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// Copyright 2017 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.
//
// Simplified dead code detector. Used for skipping certain checks
// on unreachable code (for instance, shift checks on arch-specific code).
package main
import (
"go/ast"
"go/constant"
)
// updateDead puts unreachable "if" and "case" nodes into f.dead.
func (f *File) updateDead(node ast.Node) {
if f.dead[node] {
// The node is already marked as dead.
return
}
switch stmt := node.(type) {
case *ast.IfStmt:
// "if" branch is dead if its condition evaluates
// to constant false.
v := f.pkg.types[stmt.Cond].Value
if v == nil {
return
}
if !constant.BoolVal(v) {
f.setDead(stmt.Body)
return
}
f.setDead(stmt.Else)
case *ast.SwitchStmt:
// Case clause with empty switch tag is dead if it evaluates
// to constant false.
if stmt.Tag == nil {
BodyLoopBool:
for _, stmt := range stmt.Body.List {
cc := stmt.(*ast.CaseClause)
if cc.List == nil {
// Skip default case.
continue
}
for _, expr := range cc.List {
v := f.pkg.types[expr].Value
if v == nil || v.Kind() != constant.Bool || constant.BoolVal(v) {
continue BodyLoopBool
}
}
f.setDead(cc)
}
return
}
// Case clause is dead if its constant value doesn't match
// the constant value from the switch tag.
// TODO: This handles integer comparisons only.
v := f.pkg.types[stmt.Tag].Value
if v == nil || v.Kind() != constant.Int {
return
}
tagN, ok := constant.Uint64Val(v)
if !ok {
return
}
BodyLoopInt:
for _, x := range stmt.Body.List {
cc := x.(*ast.CaseClause)
if cc.List == nil {
// Skip default case.
continue
}
for _, expr := range cc.List {
v := f.pkg.types[expr].Value
if v == nil {
continue BodyLoopInt
}
n, ok := constant.Uint64Val(v)
if !ok || tagN == n {
continue BodyLoopInt
}
}
f.setDead(cc)
}
}
}
// setDead marks the node and all the children as dead.
func (f *File) setDead(node ast.Node) {
dv := deadVisitor{
f: f,
}
ast.Walk(dv, node)
}
type deadVisitor struct {
f *File
}
func (dv deadVisitor) Visit(node ast.Node) ast.Visitor {
if node == nil {
return nil
}
dv.f.dead[node] = true
return dv
}