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// Copyright 2016 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 cfg
// This file implements the CFG construction pass.
import (
"fmt"
"go/ast"
"go/token"
)
type builder struct {
cfg *CFG
mayReturn func(*ast.CallExpr) bool
current *Block
lblocks map[*ast.Object]*lblock // labeled blocks
targets *targets // linked stack of branch targets
}
func (b *builder) stmt(_s ast.Stmt) {
// The label of the current statement. If non-nil, its _goto
// target is always set; its _break and _continue are set only
// within the body of switch/typeswitch/select/for/range.
// It is effectively an additional default-nil parameter of stmt().
var label *lblock
start:
switch s := _s.(type) {
case *ast.BadStmt,
*ast.SendStmt,
*ast.IncDecStmt,
*ast.GoStmt,
*ast.DeferStmt,
*ast.EmptyStmt,
*ast.AssignStmt:
// No effect on control flow.
b.add(s)
case *ast.ExprStmt:
b.add(s)
if call, ok := s.X.(*ast.CallExpr); ok && !b.mayReturn(call) {
// Calls to panic, os.Exit, etc, never return.
b.current = b.newBlock("unreachable.call")
}
case *ast.DeclStmt:
// Treat each var ValueSpec as a separate statement.
d := s.Decl.(*ast.GenDecl)
if d.Tok == token.VAR {
for _, spec := range d.Specs {
if spec, ok := spec.(*ast.ValueSpec); ok {
b.add(spec)
}
}
}
case *ast.LabeledStmt:
label = b.labeledBlock(s.Label)
b.jump(label._goto)
b.current = label._goto
_s = s.Stmt
goto start // effectively: tailcall stmt(g, s.Stmt, label)
case *ast.ReturnStmt:
b.add(s)
b.current = b.newBlock("unreachable.return")
case *ast.BranchStmt:
b.branchStmt(s)
case *ast.BlockStmt:
b.stmtList(s.List)
case *ast.IfStmt:
if s.Init != nil {
b.stmt(s.Init)
}
then := b.newBlock("if.then")
done := b.newBlock("if.done")
_else := done
if s.Else != nil {
_else = b.newBlock("if.else")
}
b.add(s.Cond)
b.ifelse(then, _else)
b.current = then
b.stmt(s.Body)
b.jump(done)
if s.Else != nil {
b.current = _else
b.stmt(s.Else)
b.jump(done)
}
b.current = done
case *ast.SwitchStmt:
b.switchStmt(s, label)
case *ast.TypeSwitchStmt:
b.typeSwitchStmt(s, label)
case *ast.SelectStmt:
b.selectStmt(s, label)
case *ast.ForStmt:
b.forStmt(s, label)
case *ast.RangeStmt:
b.rangeStmt(s, label)
default:
panic(fmt.Sprintf("unexpected statement kind: %T", s))
}
}
func (b *builder) stmtList(list []ast.Stmt) {
for _, s := range list {
b.stmt(s)
}
}
func (b *builder) branchStmt(s *ast.BranchStmt) {
var block *Block
switch s.Tok {
case token.BREAK:
if s.Label != nil {
if lb := b.labeledBlock(s.Label); lb != nil {
block = lb._break
}
} else {
for t := b.targets; t != nil && block == nil; t = t.tail {
block = t._break
}
}
case token.CONTINUE:
if s.Label != nil {
if lb := b.labeledBlock(s.Label); lb != nil {
block = lb._continue
}
} else {
for t := b.targets; t != nil && block == nil; t = t.tail {
block = t._continue
}
}
case token.FALLTHROUGH:
for t := b.targets; t != nil && block == nil; t = t.tail {
block = t._fallthrough
}
case token.GOTO:
if s.Label != nil {
block = b.labeledBlock(s.Label)._goto
}
}
if block == nil {
block = b.newBlock("undefined.branch")
}
b.jump(block)
b.current = b.newBlock("unreachable.branch")
}
func (b *builder) switchStmt(s *ast.SwitchStmt, label *lblock) {
if s.Init != nil {
b.stmt(s.Init)
}
if s.Tag != nil {
b.add(s.Tag)
}
done := b.newBlock("switch.done")
if label != nil {
label._break = done
}
// We pull the default case (if present) down to the end.
// But each fallthrough label must point to the next
// body block in source order, so we preallocate a
// body block (fallthru) for the next case.
// Unfortunately this makes for a confusing block order.
var defaultBody *[]ast.Stmt
var defaultFallthrough *Block
var fallthru, defaultBlock *Block
ncases := len(s.Body.List)
for i, clause := range s.Body.List {
body := fallthru
if body == nil {
body = b.newBlock("switch.body") // first case only
}
// Preallocate body block for the next case.
fallthru = done
if i+1 < ncases {
fallthru = b.newBlock("switch.body")
}
cc := clause.(*ast.CaseClause)
if cc.List == nil {
// Default case.
defaultBody = &cc.Body
defaultFallthrough = fallthru
defaultBlock = body
continue
}
var nextCond *Block
for _, cond := range cc.List {
nextCond = b.newBlock("switch.next")
b.add(cond) // one half of the tag==cond condition
b.ifelse(body, nextCond)
b.current = nextCond
}
b.current = body
b.targets = &targets{
tail: b.targets,
_break: done,
_fallthrough: fallthru,
}
b.stmtList(cc.Body)
b.targets = b.targets.tail
b.jump(done)
b.current = nextCond
}
if defaultBlock != nil {
b.jump(defaultBlock)
b.current = defaultBlock
b.targets = &targets{
tail: b.targets,
_break: done,
_fallthrough: defaultFallthrough,
}
b.stmtList(*defaultBody)
b.targets = b.targets.tail
}
b.jump(done)
b.current = done
}
func (b *builder) typeSwitchStmt(s *ast.TypeSwitchStmt, label *lblock) {
if s.Init != nil {
b.stmt(s.Init)
}
if s.Assign != nil {
b.add(s.Assign)
}
done := b.newBlock("typeswitch.done")
if label != nil {
label._break = done
}
var default_ *ast.CaseClause
for _, clause := range s.Body.List {
cc := clause.(*ast.CaseClause)
if cc.List == nil {
default_ = cc
continue
}
body := b.newBlock("typeswitch.body")
var next *Block
for _, casetype := range cc.List {
next = b.newBlock("typeswitch.next")
// casetype is a type, so don't call b.add(casetype).
// This block logically contains a type assertion,
// x.(casetype), but it's unclear how to represent x.
_ = casetype
b.ifelse(body, next)
b.current = next
}
b.current = body
b.typeCaseBody(cc, done)
b.current = next
}
if default_ != nil {
b.typeCaseBody(default_, done)
} else {
b.jump(done)
}
b.current = done
}
func (b *builder) typeCaseBody(cc *ast.CaseClause, done *Block) {
b.targets = &targets{
tail: b.targets,
_break: done,
}
b.stmtList(cc.Body)
b.targets = b.targets.tail
b.jump(done)
}
func (b *builder) selectStmt(s *ast.SelectStmt, label *lblock) {
// First evaluate channel expressions.
// TODO(adonovan): fix: evaluate only channel exprs here.
for _, clause := range s.Body.List {
if comm := clause.(*ast.CommClause).Comm; comm != nil {
b.stmt(comm)
}
}
done := b.newBlock("select.done")
if label != nil {
label._break = done
}
var defaultBody *[]ast.Stmt
for _, cc := range s.Body.List {
clause := cc.(*ast.CommClause)
if clause.Comm == nil {
defaultBody = &clause.Body
continue
}
body := b.newBlock("select.body")
next := b.newBlock("select.next")
b.ifelse(body, next)
b.current = body
b.targets = &targets{
tail: b.targets,
_break: done,
}
switch comm := clause.Comm.(type) {
case *ast.ExprStmt: // <-ch
// nop
case *ast.AssignStmt: // x := <-states[state].Chan
b.add(comm.Lhs[0])
}
b.stmtList(clause.Body)
b.targets = b.targets.tail
b.jump(done)
b.current = next
}
if defaultBody != nil {
b.targets = &targets{
tail: b.targets,
_break: done,
}
b.stmtList(*defaultBody)
b.targets = b.targets.tail
b.jump(done)
}
b.current = done
}
func (b *builder) forStmt(s *ast.ForStmt, label *lblock) {
// ...init...
// jump loop
// loop:
// if cond goto body else done
// body:
// ...body...
// jump post
// post: (target of continue)
// ...post...
// jump loop
// done: (target of break)
if s.Init != nil {
b.stmt(s.Init)
}
body := b.newBlock("for.body")
done := b.newBlock("for.done") // target of 'break'
loop := body // target of back-edge
if s.Cond != nil {
loop = b.newBlock("for.loop")
}
cont := loop // target of 'continue'
if s.Post != nil {
cont = b.newBlock("for.post")
}
if label != nil {
label._break = done
label._continue = cont
}
b.jump(loop)
b.current = loop
if loop != body {
b.add(s.Cond)
b.ifelse(body, done)
b.current = body
}
b.targets = &targets{
tail: b.targets,
_break: done,
_continue: cont,
}
b.stmt(s.Body)
b.targets = b.targets.tail
b.jump(cont)
if s.Post != nil {
b.current = cont
b.stmt(s.Post)
b.jump(loop) // back-edge
}
b.current = done
}
func (b *builder) rangeStmt(s *ast.RangeStmt, label *lblock) {
b.add(s.X)
if s.Key != nil {
b.add(s.Key)
}
if s.Value != nil {
b.add(s.Value)
}
// ...
// loop: (target of continue)
// if ... goto body else done
// body:
// ...
// jump loop
// done: (target of break)
loop := b.newBlock("range.loop")
b.jump(loop)
b.current = loop
body := b.newBlock("range.body")
done := b.newBlock("range.done")
b.ifelse(body, done)
b.current = body
if label != nil {
label._break = done
label._continue = loop
}
b.targets = &targets{
tail: b.targets,
_break: done,
_continue: loop,
}
b.stmt(s.Body)
b.targets = b.targets.tail
b.jump(loop) // back-edge
b.current = done
}
// -------- helpers --------
// Destinations associated with unlabeled for/switch/select stmts.
// We push/pop one of these as we enter/leave each construct and for
// each BranchStmt we scan for the innermost target of the right type.
//
type targets struct {
tail *targets // rest of stack
_break *Block
_continue *Block
_fallthrough *Block
}
// Destinations associated with a labeled block.
// We populate these as labels are encountered in forward gotos or
// labeled statements.
//
type lblock struct {
_goto *Block
_break *Block
_continue *Block
}
// labeledBlock returns the branch target associated with the
// specified label, creating it if needed.
//
func (b *builder) labeledBlock(label *ast.Ident) *lblock {
lb := b.lblocks[label.Obj]
if lb == nil {
lb = &lblock{_goto: b.newBlock(label.Name)}
if b.lblocks == nil {
b.lblocks = make(map[*ast.Object]*lblock)
}
b.lblocks[label.Obj] = lb
}
return lb
}
// newBlock appends a new unconnected basic block to b.cfg's block
// slice and returns it.
// It does not automatically become the current block.
// comment is an optional string for more readable debugging output.
func (b *builder) newBlock(comment string) *Block {
g := b.cfg
block := &Block{
Index: int32(len(g.Blocks)),
comment: comment,
}
block.Succs = block.succs2[:0]
g.Blocks = append(g.Blocks, block)
return block
}
func (b *builder) add(n ast.Node) {
b.current.Nodes = append(b.current.Nodes, n)
}
// jump adds an edge from the current block to the target block,
// and sets b.current to nil.
func (b *builder) jump(target *Block) {
b.current.Succs = append(b.current.Succs, target)
b.current = nil
}
// ifelse emits edges from the current block to the t and f blocks,
// and sets b.current to nil.
func (b *builder) ifelse(t, f *Block) {
b.current.Succs = append(b.current.Succs, t, f)
b.current = nil
}