github.com/sbinet/go@v0.0.0-20160827155028-54d7de7dd62b/src/cmd/compile/internal/gc/swt.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 gc

import "sort"

const (
	// expression switch
	switchKindExpr  = iota // switch a {...} or switch 5 {...}
	switchKindTrue         // switch true {...} or switch {...}
	switchKindFalse        // switch false {...}

	// type switch
	switchKindType // switch a.(type) {...}
)

const binarySearchMin = 4 // minimum number of cases for binary search

// An exprSwitch walks an expression switch.
type exprSwitch struct {
	exprname *Node // node for the expression being switched on
	kind     int   // kind of switch statement (switchKind*)
}

// A typeSwitch walks a type switch.
type typeSwitch struct {
	hashname *Node // node for the hash of the type of the variable being switched on
	facename *Node // node for the concrete type of the variable being switched on
	okname   *Node // boolean node used for comma-ok type assertions
}

// A caseClause is a single case clause in a switch statement.
type caseClause struct {
	node    *Node  // points at case statement
	ordinal int    // position in switch
	hash    uint32 // hash of a type switch
	// isconst indicates whether this case clause is a constant,
	// for the purposes of the switch code generation.
	// For expression switches, that's generally literals (case 5:, not case x:).
	// For type switches, that's concrete types (case time.Time:), not interfaces (case io.Reader:).
	isconst bool
}

// caseClauses are all the case clauses in a switch statement.
type caseClauses struct {
	list   []caseClause // general cases
	defjmp *Node        // OGOTO for default case or OBREAK if no default case present
	niljmp *Node        // OGOTO for nil type case in a type switch
}

// typecheckswitch typechecks a switch statement.
func typecheckswitch(n *Node) {
	lno := lineno
	typecheckslice(n.Ninit.Slice(), Etop)

	var nilonly string
	var top int
	var t *Type

	if n.Left != nil && n.Left.Op == OTYPESW {
		// type switch
		top = Etype
		n.Left.Right = typecheck(n.Left.Right, Erv)
		t = n.Left.Right.Type
		if t != nil && !t.IsInterface() {
			Yyerror("cannot type switch on non-interface value %v", Nconv(n.Left.Right, FmtLong))
		}
	} else {
		// expression switch
		top = Erv
		if n.Left != nil {
			n.Left = typecheck(n.Left, Erv)
			n.Left = defaultlit(n.Left, nil)
			t = n.Left.Type
		} else {
			t = Types[TBOOL]
		}
		if t != nil {
			switch {
			case !okforeq[t.Etype]:
				Yyerror("cannot switch on %v", Nconv(n.Left, FmtLong))
			case t.IsSlice():
				nilonly = "slice"
			case t.IsArray() && !t.IsComparable():
				Yyerror("cannot switch on %v", Nconv(n.Left, FmtLong))
			case t.IsStruct():
				if f := t.IncomparableField(); f != nil {
					Yyerror("cannot switch on %v (struct containing %v cannot be compared)", Nconv(n.Left, FmtLong), f.Type)
				}
			case t.Etype == TFUNC:
				nilonly = "func"
			case t.IsMap():
				nilonly = "map"
			}
		}
	}

	n.Type = t

	var def, niltype *Node
	for _, ncase := range n.List.Slice() {
		setlineno(n)
		if ncase.List.Len() == 0 {
			// default
			if def != nil {
				setlineno(ncase)
				Yyerror("multiple defaults in switch (first at %v)", def.Line())
			} else {
				def = ncase
			}
		} else {
			ls := ncase.List.Slice()
			for i1, n1 := range ls {
				setlineno(n1)
				ls[i1] = typecheck(ls[i1], Erv|Etype)
				n1 = ls[i1]
				if n1.Type == nil || t == nil {
					continue
				}
				setlineno(ncase)
				switch top {
				// expression switch
				case Erv:
					ls[i1] = defaultlit(ls[i1], t)
					n1 = ls[i1]
					switch {
					case n1.Op == OTYPE:
						Yyerror("type %v is not an expression", n1.Type)
					case n1.Type != nil && assignop(n1.Type, t, nil) == 0 && assignop(t, n1.Type, nil) == 0:
						if n.Left != nil {
							Yyerror("invalid case %v in switch on %v (mismatched types %v and %v)", n1, n.Left, n1.Type, t)
						} else {
							Yyerror("invalid case %v in switch (mismatched types %v and bool)", n1, n1.Type)
						}
					case nilonly != "" && !isnil(n1):
						Yyerror("invalid case %v in switch (can only compare %s %v to nil)", n1, nilonly, n.Left)
					case t.IsInterface() && !n1.Type.IsInterface() && !n1.Type.IsComparable():
						Yyerror("invalid case %v in switch (incomparable type)", Nconv(n1, FmtLong))
					}

				// type switch
				case Etype:
					var missing, have *Field
					var ptr int
					switch {
					case n1.Op == OLITERAL && n1.Type.IsKind(TNIL):
						// case nil:
						if niltype != nil {
							Yyerror("multiple nil cases in type switch (first at %v)", niltype.Line())
						} else {
							niltype = ncase
						}
					case n1.Op != OTYPE && n1.Type != nil: // should this be ||?
						Yyerror("%v is not a type", Nconv(n1, FmtLong))
						// reset to original type
						n1 = n.Left.Right
						ls[i1] = n1
					case !n1.Type.IsInterface() && t.IsInterface() && !implements(n1.Type, t, &missing, &have, &ptr):
						if have != nil && !missing.Broke && !have.Broke {
							Yyerror("impossible type switch case: %v cannot have dynamic type %v"+" (wrong type for %v method)\n\thave %v%v\n\twant %v%v", Nconv(n.Left.Right, FmtLong), n1.Type, missing.Sym, have.Sym, Tconv(have.Type, FmtShort), missing.Sym, Tconv(missing.Type, FmtShort))
						} else if !missing.Broke {
							Yyerror("impossible type switch case: %v cannot have dynamic type %v"+" (missing %v method)", Nconv(n.Left.Right, FmtLong), n1.Type, missing.Sym)
						}
					}
				}
			}
		}

		if top == Etype && n.Type != nil {
			ll := ncase.List
			if ncase.Rlist.Len() != 0 {
				nvar := ncase.Rlist.First()
				if ll.Len() == 1 && ll.First().Type != nil && !ll.First().Type.IsKind(TNIL) {
					// single entry type switch
					nvar.Name.Param.Ntype = typenod(ll.First().Type)
				} else {
					// multiple entry type switch or default
					nvar.Name.Param.Ntype = typenod(n.Type)
				}

				nvar = typecheck(nvar, Erv|Easgn)
				ncase.Rlist.SetIndex(0, nvar)
			}
		}

		typecheckslice(ncase.Nbody.Slice(), Etop)
	}

	lineno = lno
}

// walkswitch walks a switch statement.
func walkswitch(sw *Node) {
	// convert switch {...} to switch true {...}
	if sw.Left == nil {
		sw.Left = Nodbool(true)
		sw.Left = typecheck(sw.Left, Erv)
	}

	if sw.Left.Op == OTYPESW {
		var s typeSwitch
		s.walk(sw)
	} else {
		var s exprSwitch
		s.walk(sw)
	}
}

// walk generates an AST implementing sw.
// sw is an expression switch.
// The AST is generally of the form of a linear
// search using if..goto, although binary search
// is used with long runs of constants.
func (s *exprSwitch) walk(sw *Node) {
	casebody(sw, nil)

	cond := sw.Left
	sw.Left = nil

	s.kind = switchKindExpr
	if Isconst(cond, CTBOOL) {
		s.kind = switchKindTrue
		if !cond.Val().U.(bool) {
			s.kind = switchKindFalse
		}
	}

	cond = walkexpr(cond, &sw.Ninit)
	t := sw.Type
	if t == nil {
		return
	}

	// convert the switch into OIF statements
	var cas []*Node
	if s.kind == switchKindTrue || s.kind == switchKindFalse {
		s.exprname = Nodbool(s.kind == switchKindTrue)
	} else if consttype(cond) >= 0 {
		// leave constants to enable dead code elimination (issue 9608)
		s.exprname = cond
	} else {
		s.exprname = temp(cond.Type)
		cas = []*Node{Nod(OAS, s.exprname, cond)}
		typecheckslice(cas, Etop)
	}

	// Enumerate the cases and prepare the default case.
	clauses := s.genCaseClauses(sw.List.Slice())
	sw.List.Set(nil)
	cc := clauses.list

	// handle the cases in order
	for len(cc) > 0 {
		// deal with expressions one at a time
		if !okforcmp[t.Etype] || !cc[0].isconst {
			a := s.walkCases(cc[:1])
			cas = append(cas, a)
			cc = cc[1:]
			continue
		}

		// do binary search on runs of constants
		var run int
		for run = 1; run < len(cc) && cc[run].isconst; run++ {
		}

		// sort and compile constants
		sort.Sort(caseClauseByConstVal(cc[:run]))
		a := s.walkCases(cc[:run])
		cas = append(cas, a)
		cc = cc[run:]
	}

	// handle default case
	if nerrors == 0 {
		cas = append(cas, clauses.defjmp)
		sw.Nbody.Set(append(cas, sw.Nbody.Slice()...))
		walkstmtlist(sw.Nbody.Slice())
	}
}

// walkCases generates an AST implementing the cases in cc.
func (s *exprSwitch) walkCases(cc []caseClause) *Node {
	if len(cc) < binarySearchMin {
		// linear search
		var cas []*Node
		for _, c := range cc {
			n := c.node
			lno := setlineno(n)

			a := Nod(OIF, nil, nil)
			if (s.kind != switchKindTrue && s.kind != switchKindFalse) || assignop(n.Left.Type, s.exprname.Type, nil) == OCONVIFACE || assignop(s.exprname.Type, n.Left.Type, nil) == OCONVIFACE {
				a.Left = Nod(OEQ, s.exprname, n.Left) // if name == val
				a.Left = typecheck(a.Left, Erv)
			} else if s.kind == switchKindTrue {
				a.Left = n.Left // if val
			} else {
				// s.kind == switchKindFalse
				a.Left = Nod(ONOT, n.Left, nil) // if !val
				a.Left = typecheck(a.Left, Erv)
			}
			a.Nbody.Set1(n.Right) // goto l

			cas = append(cas, a)
			lineno = lno
		}
		return liststmt(cas)
	}

	// find the middle and recur
	half := len(cc) / 2
	a := Nod(OIF, nil, nil)
	mid := cc[half-1].node.Left
	le := Nod(OLE, s.exprname, mid)
	if Isconst(mid, CTSTR) {
		// Search by length and then by value; see caseClauseByConstVal.
		lenlt := Nod(OLT, Nod(OLEN, s.exprname, nil), Nod(OLEN, mid, nil))
		leneq := Nod(OEQ, Nod(OLEN, s.exprname, nil), Nod(OLEN, mid, nil))
		a.Left = Nod(OOROR, lenlt, Nod(OANDAND, leneq, le))
	} else {
		a.Left = le
	}
	a.Left = typecheck(a.Left, Erv)
	a.Nbody.Set1(s.walkCases(cc[:half]))
	a.Rlist.Set1(s.walkCases(cc[half:]))
	return a
}

// casebody builds separate lists of statements and cases.
// It makes labels between cases and statements
// and deals with fallthrough, break, and unreachable statements.
func casebody(sw *Node, typeswvar *Node) {
	if sw.List.Len() == 0 {
		return
	}

	lno := setlineno(sw)

	var cas []*Node  // cases
	var stat []*Node // statements
	var def *Node    // defaults
	br := Nod(OBREAK, nil, nil)

	for i, n := range sw.List.Slice() {
		setlineno(n)
		if n.Op != OXCASE {
			Fatalf("casebody %v", n.Op)
		}
		n.Op = OCASE
		needvar := n.List.Len() != 1 || n.List.First().Op == OLITERAL

		jmp := Nod(OGOTO, autolabel(".s"), nil)
		switch n.List.Len() {
		case 0:
			// default
			if def != nil {
				Yyerror("more than one default case")
			}
			// reuse original default case
			n.Right = jmp
			def = n
		case 1:
			// one case -- reuse OCASE node
			n.Left = n.List.First()
			n.Right = jmp
			n.List.Set(nil)
			cas = append(cas, n)
		default:
			// expand multi-valued cases
			for _, n1 := range n.List.Slice() {
				cas = append(cas, Nod(OCASE, n1, jmp))
			}
		}

		stat = append(stat, Nod(OLABEL, jmp.Left, nil))
		if typeswvar != nil && needvar && n.Rlist.Len() != 0 {
			l := []*Node{
				Nod(ODCL, n.Rlist.First(), nil),
				Nod(OAS, n.Rlist.First(), typeswvar),
			}
			typecheckslice(l, Etop)
			stat = append(stat, l...)
		}
		stat = append(stat, n.Nbody.Slice()...)

		// botch - shouldn't fall through declaration
		last := stat[len(stat)-1]
		if last.Xoffset == n.Xoffset && last.Op == OXFALL {
			if typeswvar != nil {
				setlineno(last)
				Yyerror("cannot fallthrough in type switch")
			}

			if i+1 >= sw.List.Len() {
				setlineno(last)
				Yyerror("cannot fallthrough final case in switch")
			}

			last.Op = OFALL
		} else {
			stat = append(stat, br)
		}
	}

	stat = append(stat, br)
	if def != nil {
		cas = append(cas, def)
	}

	sw.List.Set(cas)
	sw.Nbody.Set(stat)
	lineno = lno
}

// genCaseClauses generates the caseClauses value for clauses.
func (s *exprSwitch) genCaseClauses(clauses []*Node) caseClauses {
	var cc caseClauses
	for _, n := range clauses {
		if n.Left == nil {
			// default case
			if cc.defjmp != nil {
				Fatalf("duplicate default case not detected during typechecking")
			}
			cc.defjmp = n.Right
			continue
		}
		c := caseClause{node: n, ordinal: len(cc.list)}
		switch consttype(n.Left) {
		case CTFLT, CTINT, CTRUNE, CTSTR:
			c.isconst = true
		}
		cc.list = append(cc.list, c)
	}

	if cc.defjmp == nil {
		cc.defjmp = Nod(OBREAK, nil, nil)
	}

	// diagnose duplicate cases
	s.checkDupCases(cc.list)
	return cc
}

// genCaseClauses generates the caseClauses value for clauses.
func (s *typeSwitch) genCaseClauses(clauses []*Node) caseClauses {
	var cc caseClauses
	for _, n := range clauses {
		switch {
		case n.Left == nil:
			// default case
			if cc.defjmp != nil {
				Fatalf("duplicate default case not detected during typechecking")
			}
			cc.defjmp = n.Right
			continue
		case n.Left.Op == OLITERAL:
			// nil case in type switch
			if cc.niljmp != nil {
				Fatalf("duplicate nil case not detected during typechecking")
			}
			cc.niljmp = n.Right
			continue
		}

		// general case
		c := caseClause{
			node:    n,
			ordinal: len(cc.list),
			isconst: !n.Left.Type.IsInterface(),
			hash:    typehash(n.Left.Type),
		}
		cc.list = append(cc.list, c)
	}

	if cc.defjmp == nil {
		cc.defjmp = Nod(OBREAK, nil, nil)
	}

	// diagnose duplicate cases
	s.checkDupCases(cc.list)
	return cc
}

func (s *typeSwitch) checkDupCases(cc []caseClause) {
	if len(cc) < 2 {
		return
	}
	// We store seen types in a map keyed by type hash.
	// It is possible, but very unlikely, for multiple distinct types to have the same hash.
	seen := make(map[uint32][]*Node)
	// To avoid many small allocations of length 1 slices,
	// also set up a single large slice to slice into.
	nn := make([]*Node, 0, len(cc))
Outer:
	for _, c := range cc {
		prev, ok := seen[c.hash]
		if !ok {
			// First entry for this hash.
			nn = append(nn, c.node)
			seen[c.hash] = nn[len(nn)-1 : len(nn):len(nn)]
			continue
		}
		for _, n := range prev {
			if Eqtype(n.Left.Type, c.node.Left.Type) {
				yyerrorl(c.node.Lineno, "duplicate case %v in type switch\n\tprevious case at %v", c.node.Left.Type, n.Line())
				// avoid double-reporting errors
				continue Outer
			}
		}
		seen[c.hash] = append(seen[c.hash], c.node)
	}
}

func (s *exprSwitch) checkDupCases(cc []caseClause) {
	if len(cc) < 2 {
		return
	}
	// The common case is that s's expression is not an interface.
	// In that case, all constant clauses have the same type,
	// so checking for duplicates can be done solely by value.
	if !s.exprname.Type.IsInterface() {
		seen := make(map[interface{}]*Node)
		for _, c := range cc {
			// Can't check for duplicates that aren't constants, per the spec. Issue 15896.
			// Don't check for duplicate bools. Although the spec allows it,
			// (1) the compiler hasn't checked it in the past, so compatibility mandates it, and
			// (2) it would disallow useful things like
			//       case GOARCH == "arm" && GOARM == "5":
			//       case GOARCH == "arm":
			//     which would both evaluate to false for non-ARM compiles.
			if ct := consttype(c.node.Left); ct < 0 || ct == CTBOOL {
				continue
			}
			val := c.node.Left.Val().Interface()
			prev, dup := seen[val]
			if !dup {
				seen[val] = c.node
				continue
			}
			setlineno(c.node)
			Yyerror("duplicate case %v in switch\n\tprevious case at %v", prev.Left, prev.Line())
		}
		return
	}
	// s's expression is an interface. This is fairly rare, so keep this simple.
	// Duplicates are only duplicates if they have the same type and the same value.
	type typeVal struct {
		typ string
		val interface{}
	}
	seen := make(map[typeVal]*Node)
	for _, c := range cc {
		if ct := consttype(c.node.Left); ct < 0 || ct == CTBOOL {
			continue
		}
		n := c.node.Left
		tv := typeVal{
			// Tconv here serves to completely describe the type.
			// See the comments in func typehash.
			typ: Tconv(n.Type, FmtLeft|FmtUnsigned),
			val: n.Val().Interface(),
		}
		prev, dup := seen[tv]
		if !dup {
			seen[tv] = c.node
			continue
		}
		setlineno(c.node)
		Yyerror("duplicate case %v in switch\n\tprevious case at %v", prev.Left, prev.Line())
	}
}

// walk generates an AST that implements sw,
// where sw is a type switch.
// The AST is generally of the form of a linear
// search using if..goto, although binary search
// is used with long runs of concrete types.
func (s *typeSwitch) walk(sw *Node) {
	cond := sw.Left
	sw.Left = nil

	if cond == nil {
		sw.List.Set(nil)
		return
	}
	if cond.Right == nil {
		setlineno(sw)
		Yyerror("type switch must have an assignment")
		return
	}

	cond.Right = walkexpr(cond.Right, &sw.Ninit)
	if !cond.Right.Type.IsInterface() {
		Yyerror("type switch must be on an interface")
		return
	}

	var cas []*Node

	// predeclare temporary variables and the boolean var
	s.facename = temp(cond.Right.Type)

	a := Nod(OAS, s.facename, cond.Right)
	a = typecheck(a, Etop)
	cas = append(cas, a)

	s.okname = temp(Types[TBOOL])
	s.okname = typecheck(s.okname, Erv)

	s.hashname = temp(Types[TUINT32])
	s.hashname = typecheck(s.hashname, Erv)

	// set up labels and jumps
	casebody(sw, s.facename)

	clauses := s.genCaseClauses(sw.List.Slice())
	sw.List.Set(nil)
	def := clauses.defjmp

	// 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.

	// Get interface descriptor word.
	typ := Nod(OITAB, s.facename, nil)

	// Check for nil first.
	i := Nod(OIF, nil, nil)
	i.Left = Nod(OEQ, typ, nodnil())
	if clauses.niljmp != nil {
		// Do explicit nil case right here.
		i.Nbody.Set1(clauses.niljmp)
	} else {
		// Jump to default case.
		lbl := autolabel(".s")
		i.Nbody.Set1(Nod(OGOTO, lbl, nil))
		// Wrap default case with label.
		blk := Nod(OBLOCK, nil, nil)
		blk.List.Set([]*Node{Nod(OLABEL, lbl, nil), def})
		def = blk
	}
	i.Left = typecheck(i.Left, Erv)
	cas = append(cas, i)

	if !cond.Right.Type.IsEmptyInterface() {
		// Load type from itab.
		typ = itabType(typ)
	}
	// Load hash from type.
	h := NodSym(ODOTPTR, typ, nil)
	h.Type = Types[TUINT32]
	h.Typecheck = 1
	h.Xoffset = int64(2 * Widthptr) // offset of hash in runtime._type
	h.Bounded = true                // guaranteed not to fault
	a = Nod(OAS, s.hashname, h)
	a = typecheck(a, Etop)
	cas = append(cas, a)

	cc := clauses.list

	// insert type equality check into each case block
	for _, c := range cc {
		c.node.Right = s.typeone(c.node)
	}

	// generate list of if statements, binary search for constant sequences
	for len(cc) > 0 {
		if !cc[0].isconst {
			n := cc[0].node
			cas = append(cas, n.Right)
			cc = cc[1:]
			continue
		}

		// identify run of constants
		var run int
		for run = 1; run < len(cc) && cc[run].isconst; run++ {
		}

		// sort by hash
		sort.Sort(caseClauseByType(cc[:run]))

		// for debugging: linear search
		if false {
			for i := 0; i < run; i++ {
				n := cc[i].node
				cas = append(cas, n.Right)
			}
			continue
		}

		// combine adjacent cases with the same hash
		ncase := 0
		for i := 0; i < run; i++ {
			ncase++
			hash := []*Node{cc[i].node.Right}
			for j := i + 1; j < run && cc[i].hash == cc[j].hash; j++ {
				hash = append(hash, cc[j].node.Right)
			}
			cc[i].node.Right = liststmt(hash)
		}

		// binary search among cases to narrow by hash
		cas = append(cas, s.walkCases(cc[:ncase]))
		cc = cc[ncase:]
	}

	// handle default case
	if nerrors == 0 {
		cas = append(cas, def)
		sw.Nbody.Set(append(cas, sw.Nbody.Slice()...))
		sw.List.Set(nil)
		walkstmtlist(sw.Nbody.Slice())
	}
}

// typeone generates an AST that jumps to the
// case body if the variable is of type t.
func (s *typeSwitch) typeone(t *Node) *Node {
	var name *Node
	var init []*Node
	if t.Rlist.Len() == 0 {
		name = nblank
		nblank = typecheck(nblank, Erv|Easgn)
	} else {
		name = t.Rlist.First()
		init = []*Node{Nod(ODCL, name, nil)}
		a := Nod(OAS, name, nil)
		a = typecheck(a, Etop)
		init = append(init, a)
	}

	a := Nod(OAS2, nil, nil)
	a.List.Set([]*Node{name, s.okname}) // name, ok =
	b := Nod(ODOTTYPE, s.facename, nil)
	b.Type = t.Left.Type // interface.(type)
	a.Rlist.Set1(b)
	a = typecheck(a, Etop)
	init = append(init, a)

	c := Nod(OIF, nil, nil)
	c.Left = s.okname
	c.Nbody.Set1(t.Right) // if ok { goto l }

	return liststmt(append(init, c))
}

// walkCases generates an AST implementing the cases in cc.
func (s *typeSwitch) walkCases(cc []caseClause) *Node {
	if len(cc) < binarySearchMin {
		var cas []*Node
		for _, c := range cc {
			n := c.node
			if !c.isconst {
				Fatalf("typeSwitch walkCases")
			}
			a := Nod(OIF, nil, nil)
			a.Left = Nod(OEQ, s.hashname, Nodintconst(int64(c.hash)))
			a.Left = typecheck(a.Left, Erv)
			a.Nbody.Set1(n.Right)
			cas = append(cas, a)
		}
		return liststmt(cas)
	}

	// find the middle and recur
	half := len(cc) / 2
	a := Nod(OIF, nil, nil)
	a.Left = Nod(OLE, s.hashname, Nodintconst(int64(cc[half-1].hash)))
	a.Left = typecheck(a.Left, Erv)
	a.Nbody.Set1(s.walkCases(cc[:half]))
	a.Rlist.Set1(s.walkCases(cc[half:]))
	return a
}

// caseClauseByConstVal sorts clauses by constant value to enable binary search.
type caseClauseByConstVal []caseClause

func (x caseClauseByConstVal) Len() int      { return len(x) }
func (x caseClauseByConstVal) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
func (x caseClauseByConstVal) Less(i, j int) bool {
	v1 := x[i].node.Left.Val().U
	v2 := x[j].node.Left.Val().U

	switch v1 := v1.(type) {
	case *Mpflt:
		return v1.Cmp(v2.(*Mpflt)) < 0
	case *Mpint:
		return v1.Cmp(v2.(*Mpint)) < 0
	case string:
		// 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 in exprSwitch.walkCases.
		a := v1
		b := v2.(string)
		if len(a) != len(b) {
			return len(a) < len(b)
		}
		return a < b
	}

	Fatalf("caseClauseByConstVal passed bad clauses %v < %v", x[i].node.Left, x[j].node.Left)
	return false
}

type caseClauseByType []caseClause

func (x caseClauseByType) Len() int      { return len(x) }
func (x caseClauseByType) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
func (x caseClauseByType) Less(i, j int) bool {
	c1, c2 := x[i], x[j]
	// sort by hash code, then ordinal (for the rare case of hash collisions)
	if c1.hash != c2.hash {
		return c1.hash < c2.hash
	}
	return c1.ordinal < c2.ordinal
}