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)
}