Current File : //usr/local/go/src/cmd/compile/internal/deadcode/deadcode.go
// Copyright 2009 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 deadcode
import (
"go/constant"
"cmd/compile/internal/base"
"cmd/compile/internal/ir"
)
func Func(fn *ir.Func) {
stmts(&fn.Body)
if len(fn.Body) == 0 {
return
}
for _, n := range fn.Body {
if len(n.Init()) > 0 {
return
}
switch n.Op() {
case ir.OIF:
n := n.(*ir.IfStmt)
if !ir.IsConst(n.Cond, constant.Bool) || len(n.Body) > 0 || len(n.Else) > 0 {
return
}
case ir.OFOR:
n := n.(*ir.ForStmt)
if !ir.IsConst(n.Cond, constant.Bool) || ir.BoolVal(n.Cond) {
return
}
default:
return
}
}
fn.Body = []ir.Node{ir.NewBlockStmt(base.Pos, nil)}
}
func stmts(nn *ir.Nodes) {
var lastLabel = -1
for i, n := range *nn {
if n != nil && n.Op() == ir.OLABEL {
lastLabel = i
}
}
for i, n := range *nn {
// Cut is set to true when all nodes after i'th position
// should be removed.
// In other words, it marks whole slice "tail" as dead.
cut := false
if n == nil {
continue
}
if n.Op() == ir.OIF {
n := n.(*ir.IfStmt)
n.Cond = expr(n.Cond)
if ir.IsConst(n.Cond, constant.Bool) {
var body ir.Nodes
if ir.BoolVal(n.Cond) {
n.Else = ir.Nodes{}
body = n.Body
} else {
n.Body = ir.Nodes{}
body = n.Else
}
// If "then" or "else" branch ends with panic or return statement,
// it is safe to remove all statements after this node.
// isterminating is not used to avoid goto-related complications.
// We must be careful not to deadcode-remove labels, as they
// might be the target of a goto. See issue 28616.
if body := body; len(body) != 0 {
switch body[(len(body) - 1)].Op() {
case ir.ORETURN, ir.OTAILCALL, ir.OPANIC:
if i > lastLabel {
cut = true
}
}
}
}
}
if len(n.Init()) != 0 {
stmts(n.(ir.InitNode).PtrInit())
}
switch n.Op() {
case ir.OBLOCK:
n := n.(*ir.BlockStmt)
stmts(&n.List)
case ir.OFOR:
n := n.(*ir.ForStmt)
stmts(&n.Body)
case ir.OIF:
n := n.(*ir.IfStmt)
stmts(&n.Body)
stmts(&n.Else)
case ir.ORANGE:
n := n.(*ir.RangeStmt)
stmts(&n.Body)
case ir.OSELECT:
n := n.(*ir.SelectStmt)
for _, cas := range n.Cases {
stmts(&cas.Body)
}
case ir.OSWITCH:
n := n.(*ir.SwitchStmt)
for _, cas := range n.Cases {
stmts(&cas.Body)
}
}
if cut {
*nn = (*nn)[:i+1]
break
}
}
}
func expr(n ir.Node) ir.Node {
// Perform dead-code elimination on short-circuited boolean
// expressions involving constants with the intent of
// producing a constant 'if' condition.
switch n.Op() {
case ir.OANDAND:
n := n.(*ir.LogicalExpr)
n.X = expr(n.X)
n.Y = expr(n.Y)
if ir.IsConst(n.X, constant.Bool) {
if ir.BoolVal(n.X) {
return n.Y // true && x => x
} else {
return n.X // false && x => false
}
}
case ir.OOROR:
n := n.(*ir.LogicalExpr)
n.X = expr(n.X)
n.Y = expr(n.Y)
if ir.IsConst(n.X, constant.Bool) {
if ir.BoolVal(n.X) {
return n.X // true || x => true
} else {
return n.Y // false || x => x
}
}
}
return n
}
Mini Shell