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 (
	"fmt"
	"go/constant"
	"go/token"
	"math/bits"
	"sort"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/objw"
	"cmd/compile/internal/reflectdata"
	"cmd/compile/internal/rttype"
	"cmd/compile/internal/ssagen"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
	"cmd/internal/obj"
	"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(base.Pos, 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{
		pos:      lno,
		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 i, n1 := range ncase.List {
			var rtype ir.Node
			if i < len(ncase.RTypes) {
				rtype = ncase.RTypes[i]
			}
			s.Add(ncase.Pos(), n1, rtype, 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 {
	pos      src.XPos
	exprname ir.Node // value being switched on

	done    ir.Nodes
	clauses []exprClause
}

type exprClause struct {
	pos    src.XPos
	lo, hi ir.Node
	rtype  ir.Node // *runtime._type for OEQ node
	jmp    ir.Node
}

func (s *exprSwitch) Add(pos src.XPos, expr, rtype, jmp ir.Node) {
	c := exprClause{pos: pos, lo: expr, hi: expr, rtype: rtype, 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:])

		// We have strings of more than one length. Generate an
		// outer switch which switches on the length of the string
		// and an inner switch in each case which resolves all the
		// strings of the same length. The code looks something like this:

		// goto outerLabel
		// len5:
		//   ... search among length 5 strings ...
		//   goto endLabel
		// len8:
		//   ... search among length 8 strings ...
		//   goto endLabel
		// ... other lengths ...
		// outerLabel:
		// switch len(s) {
		//   case 5: goto len5
		//   case 8: goto len8
		//   ... other lengths ...
		// }
		// endLabel:

		outerLabel := typecheck.AutoLabel(".s")
		endLabel := typecheck.AutoLabel(".s")

		// Jump around all the individual switches for each length.
		s.done.Append(ir.NewBranchStmt(s.pos, ir.OGOTO, outerLabel))

		var outer exprSwitch
		outer.exprname = ir.NewUnaryExpr(s.pos, ir.OLEN, s.exprname)
		outer.exprname.SetType(types.Types[types.TINT])

		for _, run := range runs {
			// Target label to jump to when we match this length.
			label := typecheck.AutoLabel(".s")

			// Search within this run of same-length strings.
			pos := run[0].pos
			s.done.Append(ir.NewLabelStmt(pos, label))
			stringSearch(s.exprname, run, &s.done)
			s.done.Append(ir.NewBranchStmt(pos, ir.OGOTO, endLabel))

			// Add length case to outer switch.
			cas := ir.NewInt(pos, runLen(run))
			jmp := ir.NewBranchStmt(pos, ir.OGOTO, label)
			outer.Add(pos, cas, nil, jmp)
		}
		s.done.Append(ir.NewLabelStmt(s.pos, outerLabel))
		outer.Emit(&s.done)
		s.done.Append(ir.NewLabelStmt(s.pos, endLabel))
		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) {
	if s.tryJumpTable(cc, out) {
		return
	}
	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}
		},
	)
}

// Try to implement the clauses with a jump table. Returns true if successful.
func (s *exprSwitch) tryJumpTable(cc []exprClause, out *ir.Nodes) bool {
	const minCases = 8   // have at least minCases cases in the switch
	const minDensity = 4 // use at least 1 out of every minDensity entries

	if base.Flag.N != 0 || !ssagen.Arch.LinkArch.CanJumpTable || base.Ctxt.Retpoline {
		return false
	}
	if len(cc) < minCases {
		return false // not enough cases for it to be worth it
	}
	if cc[0].lo.Val().Kind() != constant.Int {
		return false // e.g. float
	}
	if s.exprname.Type().Size() > int64(types.PtrSize) {
		return false // 64-bit switches on 32-bit archs
	}
	min := cc[0].lo.Val()
	max := cc[len(cc)-1].hi.Val()
	width := constant.BinaryOp(constant.BinaryOp(max, token.SUB, min), token.ADD, constant.MakeInt64(1))
	limit := constant.MakeInt64(int64(len(cc)) * minDensity)
	if constant.Compare(width, token.GTR, limit) {
		// We disable jump tables if we use less than a minimum fraction of the entries.
		// i.e. for switch x {case 0: case 1000: case 2000:} we don't want to use a jump table.
		return false
	}
	jt := ir.NewJumpTableStmt(base.Pos, s.exprname)
	for _, c := range cc {
		jmp := c.jmp.(*ir.BranchStmt)
		if jmp.Op() != ir.OGOTO || jmp.Label == nil {
			panic("bad switch case body")
		}
		for i := c.lo.Val(); constant.Compare(i, token.LEQ, c.hi.Val()); i = constant.BinaryOp(i, token.ADD, constant.MakeInt64(1)) {
			jt.Cases = append(jt.Cases, i)
			jt.Targets = append(jt.Targets, jmp.Label)
		}
	}
	out.Append(jt)
	return true
}

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)
		}
	}

	n := ir.NewBinaryExpr(c.pos, ir.OEQ, exprname, c.lo)
	n.RType = c.rtype
	return n
}

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) {
	if len(stmts) == 0 {
		return false, src.NoXPos
	}
	i := len(stmts) - 1
	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.srcName = sw.Tag.(*ir.TypeSwitchGuard).X
	s.srcName = walkExpr(s.srcName, sw.PtrInit())
	s.srcName = copyExpr(s.srcName, s.srcName.Type(), &sw.Compiled)
	s.okName = typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TBOOL])
	s.itabName = typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TUINT8].PtrTo())

	// Get interface descriptor word.
	// For empty interfaces this will be the type.
	// For non-empty interfaces this will be the itab.
	srcItab := ir.NewUnaryExpr(base.Pos, ir.OITAB, s.srcName)
	srcData := ir.NewUnaryExpr(base.Pos, ir.OIDATA, s.srcName)
	srcData.SetType(types.Types[types.TUINT8].PtrTo())
	srcData.SetTypecheck(1)

	// 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, srcItab, 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 later.
	sw.Compiled.Append(ifNil)

	// Load hash from type or itab.
	dotHash := typeHashFieldOf(base.Pos, srcItab)
	s.hashName = copyExpr(dotHash, dotHash.Type(), &sw.Compiled)

	// Make a label for each case body.
	labels := make([]*types.Sym, len(sw.Cases))
	for i := range sw.Cases {
		labels[i] = typecheck.AutoLabel(".s")
	}

	// "jump" to execute if no case matches.
	br := ir.NewBranchStmt(base.Pos, ir.OBREAK, nil)

	// Assemble a list of all the types we're looking for.
	// This pass flattens the case lists, as well as handles
	// some unusual cases, like default and nil cases.
	type oneCase struct {
		pos src.XPos
		jmp ir.Node // jump to body of selected case

		// The case we're matching. Normally the type we're looking for
		// is typ.Type(), but when typ is ODYNAMICTYPE the actual type
		// we're looking for is not a compile-time constant (typ.Type()
		// will be its shape).
		typ ir.Node
	}
	var cases []oneCase
	var defaultGoto, nilGoto ir.Node
	for i, ncase := range sw.Cases {
		jmp := ir.NewBranchStmt(ncase.Pos(), ir.OGOTO, labels[i])
		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 n1.Op() == ir.ODYNAMICTYPE {
				// Convert dynamic to static, if the dynamic is actually static.
				// TODO: why isn't this OTYPE to begin with?
				dt := n1.(*ir.DynamicType)
				if dt.RType != nil && dt.RType.Op() == ir.OADDR {
					addr := dt.RType.(*ir.AddrExpr)
					if addr.X.Op() == ir.OLINKSYMOFFSET {
						n1 = ir.TypeNode(n1.Type())
					}
				}
				if dt.ITab != nil && dt.ITab.Op() == ir.OADDR {
					addr := dt.ITab.(*ir.AddrExpr)
					if addr.X.Op() == ir.OLINKSYMOFFSET {
						n1 = ir.TypeNode(n1.Type())
					}
				}
			}
			cases = append(cases, oneCase{
				pos: ncase.Pos(),
				typ: n1,
				jmp: jmp,
			})
		}
	}
	if defaultGoto == nil {
		defaultGoto = br
	}
	if nilGoto == nil {
		nilGoto = defaultGoto
	}
	ifNil.Body = []ir.Node{nilGoto}

	// Now go through the list of cases, processing groups as we find them.
	var concreteCases []oneCase
	var interfaceCases []oneCase
	flush := func() {
		// Process all the concrete types first. Because we handle shadowing
		// below, it is correct to do all the concrete types before all of
		// the interface types.
		// The concrete cases can all be handled without a runtime call.
		if len(concreteCases) > 0 {
			var clauses []typeClause
			for _, c := range concreteCases {
				as := ir.NewAssignListStmt(c.pos, ir.OAS2,
					[]ir.Node{ir.BlankNode, s.okName},                               // _, ok =
					[]ir.Node{ir.NewTypeAssertExpr(c.pos, s.srcName, c.typ.Type())}) // iface.(type)
				nif := ir.NewIfStmt(c.pos, s.okName, []ir.Node{c.jmp}, nil)
				clauses = append(clauses, typeClause{
					hash: types.TypeHash(c.typ.Type()),
					body: []ir.Node{typecheck.Stmt(as), typecheck.Stmt(nif)},
				})
			}
			s.flush(clauses, &sw.Compiled)
			concreteCases = concreteCases[:0]
		}

		// The "any" case, if it exists, must be the last interface case, because
		// it would shadow all subsequent cases. Strip it off here so the runtime
		// call only needs to handle non-empty interfaces.
		var anyGoto ir.Node
		if len(interfaceCases) > 0 && interfaceCases[len(interfaceCases)-1].typ.Type().IsEmptyInterface() {
			anyGoto = interfaceCases[len(interfaceCases)-1].jmp
			interfaceCases = interfaceCases[:len(interfaceCases)-1]
		}

		// Next, process all the interface types with a single call to the runtime.
		if len(interfaceCases) > 0 {

			// Build an internal/abi.InterfaceSwitch descriptor to pass to the runtime.
			lsym := types.LocalPkg.Lookup(fmt.Sprintf(".interfaceSwitch.%d", interfaceSwitchGen)).LinksymABI(obj.ABI0)
			interfaceSwitchGen++
			c := rttype.NewCursor(lsym, 0, rttype.InterfaceSwitch)
			c.Field("Cache").WritePtr(typecheck.LookupRuntimeVar("emptyInterfaceSwitchCache"))
			c.Field("NCases").WriteInt(int64(len(interfaceCases)))
			array, sizeDelta := c.Field("Cases").ModifyArray(len(interfaceCases))
			for i, c := range interfaceCases {
				array.Elem(i).WritePtr(reflectdata.TypeSym(c.typ.Type()).Linksym())
			}
			objw.Global(lsym, int32(rttype.InterfaceSwitch.Size()+sizeDelta), obj.LOCAL)
			// The GC only needs to see the first pointer in the structure (all the others
			// are to static locations). So the InterfaceSwitch type itself is fine, even
			// though it might not cover the whole array we wrote above.
			lsym.Gotype = reflectdata.TypeLinksym(rttype.InterfaceSwitch)

			// Call runtime to do switch
			// case, itab = runtime.interfaceSwitch(&descriptor, typeof(arg))
			var typeArg ir.Node
			if s.srcName.Type().IsEmptyInterface() {
				typeArg = ir.NewConvExpr(base.Pos, ir.OCONVNOP, types.Types[types.TUINT8].PtrTo(), srcItab)
			} else {
				typeArg = itabType(srcItab)
			}
			caseVar := typecheck.TempAt(base.Pos, ir.CurFunc, types.Types[types.TINT])
			isw := ir.NewInterfaceSwitchStmt(base.Pos, caseVar, s.itabName, typeArg, dotHash, lsym)
			sw.Compiled.Append(isw)

			// Switch on the result of the call (or cache lookup).
			var newCases []*ir.CaseClause
			for i, c := range interfaceCases {
				newCases = append(newCases, &ir.CaseClause{
					List: []ir.Node{ir.NewInt(base.Pos, int64(i))},
					Body: []ir.Node{c.jmp},
				})
			}
			// TODO: add len(newCases) case, mark switch as bounded
			sw2 := ir.NewSwitchStmt(base.Pos, caseVar, newCases)
			sw.Compiled.Append(typecheck.Stmt(sw2))
			interfaceCases = interfaceCases[:0]
		}

		if anyGoto != nil {
			// We've already handled the nil case, so everything
			// that reaches here matches the "any" case.
			sw.Compiled.Append(anyGoto)
		}
	}
caseLoop:
	for _, c := range cases {
		if c.typ.Op() == ir.ODYNAMICTYPE {
			flush() // process all previous cases
			dt := c.typ.(*ir.DynamicType)
			dot := ir.NewDynamicTypeAssertExpr(c.pos, ir.ODYNAMICDOTTYPE, s.srcName, dt.RType)
			dot.ITab = dt.ITab
			dot.SetType(c.typ.Type())
			dot.SetTypecheck(1)

			as := ir.NewAssignListStmt(c.pos, ir.OAS2, nil, nil)
			as.Lhs = []ir.Node{ir.BlankNode, s.okName} // _, ok =
			as.Rhs = []ir.Node{dot}
			typecheck.Stmt(as)

			nif := ir.NewIfStmt(c.pos, s.okName, []ir.Node{c.jmp}, nil)
			sw.Compiled.Append(as, nif)
			continue
		}

		// Check for shadowing (a case that will never fire because
		// a previous case would have always fired first). This check
		// allows us to reorder concrete and interface cases.
		// (TODO: these should be vet failures, maybe?)
		for _, ic := range interfaceCases {
			// An interface type case will shadow all
			// subsequent types that implement that interface.
			if typecheck.Implements(c.typ.Type(), ic.typ.Type()) {
				continue caseLoop
			}
			// Note that we don't need to worry about:
			// 1. Two concrete types shadowing each other. That's
			//    disallowed by the spec.
			// 2. A concrete type shadowing an interface type.
			//    That can never happen, as interface types can
			//    be satisfied by an infinite set of concrete types.
			// The correctness of this step also depends on handling
			// the dynamic type cases separately, as we do above.
		}

		if c.typ.Type().IsInterface() {
			interfaceCases = append(interfaceCases, c)
		} else {
			concreteCases = append(concreteCases, c)
		}
	}
	flush()

	sw.Compiled.Append(defaultGoto) // if none of the cases matched

	// Now generate all the case bodies
	for i, ncase := range sw.Cases {
		sw.Compiled.Append(ir.NewLabelStmt(ncase.Pos(), labels[i]))
		if caseVar := ncase.Var; caseVar != nil {
			val := s.srcName
			if len(ncase.List) == 1 {
				// single type. We have to downcast the input value to the target type.
				if ncase.List[0].Op() == ir.OTYPE { // single compile-time known type
					t := ncase.List[0].Type()
					if t.IsInterface() {
						// This case is an interface. Build case value from input interface.
						// The data word will always be the same, but the itab/type changes.
						if t.IsEmptyInterface() {
							var typ ir.Node
							if s.srcName.Type().IsEmptyInterface() {
								// E->E, nothing to do, type is already correct.
								typ = srcItab
							} else {
								// I->E, load type out of itab
								typ = itabType(srcItab)
								typ.SetPos(ncase.Pos())
							}
							val = ir.NewBinaryExpr(ncase.Pos(), ir.OMAKEFACE, typ, srcData)
						} else {
							// The itab we need was returned by a runtime.interfaceSwitch call.
							val = ir.NewBinaryExpr(ncase.Pos(), ir.OMAKEFACE, s.itabName, srcData)
						}
					} else {
						// This case is a concrete type, just read its value out of the interface.
						val = ifaceData(ncase.Pos(), s.srcName, t)
					}
				} else if ncase.List[0].Op() == ir.ODYNAMICTYPE { // single runtime known type
					dt := ncase.List[0].(*ir.DynamicType)
					x := ir.NewDynamicTypeAssertExpr(ncase.Pos(), ir.ODYNAMICDOTTYPE, val, dt.RType)
					x.ITab = dt.ITab
					val = x
				} else if ir.IsNil(ncase.List[0]) {
				} else {
					base.Fatalf("unhandled type switch case %v", ncase.List[0])
				}
				val.SetType(caseVar.Type())
				val.SetTypecheck(1)
			}
			l := []ir.Node{
				ir.NewDecl(ncase.Pos(), ir.ODCL, caseVar),
				ir.NewAssignStmt(ncase.Pos(), caseVar, val),
			}
			typecheck.Stmts(l)
			sw.Compiled.Append(l...)
		}
		sw.Compiled.Append(ncase.Body...)
		sw.Compiled.Append(br)
	}

	walkStmtList(sw.Compiled)
	sw.Tag = nil
	sw.Cases = nil
}

var interfaceSwitchGen int

// 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", rttype.Type.OffsetOf("Hash"), 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:
	srcName  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
	itabName ir.Node // itab value to use for first word of non-empty interface
}

type typeClause struct {
	hash uint32
	body ir.Nodes
}

func (s *typeSwitch) flush(cc []typeClause, compiled *ir.Nodes) {
	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

	if s.tryJumpTable(cc, compiled) {
		return
	}
	binarySearch(len(cc), compiled,
		func(i int) ir.Node {
			return ir.NewBinaryExpr(base.Pos, ir.OLE, s.hashName, ir.NewInt(base.Pos, 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(base.Pos, int64(c.hash)))
			nif.Body.Append(c.body.Take()...)
		},
	)
}

// Try to implement the clauses with a jump table. Returns true if successful.
func (s *typeSwitch) tryJumpTable(cc []typeClause, out *ir.Nodes) bool {
	const minCases = 5 // have at least minCases cases in the switch
	if base.Flag.N != 0 || !ssagen.Arch.LinkArch.CanJumpTable || base.Ctxt.Retpoline {
		return false
	}
	if len(cc) < minCases {
		return false // not enough cases for it to be worth it
	}
	hashes := make([]uint32, len(cc))
	// b = # of bits to use. Start with the minimum number of
	// bits possible, but try a few larger sizes if needed.
	b0 := bits.Len(uint(len(cc) - 1))
	for b := b0; b < b0+3; b++ {
	pickI:
		for i := 0; i <= 32-b; i++ { // starting bit position
			// Compute the hash we'd get from all the cases,
			// selecting b bits starting at bit i.
			hashes = hashes[:0]
			for _, c := range cc {
				h := c.hash >> i & (1<<b - 1)
				hashes = append(hashes, h)
			}
			// Order by increasing hash.
			sort.Slice(hashes, func(j, k int) bool {
				return hashes[j] < hashes[k]
			})
			for j := 1; j < len(hashes); j++ {
				if hashes[j] == hashes[j-1] {
					// There is a duplicate hash; try a different b/i pair.
					continue pickI
				}
			}

			// All hashes are distinct. Use these values of b and i.
			h := s.hashName
			if i != 0 {
				h = ir.NewBinaryExpr(base.Pos, ir.ORSH, h, ir.NewInt(base.Pos, int64(i)))
			}
			h = ir.NewBinaryExpr(base.Pos, ir.OAND, h, ir.NewInt(base.Pos, int64(1<<b-1)))
			h = typecheck.Expr(h)

			// Build jump table.
			jt := ir.NewJumpTableStmt(base.Pos, h)
			jt.Cases = make([]constant.Value, 1<<b)
			jt.Targets = make([]*types.Sym, 1<<b)
			out.Append(jt)

			// Start with all hashes going to the didn't-match target.
			noMatch := typecheck.AutoLabel(".s")
			for j := 0; j < 1<<b; j++ {
				jt.Cases[j] = constant.MakeInt64(int64(j))
				jt.Targets[j] = noMatch
			}
			// This statement is not reachable, but it will make it obvious that we don't
			// fall through to the first case.
			out.Append(ir.NewBranchStmt(base.Pos, ir.OGOTO, noMatch))

			// Emit each of the actual cases.
			for _, c := range cc {
				h := c.hash >> i & (1<<b - 1)
				label := typecheck.AutoLabel(".s")
				jt.Targets[h] = label
				out.Append(ir.NewLabelStmt(base.Pos, label))
				out.Append(c.body...)
				// We reach here if the hash matches but the type equality test fails.
				out.Append(ir.NewBranchStmt(base.Pos, ir.OGOTO, noMatch))
			}
			// Emit point to go to if type doesn't match any case.
			out.Append(ir.NewLabelStmt(base.Pos, noMatch))
			return true
		}
	}
	// Couldn't find a perfect hash. Fall back to binary search.
	return false
}

// 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.Cond and nif.Body.
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)
}

func stringSearch(expr ir.Node, cc []exprClause, out *ir.Nodes) {
	if len(cc) < 4 {
		// Short list, just do brute force equality checks.
		for _, c := range cc {
			nif := ir.NewIfStmt(base.Pos.WithNotStmt(), typecheck.DefaultLit(typecheck.Expr(c.test(expr)), nil), []ir.Node{c.jmp}, nil)
			out.Append(nif)
			out = &nif.Else
		}
		return
	}

	// The strategy here is to find a simple test to divide the set of possible strings
	// that might match expr approximately in half.
	// The test we're going to use is to do an ordered comparison of a single byte
	// of expr to a constant. We will pick the index of that byte and the value we're
	// comparing against to make the split as even as possible.
	//   if expr[3] <= 'd' { ... search strings with expr[3] at 'd' or lower  ... }
	//   else              { ... search strings with expr[3] at 'e' or higher ... }
	//
	// To add complication, we will do the ordered comparison in the signed domain.
	// The reason for this is to prevent CSE from merging the load used for the
	// ordered comparison with the load used for the later equality check.
	//   if expr[3] <= 'd' { ... if expr[0] == 'f' && expr[1] == 'o' && expr[2] == 'o' && expr[3] == 'd' { ... } }
	// If we did both expr[3] loads in the unsigned domain, they would be CSEd, and that
	// would in turn defeat the combining of expr[0]...expr[3] into a single 4-byte load.
	// See issue 48222.
	// By using signed loads for the ordered comparison and unsigned loads for the
	// equality comparison, they don't get CSEd and the equality comparisons will be
	// done using wider loads.

	n := len(ir.StringVal(cc[0].lo)) // Length of the constant strings.
	bestScore := int64(0)            // measure of how good the split is.
	bestIdx := 0                     // split using expr[bestIdx]
	bestByte := int8(0)              // compare expr[bestIdx] against bestByte
	for idx := 0; idx < n; idx++ {
		for b := int8(-128); b < 127; b++ {
			le := 0
			for _, c := range cc {
				s := ir.StringVal(c.lo)
				if int8(s[idx]) <= b {
					le++
				}
			}
			score := int64(le) * int64(len(cc)-le)
			if score > bestScore {
				bestScore = score
				bestIdx = idx
				bestByte = b
			}
		}
	}

	// The split must be at least 1:n-1 because we have at least 2 distinct strings; they
	// have to be different somewhere.
	// TODO: what if the best split is still pretty bad?
	if bestScore == 0 {
		base.Fatalf("unable to split string set")
	}

	// Convert expr to a []int8
	slice := ir.NewConvExpr(base.Pos, ir.OSTR2BYTESTMP, types.NewSlice(types.Types[types.TINT8]), expr)
	slice.SetTypecheck(1) // legacy typechecker doesn't handle this op
	slice.MarkNonNil()
	// Load the byte we're splitting on.
	load := ir.NewIndexExpr(base.Pos, slice, ir.NewInt(base.Pos, int64(bestIdx)))
	// Compare with the value we're splitting on.
	cmp := ir.Node(ir.NewBinaryExpr(base.Pos, ir.OLE, load, ir.NewInt(base.Pos, int64(bestByte))))
	cmp = typecheck.DefaultLit(typecheck.Expr(cmp), nil)
	nif := ir.NewIfStmt(base.Pos, cmp, nil, nil)

	var le []exprClause
	var gt []exprClause
	for _, c := range cc {
		s := ir.StringVal(c.lo)
		if int8(s[bestIdx]) <= bestByte {
			le = append(le, c)
		} else {
			gt = append(gt, c)
		}
	}
	stringSearch(expr, le, &nif.Body)
	stringSearch(expr, gt, &nif.Else)
	out.Append(nif)

	// TODO: if expr[bestIdx] has enough different possible values, use a jump table.
}