Current File : //usr/local/go/src/cmd/vendor/golang.org/x/tools/go/analysis/passes/shift/dead.go
// 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.
package shift
// Simplified dead code detector.
// Used for skipping shift checks on unreachable arch-specific code.
import (
"go/ast"
"go/constant"
"go/types"
)
// updateDead puts unreachable "if" and "case" nodes into dead.
func updateDead(info *types.Info, dead map[ast.Node]bool, node ast.Node) {
if dead[node] {
// The node is already marked as dead.
return
}
// setDead marks the node and all the children as dead.
setDead := func(n ast.Node) {
ast.Inspect(n, func(node ast.Node) bool {
if node != nil {
dead[node] = true
}
return true
})
}
switch stmt := node.(type) {
case *ast.IfStmt:
// "if" branch is dead if its condition evaluates
// to constant false.
v := info.Types[stmt.Cond].Value
if v == nil {
return
}
if !constant.BoolVal(v) {
setDead(stmt.Body)
return
}
if stmt.Else != nil {
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 := info.Types[expr].Value
if v == nil || v.Kind() != constant.Bool || constant.BoolVal(v) {
continue BodyLoopBool
}
}
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 := info.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 := info.Types[expr].Value
if v == nil {
continue BodyLoopInt
}
n, ok := constant.Uint64Val(v)
if !ok || tagN == n {
continue BodyLoopInt
}
}
setDead(cc)
}
}
}
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