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// Copyright 2013 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 ssa
// lvalues are the union of addressable expressions and map-index
// expressions.
import (
// An lvalue represents an assignable location that may appear on the
// left-hand side of an assignment. This is a generalization of a
// pointer to permit updates to elements of maps.
type lvalue interface {
store(fn *Function, v Value) // stores v into the location
load(fn *Function) Value // loads the contents of the location
address(fn *Function) Value // address of the location
typ() types.Type // returns the type of the location
// An address is an lvalue represented by a true pointer.
type address struct {
addr Value
pos token.Pos // source position
expr ast.Expr // source syntax of the value (not address) [debug mode]
func (a *address) load(fn *Function) Value {
load := emitLoad(fn, a.addr)
load.pos = a.pos
return load
func (a *address) store(fn *Function, v Value) {
store := emitStore(fn, a.addr, v, a.pos)
if a.expr != nil {
// store.Val is v, converted for assignability.
emitDebugRef(fn, a.expr, store.Val, false)
func (a *address) address(fn *Function) Value {
if a.expr != nil {
emitDebugRef(fn, a.expr, a.addr, true)
return a.addr
func (a *address) typ() types.Type {
return deref(a.addr.Type())
// An element is an lvalue represented by m[k], the location of an
// element of a map or string. These locations are not addressable
// since pointers cannot be formed from them, but they do support
// load(), and in the case of maps, store().
type element struct {
m, k Value // map or string
t types.Type // map element type or string byte type
pos token.Pos // source position of colon ({k:v}) or lbrack (m[k]=v)
func (e *element) load(fn *Function) Value {
l := &Lookup{
X: e.m,
Index: e.k,
return fn.emit(l)
func (e *element) store(fn *Function, v Value) {
up := &MapUpdate{
Map: e.m,
Key: e.k,
Value: emitConv(fn, v, e.t),
up.pos = e.pos
func (e *element) address(fn *Function) Value {
panic("map/string elements are not addressable")
func (e *element) typ() types.Type {
return e.t
// A blank is a dummy variable whose name is "_".
// It is not reified: loads are illegal and stores are ignored.
type blank struct{}
func (bl blank) load(fn *Function) Value {
panic("blank.load is illegal")
func (bl blank) store(fn *Function, v Value) {
// no-op
func (bl blank) address(fn *Function) Value {
panic("blank var is not addressable")
func (bl blank) typ() types.Type {
// This should be the type of the blank Ident; the typechecker
// doesn't provide this yet, but fortunately, we don't need it
// yet either.
panic("blank.typ is unimplemented")