                // return float64(*(*float32)(unsafe.Pointer(&v.scalar)))
                panic("reflect: missing flagIndir")
        case Float64:
                if v.flag&flagIndir != 0 {
                        return *(*float64)(v.ptr)
                }
                // return *(*float64)(unsafe.Pointer(&v.scalar))
                panic("reflect: missing flagIndir")
        }
        panic(&ValueError{"reflect.Value.Float", k})
}

var uint8Type = TypeOf(uint8(0)).(*rtype)

// Index returns v's i'th element.
// It panics if v's Kind is not Array, Slice, or String or i is out of range.
func (v Value) Index(i int) Value {
        k := v.kind()
        switch k {
        case Array:
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                if i < 0 || i > int(tt.len) {
                        panic("reflect: array index out of range")
                }
                typ := tt.elem
                fl := v.flag & (flagRO | flagIndir | flagAddr) // bits same as overall array
                fl |= flag(typ.Kind()) << flagKindShift
                offset := uintptr(i) * typ.size

                var val unsafe.Pointer
                switch {
                case fl&flagIndir != 0:
                        // Indirect.  Just bump pointer.
                        val = unsafe.Pointer(uintptr(v.ptr) + offset)
                case typ.pointers():
                        if offset != 0 {
                                panic("can't Index(i) with i!=0 on ptrLike value")
                        }
                        val = v.ptr
                case bigEndian:
                        // Direct.  Discard leading bytes.
                        // scalar = v.scalar << (offset * 8)
                        panic("reflect: missing flagIndir")
                default:
                        // Direct.  Discard leading bytes.
                        // scalar = v.scalar >> (offset * 8)
                        panic("reflect: missing flagIndir")
                }
                return Value{typ, val /* scalar, */, fl}

        case Slice:
                // Element flag same as Elem of Ptr.
                // Addressable, indirect, possibly read-only.
                fl := flagAddr | flagIndir | v.flag&flagRO
                s := (*sliceHeader)(v.ptr)
                if i < 0 || i >= s.Len {
                        panic("reflect: slice index out of range")
                }
                tt := (*sliceType)(unsafe.Pointer(v.typ))
                typ := tt.elem
                fl |= flag(typ.Kind()) << flagKindShift
                val := unsafe.Pointer(uintptr(s.Data) + uintptr(i)*typ.size)
                return Value{typ, val /* 0, */, fl}

        case String:
                fl := v.flag&flagRO | flag(Uint8<<flagKindShift) | flagIndir
                s := (*StringHeader)(v.ptr)
                if i < 0 || i >= s.Len {
                        panic("reflect: string index out of range")
                }
                b := uintptr(0)
                *(*byte)(unsafe.Pointer(&b)) = *(*byte)(unsafe.Pointer(uintptr(s.Data) + uintptr(i)))
                return Value{uint8Type, unsafe.Pointer(&b) /* 0, */, fl | flagIndir}
        }
        panic(&ValueError{"reflect.Value.Index", k})
}

// Int returns v's underlying value, as an int64.
// It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
func (v Value) Int() int64 {
        k := v.kind()
        var p unsafe.Pointer
        if v.flag&flagIndir != 0 {
                p = v.ptr
        } else {
                // The escape analysis is good enough that &v.scalar
                // does not trigger a heap allocation.
                // p = unsafe.Pointer(&v.scalar)
                switch k {
                case Int, Int8, Int16, Int32, Int64:
                        panic("reflect: missing flagIndir")
                }
        }
        switch k {
        case Int:
                return int64(*(*int)(p))
        case Int8:
                return int64(*(*int8)(p))
        case Int16:
                return int64(*(*int16)(p))
        case Int32:
                return int64(*(*int32)(p))
        case Int64:
                return int64(*(*int64)(p))
        }
        panic(&ValueError{"reflect.Value.Int", k})
}

// CanInterface returns true if Interface can be used without panicking.
func (v Value) CanInterface() bool {
        if v.flag == 0 {
                panic(&ValueError{"reflect.Value.CanInterface", Invalid})
        }
        return v.flag&flagRO == 0
}

// Interface returns v's current value as an interface{}.
// It is equivalent to:
//      var i interface{} = (v's underlying value)
// It panics if the Value was obtained by accessing
// unexported struct fields.
func (v Value) Interface() (i interface{}) {
        return valueInterface(v, true)
}

func valueInterface(v Value, safe bool) interface{} {
        if v.flag == 0 {
                panic(&ValueError{"reflect.Value.Interface", 0})
        }
        if safe && v.flag&flagRO != 0 {
                // Do not allow access to unexported values via Interface,
                // because they might be pointers that should not be
                // writable or methods or function that should not be callable.
                panic("reflect.Value.Interface: cannot return value obtained from unexported field or method")
        }
        if v.flag&flagMethod != 0 {
                v = makeMethodValue("Interface", v)
        }

        if v.flag&flagMethodFn != 0 {
                if v.typ.Kind() != Func {
                        panic("reflect: MethodFn of non-Func")
                }
                ft := (*funcType)(unsafe.Pointer(v.typ))
                if ft.in[0].Kind() != Ptr {
                        v = makeValueMethod(v)
                }
        }

        if v.kind() == Interface {
                // Special case: return the element inside the interface.
                // Empty interface has one layout, all interfaces with
                // methods have a second layout.
                if v.NumMethod() == 0 {
                        return *(*interface{})(v.ptr)
                }
                return *(*interface {
                        M()
                })(v.ptr)
        }

        // Non-interface value.
        var eface emptyInterface
        eface.typ = toType(v.typ).common()
        eface.word = v.iword()

        // Don't need to allocate if v is not addressable or fits in one word.
        if v.flag&flagAddr != 0 && v.kind() != Ptr && v.kind() != UnsafePointer {
                // eface.word is a pointer to the actual data,
                // which might be changed.  We need to return
                // a pointer to unchanging data, so make a copy.
                ptr := unsafe_New(v.typ)
                memmove(ptr, unsafe.Pointer(eface.word), v.typ.size)
                eface.word = iword(ptr)
        }

        if v.flag&flagIndir == 0 && v.kind() != Ptr && v.kind() != UnsafePointer {
                panic("missing flagIndir")
        }
        // TODO: pass safe to packEface so we don't need to copy if safe==true?
        return packEface(v)
}

// InterfaceData returns the interface v's value as a uintptr pair.
// It panics if v's Kind is not Interface.
func (v Value) InterfaceData() [2]uintptr {
        // TODO: deprecate this
        v.mustBe(Interface)
        // We treat this as a read operation, so we allow
        // it even for unexported data, because the caller
        // has to import "unsafe" to turn it into something
        // that can be abused.
        // Interface value is always bigger than a word; assume flagIndir.
        return *(*[2]uintptr)(v.ptr)
}

// IsNil returns true if v is a nil value.
// It panics if v's Kind is not Chan, Func, Interface, Map, Ptr, or Slice.
func (v Value) IsNil() bool {
        k := v.kind()
        switch k {
        case Chan, Func, Map, Ptr:
                if v.flag&flagMethod != 0 {
                        return false
                }
                ptr := v.ptr
                if v.flag&flagIndir != 0 {
                        ptr = *(*unsafe.Pointer)(ptr)
                }
                return ptr == nil
        case Interface, Slice:
                // Both interface and slice are nil if first word is 0.
                // Both are always bigger than a word; assume flagIndir.
                return *(*unsafe.Pointer)(v.ptr) == nil
        }
        panic(&ValueError{"reflect.Value.IsNil", k})
}

// IsValid returns true if v represents a value.
// It returns false if v is the zero Value.
// If IsValid returns false, all other methods except String panic.
// Most functions and methods never return an invalid value.
// If one does, its documentation states the conditions explicitly.
func (v Value) IsValid() bool {
        return v.flag != 0
}

// Kind returns v's Kind.
// If v is the zero Value (IsValid returns false), Kind returns Invalid.
func (v Value) Kind() Kind {
        return v.kind()
}

// Len returns v's length.
// It panics if v's Kind is not Array, Chan, Map, Slice, or String.
func (v Value) Len() int {
        k := v.kind()
        switch k {
        case Array:
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                return int(tt.len)
        case Chan:
                return chanlen(v.pointer())
        case Map:
                return maplen(v.pointer())
        case Slice:
                // Slice is bigger than a word; assume flagIndir.
                return (*sliceHeader)(v.ptr).Len
        case String:
                // String is bigger than a word; assume flagIndir.
                return (*stringHeader)(v.ptr).Len
        }
        panic(&ValueError{"reflect.Value.Len", k})
}

// MapIndex returns the value associated with key in the map v.
// It panics if v's Kind is not Map.
// It returns the zero Value if key is not found in the map or if v represents a nil map.
// As in Go, the key's value must be assignable to the map's key type.
func (v Value) MapIndex(key Value) Value {
        v.mustBe(Map)
        tt := (*mapType)(unsafe.Pointer(v.typ))

        // Do not require key to be exported, so that DeepEqual
        // and other programs can use all the keys returned by
        // MapKeys as arguments to MapIndex.  If either the map
        // or the key is unexported, though, the result will be
        // considered unexported.  This is consistent with the
        // behavior for structs, which allow read but not write
        // of unexported fields.
        key = key.assignTo("reflect.Value.MapIndex", tt.key, nil)

        var k unsafe.Pointer
        if key.flag&flagIndir != 0 {
                k = key.ptr
        } else if key.typ.pointers() {
                k = unsafe.Pointer(&key.ptr)
        } else {
                // k = unsafe.Pointer(&key.scalar)
                panic("reflect: missing flagIndir")
        }
        e := mapaccess(v.typ, v.pointer(), k)
        if e == nil {
                return Value{}
        }
        typ := tt.elem
        fl := (v.flag | key.flag) & flagRO
        fl |= flag(typ.Kind()) << flagKindShift
        if typ.Kind() != Ptr && typ.Kind() != UnsafePointer {
                // Copy result so future changes to the map
                // won't change the underlying value.
                c := unsafe_New(typ)
                memmove(c, e, typ.size)
                return Value{typ, c /* 0, */, fl | flagIndir}
        } else if typ.pointers() {
                return Value{typ, *(*unsafe.Pointer)(e) /* 0, */, fl}
        } else {
                panic("reflect: can't happen")
        }
}

// MapKeys returns a slice containing all the keys present in the map,
// in unspecified order.
// It panics if v's Kind is not Map.
// It returns an empty slice if v represents a nil map.
func (v Value) MapKeys() []Value {
        v.mustBe(Map)
        tt := (*mapType)(unsafe.Pointer(v.typ))
        keyType := tt.key

        fl := v.flag&flagRO | flag(keyType.Kind())<<flagKindShift
        if keyType.Kind() != Ptr && keyType.Kind() != UnsafePointer {
                fl |= flagIndir
        }

        m := v.pointer()
        mlen := int(0)
        if m != nil {
                mlen = maplen(m)
        }
        it := mapiterinit(v.typ, m)
        a := make([]Value, mlen)
        var i int
        for i = 0; i < len(a); i++ {
                key := mapiterkey(it)
                if key == nil {
                        // Someone deleted an entry from the map since we
                        // called maplen above.  It's a data race, but nothing
                        // we can do about it.
                        break
                }
                if keyType.Kind() != Ptr && keyType.Kind() != UnsafePointer {
                        // Copy result so future changes to the map
                        // won't change the underlying value.
                        c := unsafe_New(keyType)
                        memmove(c, key, keyType.size)
                        a[i] = Value{keyType, c /* 0, */, fl | flagIndir}
                } else if keyType.pointers() {
                        a[i] = Value{keyType, *(*unsafe.Pointer)(key) /* 0, */, fl}
                } else {
                        panic("reflect: can't happen")
                }
                mapiternext(it)
        }
        return a[:i]
}

// Method returns a function value corresponding to v's i'th method.
// The arguments to a Call on the returned function should not include
// a receiver; the returned function will always use v as the receiver.
// Method panics if i is out of range or if v is a nil interface value.
func (v Value) Method(i int) Value {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.Method", Invalid})
        }
        if v.flag&flagMethod != 0 || i < 0 || i >= v.typ.NumMethod() {
                panic("reflect: Method index out of range")
        }
        if v.typ.Kind() == Interface && v.IsNil() {
                panic("reflect: Method on nil interface value")
        }
        fl := v.flag & (flagRO | flagIndir)
        fl |= flag(Func) << flagKindShift
        fl |= flag(i)<<flagMethodShift | flagMethod
        return Value{v.typ, v.ptr /* v.scalar, */, fl}
}

// NumMethod returns the number of methods in the value's method set.
func (v Value) NumMethod() int {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.NumMethod", Invalid})
        }
        if v.flag&flagMethod != 0 {
                return 0
        }
        return v.typ.NumMethod()
}

// MethodByName returns a function value corresponding to the method
// of v with the given name.
// The arguments to a Call on the returned function should not include
// a receiver; the returned function will always use v as the receiver.
// It returns the zero Value if no method was found.
func (v Value) MethodByName(name string) Value {
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.MethodByName", Invalid})
        }
        if v.flag&flagMethod != 0 {
                return Value{}
        }
        m, ok := v.typ.MethodByName(name)
        if !ok {
                return Value{}
        }
        return v.Method(m.Index)
}

// NumField returns the number of fields in the struct v.
// It panics if v's Kind is not Struct.
func (v Value) NumField() int {
        v.mustBe(Struct)
        tt := (*structType)(unsafe.Pointer(v.typ))
        return len(tt.fields)
}

// OverflowComplex returns true if the complex128 x cannot be represented by v's type.
// It panics if v's Kind is not Complex64 or Complex128.
func (v Value) OverflowComplex(x complex128) bool {
        k := v.kind()
        switch k {
        case Complex64:
                return overflowFloat32(real(x)) || overflowFloat32(imag(x))
        case Complex128:
                return false
        }
        panic(&ValueError{"reflect.Value.OverflowComplex", k})
}

// OverflowFloat returns true if the float64 x cannot be represented by v's type.
// It panics if v's Kind is not Float32 or Float64.
func (v Value) OverflowFloat(x float64) bool {
        k := v.kind()
        switch k {
        case Float32:
                return overflowFloat32(x)
        case Float64:
                return false
        }
        panic(&ValueError{"reflect.Value.OverflowFloat", k})
}

func overflowFloat32(x float64) bool {
        if x < 0 {
                x = -x
        }
        return math.MaxFloat32 < x && x <= math.MaxFloat64
}

// OverflowInt returns true if the int64 x cannot be represented by v's type.
// It panics if v's Kind is not Int, Int8, int16, Int32, or Int64.
func (v Value) OverflowInt(x int64) bool {
        k := v.kind()
        switch k {
        case Int, Int8, Int16, Int32, Int64:
                bitSize := v.typ.size * 8
                trunc := (x << (64 - bitSize)) >> (64 - bitSize)
                return x != trunc
        }
        panic(&ValueError{"reflect.Value.OverflowInt", k})
}

// OverflowUint returns true if the uint64 x cannot be represented by v's type.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) OverflowUint(x uint64) bool {
        k := v.kind()
        switch k {
        case Uint, Uintptr, Uint8, Uint16, Uint32, Uint64:
                bitSize := v.typ.size * 8
                trunc := (x << (64 - bitSize)) >> (64 - bitSize)
                return x != trunc
        }
        panic(&ValueError{"reflect.Value.OverflowUint", k})
}

// Pointer returns v's value as a uintptr.
// It returns uintptr instead of unsafe.Pointer so that
// code using reflect cannot obtain unsafe.Pointers
// without importing the unsafe package explicitly.
// It panics if v's Kind is not Chan, Func, Map, Ptr, Slice, or UnsafePointer.
//
// If v's Kind is Func, the returned pointer is an underlying
// code pointer, but not necessarily enough to identify a
// single function uniquely. The only guarantee is that the
// result is zero if and only if v is a nil func Value.
//
// If v's Kind is Slice, the returned pointer is to the first
// element of the slice.  If the slice is nil the returned value
// is 0.  If the slice is empty but non-nil the return value is non-zero.
func (v Value) Pointer() uintptr {
        // TODO: deprecate
        k := v.kind()
        switch k {
        case Chan, Map, Ptr, UnsafePointer:
                return uintptr(v.pointer())
        case Func:
                if v.flag&flagMethod != 0 {
                        // As the doc comment says, the returned pointer is an
                        // underlying code pointer but not necessarily enough to
                        // identify a single function uniquely. All method expressions
                        // created via reflect have the same underlying code pointer,
                        // so their Pointers are equal. The function used here must
                        // match the one used in makeMethodValue.
                        f := makeFuncStub
                        return **(**uintptr)(unsafe.Pointer(&f))
                }
                p := v.pointer()
                // Non-nil func value points at data block.
                // First word of data block is actual code.
                if p != nil {
                        p = *(*unsafe.Pointer)(p)
                }
                return uintptr(p)

        case Slice:
                return (*SliceHeader)(v.ptr).Data
        }
        panic(&ValueError{"reflect.Value.Pointer", k})
}

// Recv receives and returns a value from the channel v.
// It panics if v's Kind is not Chan.
// The receive blocks until a value is ready.
// The boolean value ok is true if the value x corresponds to a send
// on the channel, false if it is a zero value received because the channel is closed.
func (v Value) Recv() (x Value, ok bool) {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.recv(false)
}

// internal recv, possibly non-blocking (nb).
// v is known to be a channel.
func (v Value) recv(nb bool) (val Value, ok bool) {
        tt := (*chanType)(unsafe.Pointer(v.typ))
        if ChanDir(tt.dir)&RecvDir == 0 {
                panic("reflect: recv on send-only channel")
        }
        word, selected, ok := chanrecv(v.typ, v.pointer(), nb)
        if selected {
                val = fromIword(tt.elem, word, 0)
        }
        return
}

// Send sends x on the channel v.
// It panics if v's kind is not Chan or if x's type is not the same type as v's element type.
// As in Go, x's value must be assignable to the channel's element type.
func (v Value) Send(x Value) {
        v.mustBe(Chan)
        v.mustBeExported()
        v.send(x, false)
}

// internal send, possibly non-blocking.
// v is known to be a channel.
func (v Value) send(x Value, nb bool) (selected bool) {
        tt := (*chanType)(unsafe.Pointer(v.typ))
        if ChanDir(tt.dir)&SendDir == 0 {
                panic("reflect: send on recv-only channel")
        }
        x.mustBeExported()
        x = x.assignTo("reflect.Value.Send", tt.elem, nil)
        return chansend(v.typ, v.pointer(), x.iword(), nb)
}

// Set assigns x to the value v.
// It panics if CanSet returns false.
// As in Go, x's value must be assignable to v's type.
func (v Value) Set(x Value) {
        v.mustBeAssignable()
        x.mustBeExported() // do not let unexported x leak
        var target *interface{}
        if v.kind() == Interface {
                target = (*interface{})(v.ptr)
        }
        x = x.assignTo("reflect.Set", v.typ, target)
        if x.flag&flagIndir != 0 {
                memmove(v.ptr, x.ptr, v.typ.size)
        } else if x.typ.pointers() {
                *(*unsafe.Pointer)(v.ptr) = x.ptr
        } else {
                // memmove(v.ptr, unsafe.Pointer(&x.scalar), v.typ.size)
                panic("reflect: missing flagIndir")
        }
}

// SetBool sets v's underlying value.
// It panics if v's Kind is not Bool or if CanSet() is false.
func (v Value) SetBool(x bool) {
        v.mustBeAssignable()
        v.mustBe(Bool)
        *(*bool)(v.ptr) = x
}

// SetBytes sets v's underlying value.
// It panics if v's underlying value is not a slice of bytes.
func (v Value) SetBytes(x []byte) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Uint8 {
                panic("reflect.Value.SetBytes of non-byte slice")
        }
        *(*[]byte)(v.ptr) = x
}

// setRunes sets v's underlying value.
// It panics if v's underlying value is not a slice of runes (int32s).
func (v Value) setRunes(x []rune) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        if v.typ.Elem().Kind() != Int32 {
                panic("reflect.Value.setRunes of non-rune slice")
        }
        *(*[]rune)(v.ptr) = x
}

// SetComplex sets v's underlying value to x.
// It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
func (v Value) SetComplex(x complex128) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetComplex", k})
        case Complex64:
                *(*complex64)(v.ptr) = complex64(x)
        case Complex128:
                *(*complex128)(v.ptr) = x
        }
}

// SetFloat sets v's underlying value to x.
// It panics if v's Kind is not Float32 or Float64, or if CanSet() is false.
func (v Value) SetFloat(x float64) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetFloat", k})
        case Float32:
                *(*float32)(v.ptr) = float32(x)
        case Float64:
                *(*float64)(v.ptr) = x
        }
}

// SetInt sets v's underlying value to x.
// It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64, or if CanSet() is false.
func (v Value) SetInt(x int64) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetInt", k})
        case Int:
                *(*int)(v.ptr) = int(x)
        case Int8:
                *(*int8)(v.ptr) = int8(x)
        case Int16:
                *(*int16)(v.ptr) = int16(x)
        case Int32:
                *(*int32)(v.ptr) = int32(x)
        case Int64:
                *(*int64)(v.ptr) = x
        }
}

// SetLen sets v's length to n.
// It panics if v's Kind is not Slice or if n is negative or
// greater than the capacity of the slice.
func (v Value) SetLen(n int) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        s := (*sliceHeader)(v.ptr)
        if n < 0 || n > int(s.Cap) {
                panic("reflect: slice length out of range in SetLen")
        }
        s.Len = n
}

// SetCap sets v's capacity to n.
// It panics if v's Kind is not Slice or if n is smaller than the length or
// greater than the capacity of the slice.
func (v Value) SetCap(n int) {
        v.mustBeAssignable()
        v.mustBe(Slice)
        s := (*sliceHeader)(v.ptr)
        if n < int(s.Len) || n > int(s.Cap) {
                panic("reflect: slice capacity out of range in SetCap")
        }
        s.Cap = n
}

// SetMapIndex sets the value associated with key in the map v to val.
// It panics if v's Kind is not Map.
// If val is the zero Value, SetMapIndex deletes the key from the map.
// As in Go, key's value must be assignable to the map's key type,
// and val's value must be assignable to the map's value type.
func (v Value) SetMapIndex(key, val Value) {
        v.mustBe(Map)
        v.mustBeExported()
        key.mustBeExported()
        tt := (*mapType)(unsafe.Pointer(v.typ))
        key = key.assignTo("reflect.Value.SetMapIndex", tt.key, nil)
        var k unsafe.Pointer
        if key.flag&flagIndir != 0 {
                k = key.ptr
        } else if key.typ.pointers() {
                k = unsafe.Pointer(&key.ptr)
        } else {
                // k = unsafe.Pointer(&key.scalar)
                panic("reflect: missing flagIndir")
        }
        if val.typ == nil {
                mapdelete(v.typ, v.pointer(), k)
                return
        }
        val.mustBeExported()
        val = val.assignTo("reflect.Value.SetMapIndex", tt.elem, nil)
        var e unsafe.Pointer
        if val.flag&flagIndir != 0 {
                e = val.ptr
        } else if val.typ.pointers() {
                e = unsafe.Pointer(&val.ptr)
        } else {
                // e = unsafe.Pointer(&val.scalar)
                panic("reflect: missing flagIndir")
        }
        mapassign(v.typ, v.pointer(), k, e)
}

// SetUint sets v's underlying value to x.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64, or if CanSet() is false.
func (v Value) SetUint(x uint64) {
        v.mustBeAssignable()
        switch k := v.kind(); k {
        default:
                panic(&ValueError{"reflect.Value.SetUint", k})
        case Uint:
                *(*uint)(v.ptr) = uint(x)
        case Uint8:
                *(*uint8)(v.ptr) = uint8(x)
        case Uint16:
                *(*uint16)(v.ptr) = uint16(x)
        case Uint32:
                *(*uint32)(v.ptr) = uint32(x)
        case Uint64:
                *(*uint64)(v.ptr) = x
        case Uintptr:
                *(*uintptr)(v.ptr) = uintptr(x)
        }
}

// SetPointer sets the unsafe.Pointer value v to x.
// It panics if v's Kind is not UnsafePointer.
func (v Value) SetPointer(x unsafe.Pointer) {
        v.mustBeAssignable()
        v.mustBe(UnsafePointer)
        *(*unsafe.Pointer)(v.ptr) = x
}

// SetString sets v's underlying value to x.
// It panics if v's Kind is not String or if CanSet() is false.
func (v Value) SetString(x string) {
        v.mustBeAssignable()
        v.mustBe(String)
        *(*string)(v.ptr) = x
}

// Slice returns v[i:j].
// It panics if v's Kind is not Array, Slice or String, or if v is an unaddressable array,
// or if the indexes are out of bounds.
func (v Value) Slice(i, j int) Value {
        var (
                cap  int
                typ  *sliceType
                base unsafe.Pointer
        )
        switch kind := v.kind(); kind {
        default:
                panic(&ValueError{"reflect.Value.Slice", kind})

        case Array:
                if v.flag&flagAddr == 0 {
                        panic("reflect.Value.Slice: slice of unaddressable array")
                }
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                cap = int(tt.len)
                typ = (*sliceType)(unsafe.Pointer(tt.slice))
                base = v.ptr

        case Slice:
                typ = (*sliceType)(unsafe.Pointer(v.typ))
                s := (*sliceHeader)(v.ptr)
                base = unsafe.Pointer(s.Data)
                cap = s.Cap

        case String:
                s := (*stringHeader)(v.ptr)
                if i < 0 || j < i || j > s.Len {
                        panic("reflect.Value.Slice: string slice index out of bounds")
                }
                t := stringHeader{unsafe.Pointer(uintptr(s.Data) + uintptr(i)), j - i}
                return Value{v.typ, unsafe.Pointer(&t) /* 0, */, v.flag}
        }

        if i < 0 || j < i || j > cap {
                panic("reflect.Value.Slice: slice index out of bounds")
        }

        // Declare slice so that gc can see the base pointer in it.
        var x []unsafe.Pointer

        // Reinterpret as *sliceHeader to edit.
        s := (*sliceHeader)(unsafe.Pointer(&x))
        s.Data = unsafe.Pointer(uintptr(base) + uintptr(i)*typ.elem.Size())
        s.Len = j - i
        s.Cap = cap - i

        fl := v.flag&flagRO | flagIndir | flag(Slice)<<flagKindShift
        return Value{typ.common(), unsafe.Pointer(&x) /* 0, */, fl}
}

// Slice3 is the 3-index form of the slice operation: it returns v[i:j:k].
// It panics if v's Kind is not Array or Slice, or if v is an unaddressable array,
// or if the indexes are out of bounds.
func (v Value) Slice3(i, j, k int) Value {
        var (
                cap  int
                typ  *sliceType
                base unsafe.Pointer
        )
        switch kind := v.kind(); kind {
        default:
                panic(&ValueError{"reflect.Value.Slice3", kind})

        case Array:
                if v.flag&flagAddr == 0 {
                        panic("reflect.Value.Slice3: slice of unaddressable array")
                }
                tt := (*arrayType)(unsafe.Pointer(v.typ))
                cap = int(tt.len)
                typ = (*sliceType)(unsafe.Pointer(tt.slice))
                base = v.ptr

        case Slice:
                typ = (*sliceType)(unsafe.Pointer(v.typ))
                s := (*sliceHeader)(v.ptr)
                base = s.Data
                cap = s.Cap
        }

        if i < 0 || j < i || k < j || k > cap {
                panic("reflect.Value.Slice3: slice index out of bounds")
        }

        // Declare slice so that the garbage collector
        // can see the base pointer in it.
        var x []unsafe.Pointer

        // Reinterpret as *sliceHeader to edit.
        s := (*sliceHeader)(unsafe.Pointer(&x))
        s.Data = unsafe.Pointer(uintptr(base) + uintptr(i)*typ.elem.Size())
        s.Len = j - i
        s.Cap = k - i

        fl := v.flag&flagRO | flagIndir | flag(Slice)<<flagKindShift
        return Value{typ.common(), unsafe.Pointer(&x) /* 0, */, fl}
}

// String returns the string v's underlying value, as a string.
// String is a special case because of Go's String method convention.
// Unlike the other getters, it does not panic if v's Kind is not String.
// Instead, it returns a string of the form "<T value>" where T is v's type.
func (v Value) String() string {
        switch k := v.kind(); k {
        case Invalid:
                return "<invalid Value>"
        case String:
                return *(*string)(v.ptr)
        }
        // If you call String on a reflect.Value of other type, it's better to
        // print something than to panic. Useful in debugging.
        return "<" + v.typ.String() + " Value>"
}

// TryRecv attempts to receive a value from the channel v but will not block.
// It panics if v's Kind is not Chan.
// If the receive cannot finish without blocking, x is the zero Value.
// The boolean ok is true if the value x corresponds to a send
// on the channel, false if it is a zero value received because the channel is closed.
func (v Value) TryRecv() (x Value, ok bool) {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.recv(true)
}

// TrySend attempts to send x on the channel v but will not block.
// It panics if v's Kind is not Chan.
// It returns true if the value was sent, false otherwise.
// As in Go, x's value must be assignable to the channel's element type.
func (v Value) TrySend(x Value) bool {
        v.mustBe(Chan)
        v.mustBeExported()
        return v.send(x, true)
}

// Type returns v's type.
func (v Value) Type() Type {
        f := v.flag
        if f == 0 {
                panic(&ValueError{"reflect.Value.Type", Invalid})
        }
        if f&flagMethod == 0 {
                // Easy case
                return toType(v.typ)
        }

        // Method value.
        // v.typ describes the receiver, not the method type.
        i := int(v.flag) >> flagMethodShift
        if v.typ.Kind() == Interface {
                // Method on interface.
                tt := (*interfaceType)(unsafe.Pointer(v.typ))
                if i < 0 || i >= len(tt.methods) {
                        panic("reflect: internal error: invalid method index")
                }
                m := &tt.methods[i]
                return toType(m.typ)
        }
        // Method on concrete type.
        ut := v.typ.uncommon()
        if ut == nil || i < 0 || i >= len(ut.methods) {
                panic("reflect: internal error: invalid method index")
        }
        m := &ut.methods[i]
        return toType(m.mtyp)
}

// Uint returns v's underlying value, as a uint64.
// It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) Uint() uint64 {
        k := v.kind()
        var p unsafe.Pointer
        if v.flag&flagIndir != 0 {
                p = v.ptr
        } else {
                // The escape analysis is good enough that &v.scalar
                // does not trigger a heap allocation.
                // p = unsafe.Pointer(&v.scalar)
                switch k {
                case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        panic("reflect: missing flagIndir")
                }
        }
        switch k {
        case Uint:
                return uint64(*(*uint)(p))
        case Uint8:
                return uint64(*(*uint8)(p))
        case Uint16:
                return uint64(*(*uint16)(p))
        case Uint32:
                return uint64(*(*uint32)(p))
        case Uint64:
                return uint64(*(*uint64)(p))
        case Uintptr:
                return uint64(*(*uintptr)(p))
        }
        panic(&ValueError{"reflect.Value.Uint", k})
}

// UnsafeAddr returns a pointer to v's data.
// It is for advanced clients that also import the "unsafe" package.
// It panics if v is not addressable.
func (v Value) UnsafeAddr() uintptr {
        // TODO: deprecate
        if v.typ == nil {
                panic(&ValueError{"reflect.Value.UnsafeAddr", Invalid})
        }
        if v.flag&flagAddr == 0 {
                panic("reflect.Value.UnsafeAddr of unaddressable value")
        }
        return uintptr(v.ptr)
}

// StringHeader is the runtime representation of a string.
// It cannot be used safely or portably and its representation may
// change in a later release.
// Moreover, the Data field is not sufficient to guarantee the data
// it references will not be garbage collected, so programs must keep
// a separate, correctly typed pointer to the underlying data.
type StringHeader struct {
        Data uintptr
        Len  int
}

// stringHeader is a safe version of StringHeader used within this package.
type stringHeader struct {
        Data unsafe.Pointer
        Len  int
}

// SliceHeader is the runtime representation of a slice.
// It cannot be used safely or portably and its representation may
// change in a later release.
// Moreover, the Data field is not sufficient to guarantee the data
// it references will not be garbage collected, so programs must keep
// a separate, correctly typed pointer to the underlying data.
type SliceHeader struct {
        Data uintptr
        Len  int
        Cap  int
}

// sliceHeader is a safe version of SliceHeader used within this package.
type sliceHeader struct {
        Data unsafe.Pointer
        Len  int
        Cap  int
}

func typesMustMatch(what string, t1, t2 Type) {
        if t1 != t2 {
                panic(what + ": " + t1.String() + " != " + t2.String())
        }
}

// grow grows the slice s so that it can hold extra more values, allocating
// more capacity if needed. It also returns the old and new slice lengths.
func grow(s Value, extra int) (Value, int, int) {
        i0 := s.Len()
        i1 := i0 + extra
        if i1 < i0 {
                panic("reflect.Append: slice overflow")
        }
        m := s.Cap()
        if i1 <= m {
                return s.Slice(0, i1), i0, i1
        }
        if m == 0 {
                m = extra
        } else {
                for m < i1 {
                        if i0 < 1024 {
                                m += m
                        } else {
                                m += m / 4
                        }
                }
        }
        t := MakeSlice(s.Type(), i1, m)
        Copy(t, s)
        return t, i0, i1
}

// Append appends the values x to a slice s and returns the resulting slice.
// As in Go, each x's value must be assignable to the slice's element type.
func Append(s Value, x ...Value) Value {
        s.mustBe(Slice)
        s, i0, i1 := grow(s, len(x))
        for i, j := i0, 0; i < i1; i, j = i+1, j+1 {
                s.Index(i).Set(x[j])
        }
        return s
}

// AppendSlice appends a slice t to a slice s and returns the resulting slice.
// The slices s and t must have the same element type.
func AppendSlice(s, t Value) Value {
        s.mustBe(Slice)
        t.mustBe(Slice)
        typesMustMatch("reflect.AppendSlice", s.Type().Elem(), t.Type().Elem())
        s, i0, i1 := grow(s, t.Len())
        Copy(s.Slice(i0, i1), t)
        return s
}

// Copy copies the contents of src into dst until either
// dst has been filled or src has been exhausted.
// It returns the number of elements copied.
// Dst and src each must have kind Slice or Array, and
// dst and src must have the same element type.
func Copy(dst, src Value) int {
        dk := dst.kind()
        if dk != Array && dk != Slice {
                panic(&ValueError{"reflect.Copy", dk})
        }
        if dk == Array {
                dst.mustBeAssignable()
        }
        dst.mustBeExported()

        sk := src.kind()
        if sk != Array && sk != Slice {
                panic(&ValueError{"reflect.Copy", sk})
        }
        src.mustBeExported()

        de := dst.typ.Elem()
        se := src.typ.Elem()
        typesMustMatch("reflect.Copy", de, se)

        n := dst.Len()
        if sn := src.Len(); n > sn {
                n = sn
        }

        // If sk is an in-line array, cannot take its address.
        // Instead, copy element by element.
        // TODO: memmove would be ok for this (sa = unsafe.Pointer(&v.scalar))
        // if we teach the compiler that ptrs don't escape from memmove.
        if src.flag&flagIndir == 0 {
                for i := 0; i < n; i++ {
                        dst.Index(i).Set(src.Index(i))
                }
                return n
        }

        // Copy via memmove.
        var da, sa unsafe.Pointer
        if dk == Array {
                da = dst.ptr
        } else {
                da = (*sliceHeader)(dst.ptr).Data
        }
        if sk == Array {
                sa = src.ptr
        } else {
                sa = (*sliceHeader)(src.ptr).Data
        }
        memmove(da, sa, uintptr(n)*de.Size())
        return n
}

// A runtimeSelect is a single case passed to rselect.
// This must match ../runtime/chan.c:/runtimeSelect
type runtimeSelect struct {
        dir uintptr // 0, SendDir, or RecvDir
        typ *rtype  // channel type
        ch  iword   // interface word for channel
        val iword   // interface word for value (for SendDir)
}

// rselect runs a select. It returns the index of the chosen case,
// and if the case was a receive, the interface word of the received
// value and the conventional OK bool to indicate whether the receive
// corresponds to a sent value.
func rselect([]runtimeSelect) (chosen int, recv iword, recvOK bool)

// A SelectDir describes the communication direction of a select case.
type SelectDir int

// NOTE: These values must match ../runtime/chan.c:/SelectDir.

const (
        _             SelectDir = iota
        SelectSend              // case Chan <- Send
        SelectRecv              // case <-Chan:
        SelectDefault           // default
)

// A SelectCase describes a single case in a select operation.
// The kind of case depends on Dir, the communication direction.
//
// If Dir is SelectDefault, the case represents a default case.
// Chan and Send must be zero Values.
//
// If Dir is SelectSend, the case represents a send operation.
// Normally Chan's underlying value must be a channel, and Send's underlying value must be
// assignable to the channel's element type. As a special case, if Chan is a zero Value,
// then the case is ignored, and the field Send will also be ignored and may be either zero
// or non-zero.
//
// If Dir is SelectRecv, the case represents a receive operation.
// Normally Chan's underlying value must be a channel and Send must be a zero Value.
// If Chan is a zero Value, then the case is ignored, but Send must still be a zero Value.
// When a receive operation is selected, the received Value is returned by Select.
//
type SelectCase struct {
        Dir  SelectDir // direction of case
        Chan Value     // channel to use (for send or receive)
        Send Value     // value to send (for send)
}

// Select executes a select operation described by the list of cases.
// Like the Go select statement, it blocks until at least one of the cases
// can proceed, makes a uniform pseudo-random choice,
// and then executes that case. It returns the index of the chosen case
// and, if that case was a receive operation, the value received and a
// boolean indicating whether the value corresponds to a send on the channel
// (as opposed to a zero value received because the channel is closed).
func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool) {
        // NOTE: Do not trust that caller is not modifying cases data underfoot.
        // The range is safe because the caller cannot modify our copy of the len
        // and each iteration makes its own copy of the value c.
        runcases := make([]runtimeSelect, len(cases))
        haveDefault := false
        for i, c := range cases {
                rc := &runcases[i]
                rc.dir = uintptr(c.Dir)
                switch c.Dir {
                default:
                        panic("reflect.Select: invalid Dir")

                case SelectDefault: // default
                        if haveDefault {
                                panic("reflect.Select: multiple default cases")
                        }
                        haveDefault = true
                        if c.Chan.IsValid() {
                                panic("reflect.Select: default case has Chan value")
                        }
                        if c.Send.IsValid() {
                                panic("reflect.Select: default case has Send value")
                        }

                case SelectSend:
                        ch := c.Chan
                        if !ch.IsValid() {
                                break
                        }
                        ch.mustBe(Chan)
                        ch.mustBeExported()
                        tt := (*chanType)(unsafe.Pointer(ch.typ))
                        if ChanDir(tt.dir)&SendDir == 0 {
                                panic("reflect.Select: SendDir case using recv-only channel")
                        }
                        rc.ch = *(*iword)(ch.iword())
                        rc.typ = &tt.rtype
                        v := c.Send
                        if !v.IsValid() {
                                panic("reflect.Select: SendDir case missing Send value")
                        }
                        v.mustBeExported()
                        v = v.assignTo("reflect.Select", tt.elem, nil)
                        rc.val = v.iword()

                case SelectRecv:
                        if c.Send.IsValid() {
                                panic("reflect.Select: RecvDir case has Send value")
                        }
                        ch := c.Chan
                        if !ch.IsValid() {
                                break
                        }
                        ch.mustBe(Chan)
                        ch.mustBeExported()
                        tt := (*chanType)(unsafe.Pointer(ch.typ))
                        rc.typ = &tt.rtype
                        if ChanDir(tt.dir)&RecvDir == 0 {
                                panic("reflect.Select: RecvDir case using send-only channel")
                        }
                        rc.ch = *(*iword)(ch.iword())
                }
        }

        chosen, word, recvOK := rselect(runcases)
        if runcases[chosen].dir == uintptr(SelectRecv) {
                tt := (*chanType)(unsafe.Pointer(runcases[chosen].typ))
                recv = fromIword(tt.elem, word, 0)
        }
        return chosen, recv, recvOK
}

/*
 * constructors
 */

// implemented in package runtime
func unsafe_New(*rtype) unsafe.Pointer
func unsafe_NewArray(*rtype, int) unsafe.Pointer

// MakeSlice creates a new zero-initialized slice value
// for the specified slice type, length, and capacity.
func MakeSlice(typ Type, len, cap int) Value {
        if typ.Kind() != Slice {
                panic("reflect.MakeSlice of non-slice type")
        }
        if len < 0 {
                panic("reflect.MakeSlice: negative len")
        }
        if cap < 0 {
                panic("reflect.MakeSlice: negative cap")
        }
        if len > cap {
                panic("reflect.MakeSlice: len > cap")
        }

        s := sliceHeader{unsafe_NewArray(typ.Elem().(*rtype), cap), len, cap}
        return Value{typ.common(), unsafe.Pointer(&s) /* 0, */, flagIndir | flag(Slice)<<flagKindShift}
}

// MakeChan creates a new channel with the specified type and buffer size.
func MakeChan(typ Type, buffer int) Value {
        if typ.Kind() != Chan {
                panic("reflect.MakeChan of non-chan type")
        }
        if buffer < 0 {
                panic("reflect.MakeChan: negative buffer size")
        }
        if typ.ChanDir() != BothDir {
                panic("reflect.MakeChan: unidirectional channel type")
        }
        ch := makechan(typ.(*rtype), uint64(buffer))
        return Value{typ.common(), unsafe.Pointer(&ch) /* 0, */, flagIndir | (flag(Chan) << flagKindShift)}
}

// MakeMap creates a new map of the specified type.
func MakeMap(typ Type) Value {
        if typ.Kind() != Map {
                panic("reflect.MakeMap of non-map type")
        }
        m := makemap(typ.(*rtype))
        return Value{typ.common(), unsafe.Pointer(&m) /* 0, */, flagIndir | (flag(Map) << flagKindShift)}
}

// Indirect returns the value that v points to.
// If v is a nil pointer, Indirect returns a zero Value.
// If v is not a pointer, Indirect returns v.
func Indirect(v Value) Value {
        if v.Kind() != Ptr {
                return v
        }
        return v.Elem()
}

// ValueOf returns a new Value initialized to the concrete value
// stored in the interface i.  ValueOf(nil) returns the zero Value.
func ValueOf(i interface{}) Value {
        if i == nil {
                return Value{}
        }

        // TODO(rsc): Eliminate this terrible hack.
        // In the call to unpackEface, i.typ doesn't escape,
        // and i.word is an integer.  So it looks like
        // i doesn't escape.  But really it does,
        // because i.word is actually a pointer.
        escapes(i)

        return unpackEface(i)
}

// Zero returns a Value representing the zero value for the specified type.
// The result is different from the zero value of the Value struct,
// which represents no value at all.
// For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0.
// The returned value is neither addressable nor settable.
func Zero(typ Type) Value {
        if typ == nil {
                panic("reflect: Zero(nil)")
        }
        t := typ.common()
        fl := flag(t.Kind()) << flagKindShift
        if t.Kind() == Ptr || t.Kind() == UnsafePointer {
                return Value{t, nil /* 0, */, fl}
        }
        return Value{t, unsafe_New(typ.(*rtype)) /* 0, */, fl | flagIndir}
}

// New returns a Value representing a pointer to a new zero value
// for the specified type.  That is, the returned Value's Type is PtrTo(t).
func New(typ Type) Value {
        if typ == nil {
                panic("reflect: New(nil)")
        }
        ptr := unsafe_New(typ.(*rtype))
        fl := flag(Ptr) << flagKindShift
        return Value{typ.common().ptrTo(), ptr /* 0, */, fl}
}

// NewAt returns a Value representing a pointer to a value of the
// specified type, using p as that pointer.
func NewAt(typ Type, p unsafe.Pointer) Value {
        fl := flag(Ptr) << flagKindShift
        return Value{typ.common().ptrTo(), p /* 0, */, fl}
}

// assignTo returns a value v that can be assigned directly to typ.
// It panics if v is not assignable to typ.
// For a conversion to an interface type, target is a suggested scratch space to use.
func (v Value) assignTo(context string, dst *rtype, target *interface{}) Value {
        if v.flag&flagMethod != 0 {
                v = makeMethodValue(context, v)
        }

        switch {
        case directlyAssignable(dst, v.typ):
                // Overwrite type so that they match.
                // Same memory layout, so no harm done.
                v.typ = dst
                fl := v.flag & (flagRO | flagAddr | flagIndir)
                fl |= flag(dst.Kind()) << flagKindShift
                return Value{dst, v.ptr /* v.scalar, */, fl}

        case implements(dst, v.typ):
                if target == nil {
                        target = new(interface{})
                }
                x := valueInterface(v, false)
                if dst.NumMethod() == 0 {
                        *target = x
                } else {
                        ifaceE2I(dst, x, unsafe.Pointer(target))
                }
                return Value{dst, unsafe.Pointer(target) /* 0, */, flagIndir | flag(Interface)<<flagKindShift}
        }

        // Failed.
        panic(context + ": value of type " + v.typ.String() + " is not assignable to type " + dst.String())
}

// Convert returns the value v converted to type t.
// If the usual Go conversion rules do not allow conversion
// of the value v to type t, Convert panics.
func (v Value) Convert(t Type) Value {
        if v.flag&flagMethod != 0 {
                v = makeMethodValue("Convert", v)
        }
        op := convertOp(t.common(), v.typ)
        if op == nil {
                panic("reflect.Value.Convert: value of type " + v.typ.String() + " cannot be converted to type " + t.String())
        }
        return op(v, t)
}

// convertOp returns the function to convert a value of type src
// to a value of type dst. If the conversion is illegal, convertOp returns nil.
func convertOp(dst, src *rtype) func(Value, Type) Value {
        switch src.Kind() {
        case Int, Int8, Int16, Int32, Int64:
                switch dst.Kind() {
                case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        return cvtInt
                case Float32, Float64:
                        return cvtIntFloat
                case String:
                        return cvtIntString
                }

        case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                switch dst.Kind() {
                case Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        return cvtUint
                case Float32, Float64:
                        return cvtUintFloat
                case String:
                        return cvtUintString
                }

        case Float32, Float64:
                switch dst.Kind() {
                case Int, Int8, Int16, Int32, Int64:
                        return cvtFloatInt
                case Uint, Uint8, Uint16, Uint32, Uint64, Uintptr:
                        return cvtFloatUint
                case Float32, Float64:
                        return cvtFloat
                }

        case Complex64, Complex128:
                switch dst.Kind() {
                case Complex64, Complex128:
                        return cvtComplex
                }

        case String:
                if dst.Kind() == Slice && dst.Elem().PkgPath() == "" {
                        switch dst.Elem().Kind() {
                        case Uint8:
                                return cvtStringBytes
                        case Int32:
                                return cvtStringRunes
                        }
                }

        case Slice:
                if dst.Kind() == String && src.Elem().PkgPath() == "" {
                        switch src.Elem().Kind() {
                        case Uint8:
                                return cvtBytesString
                        case Int32:
                                return cvtRunesString
                        }
                }
        }

        // dst and src have same underlying type.
        if haveIdenticalUnderlyingType(dst, src) {
                return cvtDirect
        }

        // dst and src are unnamed pointer types with same underlying base type.
        if dst.Kind() == Ptr && dst.Name() == "" &&
                src.Kind() == Ptr && src.Name() == "" &&
                haveIdenticalUnderlyingType(dst.Elem().common(), src.Elem().common()) {
                return cvtDirect
        }

        if implements(dst, src) {
                if src.Kind() == Interface {
                        return cvtI2I
                }
                return cvtT2I
        }

        return nil
}

// makeInt returns a Value of type t equal to bits (possibly truncated),
// where t is a signed or unsigned int type.
func makeInt(f flag, bits uint64, t Type) Value {
        typ := t.common()
        if typ.size > ptrSize {
                // Assume ptrSize >= 4, so this must be uint64.
                ptr := unsafe_New(typ)
                *(*uint64)(unsafe.Pointer(ptr)) = bits
                return Value{typ, ptr /* 0, */, f | flagIndir | flag(typ.Kind())<<flagKindShift}
        }
        var s uintptr
        switch typ.size {
        case 1:
                *(*uint8)(unsafe.Pointer(&s)) = uint8(bits)
        case 2:
                *(*uint16)(unsafe.Pointer(&s)) = uint16(bits)
        case 4:
                *(*uint32)(unsafe.Pointer(&s)) = uint32(bits)
        case 8:
                *(*uint64)(unsafe.Pointer(&s)) = uint64(bits)
        }
        return Value{typ, unsafe.Pointer(&s) /* 0, */, f | flagIndir | flag(typ.Kind())<<flagKindShift}
}

// makeFloat returns a Value of type t equal to v (possibly truncated to float32),
// where t is a float32 or float64 type.
func makeFloat(f flag, v float64, t Type) Value {
        typ := t.common()
        if typ.size > ptrSize {
                // Assume ptrSize >= 4, so this must be float64.
                ptr := unsafe_New(typ)
                *(*float64)(unsafe.Pointer(ptr)) = v
                return Value{typ, ptr /* 0, */, f | flagIndir | flag(typ.Kind())<<flagKindShift}
        }

        var s uintptr
        switch typ.size {
        case 4:
                *(*float32)(unsafe.Pointer(&s)) = float32(v)
        case 8:
                *(*float64)(unsafe.Pointer(&s)) = v
        }
        return Value{typ, unsafe.Pointer(&s) /* 0, */, f | flagIndir | flag(typ.Kind())<<flagKindShift}
}

// makeComplex returns a Value of type t equal to v (possibly truncated to complex64),
// where t is a complex64 or complex128 type.
func makeComplex(f flag, v complex128, t Type) Value {
        typ := t.common()
        if typ.size > ptrSize {
                ptr := unsafe_New(typ)
                switch typ.size {
                case 8:
                        *(*complex64)(unsafe.Pointer(ptr)) = complex64(v)
                case 16:
                        *(*complex128)(unsafe.Pointer(ptr)) = v
                }
                return Value{typ, ptr /* 0, */, f | flagIndir | flag(typ.Kind())<<flagKindShift}
        }

        // Assume ptrSize <= 8 so this must be complex64.
        var s uintptr
        *(*complex64)(unsafe.Pointer(&s)) = complex64(v)
        return Value{typ, unsafe.Pointer(&s) /* 0, */, f | flagIndir | flag(typ.Kind())<<flagKindShift}
}

func makeString(f flag, v string, t Type) Value {
        ret := New(t).Elem()
        ret.SetString(v)
        ret.flag = ret.flag&^flagAddr | f | flagIndir
        return ret
}

func makeBytes(f flag, v []byte, t Type) Value {
        ret := New(t).Elem()
        ret.SetBytes(v)
        ret.flag = ret.flag&^flagAddr | f | flagIndir
        return ret
}

func makeRunes(f flag, v []rune, t Type) Value {
        ret := New(t).Elem()
        ret.setRunes(v)
        ret.flag = ret.flag&^flagAddr | f | flagIndir
        return ret
}

// These conversion functions are returned by convertOp
// for classes of conversions. For example, the first function, cvtInt,
// takes any value v of signed int type and returns the value converted
// to type t, where t is any signed or unsigned int type.

// convertOp: intXX -> [u]intXX
func cvtInt(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, uint64(v.Int()), t)
}

// convertOp: uintXX -> [u]intXX
func cvtUint(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, v.Uint(), t)
}

// convertOp: floatXX -> intXX
func cvtFloatInt(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, uint64(int64(v.Float())), t)
}

// convertOp: floatXX -> uintXX
func cvtFloatUint(v Value, t Type) Value {
        return makeInt(v.flag&flagRO, uint64(v.Float()), t)
}

// convertOp: intXX -> floatXX
func cvtIntFloat(v Value, t Type) Value {
        return makeFloat(v.flag&flagRO, float64(v.Int()), t)
}

// convertOp: uintXX -> floatXX
func cvtUintFloat(v Value, t Type) Value {
        return makeFloat(v.flag&flagRO, float64(v.Uint()), t)
}

// convertOp: floatXX -> floatXX
func cvtFloat(v Value, t Type) Value {
        return makeFloat(v.flag&flagRO, v.Float(), t)
}

// convertOp: complexXX -> complexXX
func cvtComplex(v Value, t Type) Value {
        return makeComplex(v.flag&flagRO, v.Complex(), t)
}

// convertOp: intXX -> string
func cvtIntString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.Int()), t)
}

// convertOp: uintXX -> string
func cvtUintString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.Uint()), t)
}

// convertOp: []byte -> string
func cvtBytesString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.Bytes()), t)
}

// convertOp: string -> []byte
func cvtStringBytes(v Value, t Type) Value {
        return makeBytes(v.flag&flagRO, []byte(v.String()), t)
}

// convertOp: []rune -> string
func cvtRunesString(v Value, t Type) Value {
        return makeString(v.flag&flagRO, string(v.runes()), t)
}

// convertOp: string -> []rune
func cvtStringRunes(v Value, t Type) Value {
        return makeRunes(v.flag&flagRO, []rune(v.String()), t)
}

// convertOp: direct copy
func cvtDirect(v Value, typ Type) Value {
        f := v.flag
        t := typ.common()
        ptr := v.ptr
        if f&flagAddr != 0 {
                // indirect, mutable word - make a copy
                c := unsafe_New(t)
                memmove(c, ptr, t.size)
                ptr = c
                f &^= flagAddr
        }
        return Value{t, ptr /* v.scalar, */, v.flag&flagRO | f} // v.flag&flagRO|f == f?
}

// convertOp: concrete -> interface
func cvtT2I(v Value, typ Type) Value {
        target := new(interface{})
        x := valueInterface(v, false)
        if typ.NumMethod() == 0 {
                *target = x
        } else {
                ifaceE2I(typ.(*rtype), x, unsafe.Pointer(target))
        }
        return Value{typ.common(), unsafe.Pointer(target) /* 0, */, v.flag&flagRO | flagIndir | flag(Interface)<<flagKindShift}
}

// convertOp: interface -> interface
func cvtI2I(v Value, typ Type) Value {
        if v.IsNil() {
                ret := Zero(typ)
                ret.flag |= v.flag & flagRO
                return ret
        }
        return cvtT2I(v.Elem(), typ)
}

// implemented in ../pkg/runtime
func chancap(ch unsafe.Pointer) int
func chanclose(ch unsafe.Pointer)
func chanlen(ch unsafe.Pointer) int
func chanrecv(t *rtype, ch unsafe.Pointer, nb bool) (val iword, selected, received bool)
func chansend(t *rtype, ch unsafe.Pointer, val iword, nb bool) bool

func makechan(typ *rtype, size uint64) (ch unsafe.Pointer)
func makemap(t *rtype) (m unsafe.Pointer)
func mapaccess(t *rtype, m unsafe.Pointer, key unsafe.Pointer) (val unsafe.Pointer)
func mapassign(t *rtype, m unsafe.Pointer, key, val unsafe.Pointer)
func mapdelete(t *rtype, m unsafe.Pointer, key unsafe.Pointer)
func mapiterinit(t *rtype, m unsafe.Pointer) unsafe.Pointer
func mapiterkey(it unsafe.Pointer) (key unsafe.Pointer)
func mapiternext(it unsafe.Pointer)
func maplen(m unsafe.Pointer) int

func call(typ *rtype, fnaddr unsafe.Pointer, isInterface bool, isMethod bool, params *unsafe.Pointer, results *unsafe.Pointer)
func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)

// Dummy annotation marking that the value x escapes,
// for use in cases where the reflect code is so clever that
// the compiler cannot follow.
func escapes(x interface{}) {
        if dummy.b {
                dummy.x = x
        }
}

var dummy struct {
        b bool
        x interface{}
}