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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 types2
import (
"cmd/compile/internal/syntax"
)
// labels checks correct label use in body.
func (check *Checker) labels(body *syntax.BlockStmt) {
// set of all labels in this body
all := NewScope(nil, body.Pos(), syntax.EndPos(body), "label")
fwdJumps := check.blockBranches(all, nil, nil, body.List)
// If there are any forward jumps left, no label was found for
// the corresponding goto statements. Either those labels were
// never defined, or they are inside blocks and not reachable
// for the respective gotos.
for _, jmp := range fwdJumps {
var msg string
name := jmp.Label.Value
if alt := all.Lookup(name); alt != nil {
msg = "goto %s jumps into block"
alt.(*Label).used = true // avoid another error
} else {
msg = "label %s not declared"
}
check.errorf(jmp.Label, msg, name)
}
// spec: "It is illegal to define a label that is never used."
for name, obj := range all.elems {
obj = resolve(name, obj)
if lbl := obj.(*Label); !lbl.used {
check.softErrorf(lbl.pos, "label %s declared but not used", lbl.name)
}
}
}
// A block tracks label declarations in a block and its enclosing blocks.
type block struct {
parent *block // enclosing block
lstmt *syntax.LabeledStmt // labeled statement to which this block belongs, or nil
labels map[string]*syntax.LabeledStmt // allocated lazily
}
// insert records a new label declaration for the current block.
// The label must not have been declared before in any block.
func (b *block) insert(s *syntax.LabeledStmt) {
name := s.Label.Value
if debug {
assert(b.gotoTarget(name) == nil)
}
labels := b.labels
if labels == nil {
labels = make(map[string]*syntax.LabeledStmt)
b.labels = labels
}
labels[name] = s
}
// gotoTarget returns the labeled statement in the current
// or an enclosing block with the given label name, or nil.
func (b *block) gotoTarget(name string) *syntax.LabeledStmt {
for s := b; s != nil; s = s.parent {
if t := s.labels[name]; t != nil {
return t
}
}
return nil
}
// enclosingTarget returns the innermost enclosing labeled
// statement with the given label name, or nil.
func (b *block) enclosingTarget(name string) *syntax.LabeledStmt {
for s := b; s != nil; s = s.parent {
if t := s.lstmt; t != nil && t.Label.Value == name {
return t
}
}
return nil
}
// blockBranches processes a block's statement list and returns the set of outgoing forward jumps.
// all is the scope of all declared labels, parent the set of labels declared in the immediately
// enclosing block, and lstmt is the labeled statement this block is associated with (or nil).
func (check *Checker) blockBranches(all *Scope, parent *block, lstmt *syntax.LabeledStmt, list []syntax.Stmt) []*syntax.BranchStmt {
b := &block{parent, lstmt, nil}
var (
varDeclPos syntax.Pos
fwdJumps, badJumps []*syntax.BranchStmt
)
// All forward jumps jumping over a variable declaration are possibly
// invalid (they may still jump out of the block and be ok).
// recordVarDecl records them for the given position.
recordVarDecl := func(pos syntax.Pos) {
varDeclPos = pos
badJumps = append(badJumps[:0], fwdJumps...) // copy fwdJumps to badJumps
}
jumpsOverVarDecl := func(jmp *syntax.BranchStmt) bool {
if varDeclPos.IsKnown() {
for _, bad := range badJumps {
if jmp == bad {
return true
}
}
}
return false
}
var stmtBranches func(syntax.Stmt)
stmtBranches = func(s syntax.Stmt) {
switch s := s.(type) {
case *syntax.DeclStmt:
for _, d := range s.DeclList {
if d, _ := d.(*syntax.VarDecl); d != nil {
recordVarDecl(d.Pos())
}
}
case *syntax.LabeledStmt:
// declare non-blank label
if name := s.Label.Value; name != "_" {
lbl := NewLabel(s.Label.Pos(), check.pkg, name)
if alt := all.Insert(lbl); alt != nil {
var err error_
err.soft = true
err.errorf(lbl.pos, "label %s already declared", name)
err.recordAltDecl(alt)
check.report(&err)
// ok to continue
} else {
b.insert(s)
check.recordDef(s.Label, lbl)
}
// resolve matching forward jumps and remove them from fwdJumps
i := 0
for _, jmp := range fwdJumps {
if jmp.Label.Value == name {
// match
lbl.used = true
check.recordUse(jmp.Label, lbl)
if jumpsOverVarDecl(jmp) {
check.softErrorf(
jmp.Label,
"goto %s jumps over variable declaration at line %d",
name,
varDeclPos.Line(),
)
// ok to continue
}
} else {
// no match - record new forward jump
fwdJumps[i] = jmp
i++
}
}
fwdJumps = fwdJumps[:i]
lstmt = s
}
stmtBranches(s.Stmt)
case *syntax.BranchStmt:
if s.Label == nil {
return // checked in 1st pass (check.stmt)
}
// determine and validate target
name := s.Label.Value
switch s.Tok {
case syntax.Break:
// spec: "If there is a label, it must be that of an enclosing
// "for", "switch", or "select" statement, and that is the one
// whose execution terminates."
valid := false
if t := b.enclosingTarget(name); t != nil {
switch t.Stmt.(type) {
case *syntax.SwitchStmt, *syntax.SelectStmt, *syntax.ForStmt:
valid = true
}
}
if !valid {
check.errorf(s.Label, "invalid break label %s", name)
return
}
case syntax.Continue:
// spec: "If there is a label, it must be that of an enclosing
// "for" statement, and that is the one whose execution advances."
valid := false
if t := b.enclosingTarget(name); t != nil {
switch t.Stmt.(type) {
case *syntax.ForStmt:
valid = true
}
}
if !valid {
check.errorf(s.Label, "invalid continue label %s", name)
return
}
case syntax.Goto:
if b.gotoTarget(name) == nil {
// label may be declared later - add branch to forward jumps
fwdJumps = append(fwdJumps, s)
return
}
default:
check.errorf(s, invalidAST+"branch statement: %s %s", s.Tok, name)
return
}
// record label use
obj := all.Lookup(name)
obj.(*Label).used = true
check.recordUse(s.Label, obj)
case *syntax.AssignStmt:
if s.Op == syntax.Def {
recordVarDecl(s.Pos())
}
case *syntax.BlockStmt:
// Unresolved forward jumps inside the nested block
// become forward jumps in the current block.
fwdJumps = append(fwdJumps, check.blockBranches(all, b, lstmt, s.List)...)
case *syntax.IfStmt:
stmtBranches(s.Then)
if s.Else != nil {
stmtBranches(s.Else)
}
case *syntax.SwitchStmt:
b := &block{b, lstmt, nil}
for _, s := range s.Body {
fwdJumps = append(fwdJumps, check.blockBranches(all, b, nil, s.Body)...)
}
case *syntax.SelectStmt:
b := &block{b, lstmt, nil}
for _, s := range s.Body {
fwdJumps = append(fwdJumps, check.blockBranches(all, b, nil, s.Body)...)
}
case *syntax.ForStmt:
stmtBranches(s.Body)
}
}
for _, s := range list {
stmtBranches(s)
}
return fwdJumps
}