Current File : //usr/local/go/src/cmd/compile/internal/walk/switch.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 walk
import (
"go/constant"
"go/token"
"sort"
"cmd/compile/internal/base"
"cmd/compile/internal/ir"
"cmd/compile/internal/typecheck"
"cmd/compile/internal/types"
"cmd/internal/src"
)
// walkSwitch walks a switch statement.
func walkSwitch(sw *ir.SwitchStmt) {
// Guard against double walk, see #25776.
if sw.Walked() {
return // Was fatal, but eliminating every possible source of double-walking is hard
}
sw.SetWalked(true)
if sw.Tag != nil && sw.Tag.Op() == ir.OTYPESW {
walkSwitchType(sw)
} else {
walkSwitchExpr(sw)
}
}
// walkSwitchExpr generates an AST implementing sw. sw is an
// expression switch.
func walkSwitchExpr(sw *ir.SwitchStmt) {
lno := ir.SetPos(sw)
cond := sw.Tag
sw.Tag = nil
// convert switch {...} to switch true {...}
if cond == nil {
cond = ir.NewBool(true)
cond = typecheck.Expr(cond)
cond = typecheck.DefaultLit(cond, nil)
}
// Given "switch string(byteslice)",
// with all cases being side-effect free,
// use a zero-cost alias of the byte slice.
// Do this before calling walkExpr on cond,
// because walkExpr will lower the string
// conversion into a runtime call.
// See issue 24937 for more discussion.
if cond.Op() == ir.OBYTES2STR && allCaseExprsAreSideEffectFree(sw) {
cond := cond.(*ir.ConvExpr)
cond.SetOp(ir.OBYTES2STRTMP)
}
cond = walkExpr(cond, sw.PtrInit())
if cond.Op() != ir.OLITERAL && cond.Op() != ir.ONIL {
cond = copyExpr(cond, cond.Type(), &sw.Compiled)
}
base.Pos = lno
s := exprSwitch{
exprname: cond,
}
var defaultGoto ir.Node
var body ir.Nodes
for _, ncase := range sw.Cases {
label := typecheck.AutoLabel(".s")
jmp := ir.NewBranchStmt(ncase.Pos(), ir.OGOTO, label)
// Process case dispatch.
if len(ncase.List) == 0 {
if defaultGoto != nil {
base.Fatalf("duplicate default case not detected during typechecking")
}
defaultGoto = jmp
}
for _, n1 := range ncase.List {
s.Add(ncase.Pos(), n1, jmp)
}
// Process body.
body.Append(ir.NewLabelStmt(ncase.Pos(), label))
body.Append(ncase.Body...)
if fall, pos := endsInFallthrough(ncase.Body); !fall {
br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
br.SetPos(pos)
body.Append(br)
}
}
sw.Cases = nil
if defaultGoto == nil {
br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
br.SetPos(br.Pos().WithNotStmt())
defaultGoto = br
}
s.Emit(&sw.Compiled)
sw.Compiled.Append(defaultGoto)
sw.Compiled.Append(body.Take()...)
walkStmtList(sw.Compiled)
}
// An exprSwitch walks an expression switch.
type exprSwitch struct {
exprname ir.Node // value being switched on
done ir.Nodes
clauses []exprClause
}
type exprClause struct {
pos src.XPos
lo, hi ir.Node
jmp ir.Node
}
func (s *exprSwitch) Add(pos src.XPos, expr, jmp ir.Node) {
c := exprClause{pos: pos, lo: expr, hi: expr, jmp: jmp}
if types.IsOrdered[s.exprname.Type().Kind()] && expr.Op() == ir.OLITERAL {
s.clauses = append(s.clauses, c)
return
}
s.flush()
s.clauses = append(s.clauses, c)
s.flush()
}
func (s *exprSwitch) Emit(out *ir.Nodes) {
s.flush()
out.Append(s.done.Take()...)
}
func (s *exprSwitch) flush() {
cc := s.clauses
s.clauses = nil
if len(cc) == 0 {
return
}
// Caution: If len(cc) == 1, then cc[0] might not an OLITERAL.
// The code below is structured to implicitly handle this case
// (e.g., sort.Slice doesn't need to invoke the less function
// when there's only a single slice element).
if s.exprname.Type().IsString() && len(cc) >= 2 {
// Sort strings by length and then by value. It is
// much cheaper to compare lengths than values, and
// all we need here is consistency. We respect this
// sorting below.
sort.Slice(cc, func(i, j int) bool {
si := ir.StringVal(cc[i].lo)
sj := ir.StringVal(cc[j].lo)
if len(si) != len(sj) {
return len(si) < len(sj)
}
return si < sj
})
// runLen returns the string length associated with a
// particular run of exprClauses.
runLen := func(run []exprClause) int64 { return int64(len(ir.StringVal(run[0].lo))) }
// Collapse runs of consecutive strings with the same length.
var runs [][]exprClause
start := 0
for i := 1; i < len(cc); i++ {
if runLen(cc[start:]) != runLen(cc[i:]) {
runs = append(runs, cc[start:i])
start = i
}
}
runs = append(runs, cc[start:])
// Perform two-level binary search.
binarySearch(len(runs), &s.done,
func(i int) ir.Node {
return ir.NewBinaryExpr(base.Pos, ir.OLE, ir.NewUnaryExpr(base.Pos, ir.OLEN, s.exprname), ir.NewInt(runLen(runs[i-1])))
},
func(i int, nif *ir.IfStmt) {
run := runs[i]
nif.Cond = ir.NewBinaryExpr(base.Pos, ir.OEQ, ir.NewUnaryExpr(base.Pos, ir.OLEN, s.exprname), ir.NewInt(runLen(run)))
s.search(run, &nif.Body)
},
)
return
}
sort.Slice(cc, func(i, j int) bool {
return constant.Compare(cc[i].lo.Val(), token.LSS, cc[j].lo.Val())
})
// Merge consecutive integer cases.
if s.exprname.Type().IsInteger() {
consecutive := func(last, next constant.Value) bool {
delta := constant.BinaryOp(next, token.SUB, last)
return constant.Compare(delta, token.EQL, constant.MakeInt64(1))
}
merged := cc[:1]
for _, c := range cc[1:] {
last := &merged[len(merged)-1]
if last.jmp == c.jmp && consecutive(last.hi.Val(), c.lo.Val()) {
last.hi = c.lo
} else {
merged = append(merged, c)
}
}
cc = merged
}
s.search(cc, &s.done)
}
func (s *exprSwitch) search(cc []exprClause, out *ir.Nodes) {
binarySearch(len(cc), out,
func(i int) ir.Node {
return ir.NewBinaryExpr(base.Pos, ir.OLE, s.exprname, cc[i-1].hi)
},
func(i int, nif *ir.IfStmt) {
c := &cc[i]
nif.Cond = c.test(s.exprname)
nif.Body = []ir.Node{c.jmp}
},
)
}
func (c *exprClause) test(exprname ir.Node) ir.Node {
// Integer range.
if c.hi != c.lo {
low := ir.NewBinaryExpr(c.pos, ir.OGE, exprname, c.lo)
high := ir.NewBinaryExpr(c.pos, ir.OLE, exprname, c.hi)
return ir.NewLogicalExpr(c.pos, ir.OANDAND, low, high)
}
// Optimize "switch true { ...}" and "switch false { ... }".
if ir.IsConst(exprname, constant.Bool) && !c.lo.Type().IsInterface() {
if ir.BoolVal(exprname) {
return c.lo
} else {
return ir.NewUnaryExpr(c.pos, ir.ONOT, c.lo)
}
}
return ir.NewBinaryExpr(c.pos, ir.OEQ, exprname, c.lo)
}
func allCaseExprsAreSideEffectFree(sw *ir.SwitchStmt) bool {
// In theory, we could be more aggressive, allowing any
// side-effect-free expressions in cases, but it's a bit
// tricky because some of that information is unavailable due
// to the introduction of temporaries during order.
// Restricting to constants is simple and probably powerful
// enough.
for _, ncase := range sw.Cases {
for _, v := range ncase.List {
if v.Op() != ir.OLITERAL {
return false
}
}
}
return true
}
// endsInFallthrough reports whether stmts ends with a "fallthrough" statement.
func endsInFallthrough(stmts []ir.Node) (bool, src.XPos) {
// Search backwards for the index of the fallthrough
// statement. Do not assume it'll be in the last
// position, since in some cases (e.g. when the statement
// list contains autotmp_ variables), one or more OVARKILL
// nodes will be at the end of the list.
i := len(stmts) - 1
for i >= 0 && stmts[i].Op() == ir.OVARKILL {
i--
}
if i < 0 {
return false, src.NoXPos
}
return stmts[i].Op() == ir.OFALL, stmts[i].Pos()
}
// walkSwitchType generates an AST that implements sw, where sw is a
// type switch.
func walkSwitchType(sw *ir.SwitchStmt) {
var s typeSwitch
s.facename = sw.Tag.(*ir.TypeSwitchGuard).X
sw.Tag = nil
s.facename = walkExpr(s.facename, sw.PtrInit())
s.facename = copyExpr(s.facename, s.facename.Type(), &sw.Compiled)
s.okname = typecheck.Temp(types.Types[types.TBOOL])
// Get interface descriptor word.
// For empty interfaces this will be the type.
// For non-empty interfaces this will be the itab.
itab := ir.NewUnaryExpr(base.Pos, ir.OITAB, s.facename)
// For empty interfaces, do:
// if e._type == nil {
// do nil case if it exists, otherwise default
// }
// h := e._type.hash
// Use a similar strategy for non-empty interfaces.
ifNil := ir.NewIfStmt(base.Pos, nil, nil, nil)
ifNil.Cond = ir.NewBinaryExpr(base.Pos, ir.OEQ, itab, typecheck.NodNil())
base.Pos = base.Pos.WithNotStmt() // disable statement marks after the first check.
ifNil.Cond = typecheck.Expr(ifNil.Cond)
ifNil.Cond = typecheck.DefaultLit(ifNil.Cond, nil)
// ifNil.Nbody assigned at end.
sw.Compiled.Append(ifNil)
// Load hash from type or itab.
dotHash := typeHashFieldOf(base.Pos, itab)
s.hashname = copyExpr(dotHash, dotHash.Type(), &sw.Compiled)
br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)
var defaultGoto, nilGoto ir.Node
var body ir.Nodes
for _, ncase := range sw.Cases {
caseVar := ncase.Var
// For single-type cases with an interface type,
// we initialize the case variable as part of the type assertion.
// In other cases, we initialize it in the body.
var singleType *types.Type
if len(ncase.List) == 1 && ncase.List[0].Op() == ir.OTYPE {
singleType = ncase.List[0].Type()
}
caseVarInitialized := false
label := typecheck.AutoLabel(".s")
jmp := ir.NewBranchStmt(ncase.Pos(), ir.OGOTO, label)
if len(ncase.List) == 0 { // default:
if defaultGoto != nil {
base.Fatalf("duplicate default case not detected during typechecking")
}
defaultGoto = jmp
}
for _, n1 := range ncase.List {
if ir.IsNil(n1) { // case nil:
if nilGoto != nil {
base.Fatalf("duplicate nil case not detected during typechecking")
}
nilGoto = jmp
continue
}
if singleType != nil && singleType.IsInterface() {
s.Add(ncase.Pos(), n1.Type(), caseVar, jmp)
caseVarInitialized = true
} else {
s.Add(ncase.Pos(), n1.Type(), nil, jmp)
}
}
body.Append(ir.NewLabelStmt(ncase.Pos(), label))
if caseVar != nil && !caseVarInitialized {
val := s.facename
if singleType != nil {
// We have a single concrete type. Extract the data.
if singleType.IsInterface() {
base.Fatalf("singleType interface should have been handled in Add")
}
val = ifaceData(ncase.Pos(), s.facename, singleType)
}
l := []ir.Node{
ir.NewDecl(ncase.Pos(), ir.ODCL, caseVar),
ir.NewAssignStmt(ncase.Pos(), caseVar, val),
}
typecheck.Stmts(l)
body.Append(l...)
}
body.Append(ncase.Body...)
body.Append(br)
}
sw.Cases = nil
if defaultGoto == nil {
defaultGoto = br
}
if nilGoto == nil {
nilGoto = defaultGoto
}
ifNil.Body = []ir.Node{nilGoto}
s.Emit(&sw.Compiled)
sw.Compiled.Append(defaultGoto)
sw.Compiled.Append(body.Take()...)
walkStmtList(sw.Compiled)
}
// typeHashFieldOf returns an expression to select the type hash field
// from an interface's descriptor word (whether a *runtime._type or
// *runtime.itab pointer).
func typeHashFieldOf(pos src.XPos, itab *ir.UnaryExpr) *ir.SelectorExpr {
if itab.Op() != ir.OITAB {
base.Fatalf("expected OITAB, got %v", itab.Op())
}
var hashField *types.Field
if itab.X.Type().IsEmptyInterface() {
// runtime._type's hash field
if rtypeHashField == nil {
rtypeHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
}
hashField = rtypeHashField
} else {
// runtime.itab's hash field
if itabHashField == nil {
itabHashField = runtimeField("hash", int64(2*types.PtrSize), types.Types[types.TUINT32])
}
hashField = itabHashField
}
return boundedDotPtr(pos, itab, hashField)
}
var rtypeHashField, itabHashField *types.Field
// A typeSwitch walks a type switch.
type typeSwitch struct {
// Temporary variables (i.e., ONAMEs) used by type switch dispatch logic:
facename ir.Node // value being type-switched on
hashname ir.Node // type hash of the value being type-switched on
okname ir.Node // boolean used for comma-ok type assertions
done ir.Nodes
clauses []typeClause
}
type typeClause struct {
hash uint32
body ir.Nodes
}
func (s *typeSwitch) Add(pos src.XPos, typ *types.Type, caseVar *ir.Name, jmp ir.Node) {
var body ir.Nodes
if caseVar != nil {
l := []ir.Node{
ir.NewDecl(pos, ir.ODCL, caseVar),
ir.NewAssignStmt(pos, caseVar, nil),
}
typecheck.Stmts(l)
body.Append(l...)
} else {
caseVar = ir.BlankNode.(*ir.Name)
}
// cv, ok = iface.(type)
as := ir.NewAssignListStmt(pos, ir.OAS2, nil, nil)
as.Lhs = []ir.Node{caseVar, s.okname} // cv, ok =
dot := ir.NewTypeAssertExpr(pos, s.facename, nil)
dot.SetType(typ) // iface.(type)
as.Rhs = []ir.Node{dot}
appendWalkStmt(&body, as)
// if ok { goto label }
nif := ir.NewIfStmt(pos, nil, nil, nil)
nif.Cond = s.okname
nif.Body = []ir.Node{jmp}
body.Append(nif)
if !typ.IsInterface() {
s.clauses = append(s.clauses, typeClause{
hash: types.TypeHash(typ),
body: body,
})
return
}
s.flush()
s.done.Append(body.Take()...)
}
func (s *typeSwitch) Emit(out *ir.Nodes) {
s.flush()
out.Append(s.done.Take()...)
}
func (s *typeSwitch) flush() {
cc := s.clauses
s.clauses = nil
if len(cc) == 0 {
return
}
sort.Slice(cc, func(i, j int) bool { return cc[i].hash < cc[j].hash })
// Combine adjacent cases with the same hash.
merged := cc[:1]
for _, c := range cc[1:] {
last := &merged[len(merged)-1]
if last.hash == c.hash {
last.body.Append(c.body.Take()...)
} else {
merged = append(merged, c)
}
}
cc = merged
binarySearch(len(cc), &s.done,
func(i int) ir.Node {
return ir.NewBinaryExpr(base.Pos, ir.OLE, s.hashname, ir.NewInt(int64(cc[i-1].hash)))
},
func(i int, nif *ir.IfStmt) {
// TODO(mdempsky): Omit hash equality check if
// there's only one type.
c := cc[i]
nif.Cond = ir.NewBinaryExpr(base.Pos, ir.OEQ, s.hashname, ir.NewInt(int64(c.hash)))
nif.Body.Append(c.body.Take()...)
},
)
}
// binarySearch constructs a binary search tree for handling n cases,
// and appends it to out. It's used for efficiently implementing
// switch statements.
//
// less(i) should return a boolean expression. If it evaluates true,
// then cases before i will be tested; otherwise, cases i and later.
//
// leaf(i, nif) should setup nif (an OIF node) to test case i. In
// particular, it should set nif.Left and nif.Nbody.
func binarySearch(n int, out *ir.Nodes, less func(i int) ir.Node, leaf func(i int, nif *ir.IfStmt)) {
const binarySearchMin = 4 // minimum number of cases for binary search
var do func(lo, hi int, out *ir.Nodes)
do = func(lo, hi int, out *ir.Nodes) {
n := hi - lo
if n < binarySearchMin {
for i := lo; i < hi; i++ {
nif := ir.NewIfStmt(base.Pos, nil, nil, nil)
leaf(i, nif)
base.Pos = base.Pos.WithNotStmt()
nif.Cond = typecheck.Expr(nif.Cond)
nif.Cond = typecheck.DefaultLit(nif.Cond, nil)
out.Append(nif)
out = &nif.Else
}
return
}
half := lo + n/2
nif := ir.NewIfStmt(base.Pos, nil, nil, nil)
nif.Cond = less(half)
base.Pos = base.Pos.WithNotStmt()
nif.Cond = typecheck.Expr(nif.Cond)
nif.Cond = typecheck.DefaultLit(nif.Cond, nil)
do(lo, half, &nif.Body)
do(half, hi, &nif.Else)
out.Append(nif)
}
do(0, n, out)
}
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