cmd/compile: explicitly manage default and nil switch cases

Rather than juggle default and nil cases as part
of a slice, handle them explicitly.

Change-Id: I97b200c9d3f23fe1a438acdbf3d13b0cf7e0851e
Reviewed-on: https://go-review.googlesource.com/26761
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/swt.go

@@ -17,14 +17,11 @@ const (
 )
 
 const (
-	caseKindDefault = iota // default:
-
 	// expression switch
-	caseKindExprConst // case 5:
-	caseKindExprVar   // case x:
+	caseKindExprConst = iota // case 5:
+	caseKindExprVar          // case x:
 
 	// type switch
-	caseKindTypeNil   // case nil:
 	caseKindTypeConst // case time.Time: (concrete type, has type hash)
 	caseKindTypeVar   // case io.Reader: (interface type)
 )
@@ -52,6 +49,13 @@ type caseClause struct {
 	typ     uint8  // type of case
 }
 
+// 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
@@ -248,16 +252,10 @@ func (s *exprSwitch) walk(sw *Node) {
 		typecheckslice(cas, Etop)
 	}
 
-	// enumerate the cases, and lop off the default case
-	cc := caseClauses(sw, s.kind)
+	// Enumerate the cases and prepare the default case.
+	clauses := genCaseClauses(sw, s.kind)
 	sw.List.Set(nil)
-	var def *Node
-	if len(cc) > 0 && cc[0].typ == caseKindDefault {
-		def = cc[0].node.Right
-		cc = cc[1:]
-	} else {
-		def = Nod(OBREAK, nil, nil)
-	}
+	cc := clauses.list
 
 	// handle the cases in order
 	for len(cc) > 0 {
@@ -283,14 +281,14 @@ func (s *exprSwitch) walk(sw *Node) {
 
 	// handle default case
 	if nerrors == 0 {
-		cas = append(cas, def)
+		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 {
+func (s *exprSwitch) walkCases(cc []caseClause) *Node {
 	if len(cc) < binarySearchMin {
 		// linear search
 		var cas []*Node
@@ -422,27 +420,35 @@ func casebody(sw *Node, typeswvar *Node) {
 	lineno = lno
 }
 
-// caseClauses generates a slice of caseClauses
+// genCaseClauses generates the caseClauses value
 // corresponding to the clauses in the switch statement sw.
 // Kind is the kind of switch statement.
-func caseClauses(sw *Node, kind int) []*caseClause {
-	var cc []*caseClause
+func genCaseClauses(sw *Node, kind int) caseClauses {
+	var cc caseClauses
 	for _, n := range sw.List.Slice() {
-		c := new(caseClause)
-		cc = append(cc, c)
-		c.ordinal = len(cc)
-		c.node = n
-
 		if n.Left == nil {
-			c.typ = caseKindDefault
+			// default case
+			if cc.defjmp != nil {
+				Fatalf("duplicate default case not detected during typechecking")
+			}
+			cc.defjmp = n.Right
 			continue
 		}
 
+		if kind == switchKindType && 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)}
 		if kind == switchKindType {
 			// type switch
 			switch {
-			case n.Left.Op == OLITERAL:
-				c.typ = caseKindTypeNil
 			case n.Left.Type.IsInterface():
 				c.typ = caseKindTypeVar
 			default:
@@ -458,22 +464,24 @@ func caseClauses(sw *Node, kind int) []*caseClause {
 				c.typ = caseKindExprVar
 			}
 		}
+		cc.list = append(cc.list, c)
 	}
 
-	if cc == nil {
-		return nil
+	if cc.defjmp == nil {
+		cc.defjmp = Nod(OBREAK, nil, nil)
+	}
+
+	if cc.list == nil {
+		return cc
 	}
 
 	// sort by value and diagnose duplicate cases
 	if kind == switchKindType {
 		// type switch
-		sort.Sort(caseClauseByType(cc))
-		for i, c1 := range cc {
-			if c1.typ == caseKindTypeNil || c1.typ == caseKindDefault {
-				break
-			}
-			for _, c2 := range cc[i+1:] {
-				if c2.typ == caseKindTypeNil || c2.typ == caseKindDefault || c1.hash != c2.hash {
+		sort.Sort(caseClauseByType(cc.list))
+		for i, c1 := range cc.list {
+			for _, c2 := range cc.list[i+1:] {
+				if c1.hash != c2.hash {
 					break
 				}
 				if Eqtype(c1.node.Left.Type, c2.node.Left.Type) {
@@ -483,12 +491,12 @@ func caseClauses(sw *Node, kind int) []*caseClause {
 		}
 	} else {
 		// expression switch
-		sort.Sort(caseClauseByExpr(cc))
-		for i, c1 := range cc {
-			if i+1 == len(cc) {
+		sort.Sort(caseClauseByExpr(cc.list))
+		for i, c1 := range cc.list {
+			if i+1 == len(cc.list) {
 				break
 			}
-			c2 := cc[i+1]
+			c2 := cc.list[i+1]
 			if exprcmp(c1, c2) != 0 {
 				continue
 			}
@@ -498,7 +506,7 @@ func caseClauses(sw *Node, kind int) []*caseClause {
 	}
 
 	// put list back in processing order
-	sort.Sort(caseClauseByOrd(cc))
+	sort.Sort(caseClauseByOrd(cc.list))
 	return cc
 }
 
@@ -545,20 +553,9 @@ func (s *typeSwitch) walk(sw *Node) {
 	// set up labels and jumps
 	casebody(sw, s.facename)
 
-	cc := caseClauses(sw, switchKindType)
+	clauses := genCaseClauses(sw, switchKindType)
 	sw.List.Set(nil)
-	var def *Node
-	if len(cc) > 0 && cc[0].typ == caseKindDefault {
-		def = cc[0].node.Right
-		cc = cc[1:]
-	} else {
-		def = Nod(OBREAK, nil, nil)
-	}
-	var typenil *Node
-	if len(cc) > 0 && cc[0].typ == caseKindTypeNil {
-		typenil = cc[0].node.Right
-		cc = cc[1:]
-	}
+	def := clauses.defjmp
 
 	// For empty interfaces, do:
 	//     if e._type == nil {
@@ -573,9 +570,9 @@ func (s *typeSwitch) walk(sw *Node) {
 	// Check for nil first.
 	i := Nod(OIF, nil, nil)
 	i.Left = Nod(OEQ, typ, nodnil())
-	if typenil != nil {
+	if clauses.niljmp != nil {
 		// Do explicit nil case right here.
-		i.Nbody.Set1(typenil)
+		i.Nbody.Set1(clauses.niljmp)
 	} else {
 		// Jump to default case.
 		lbl := autolabel(".s")
@@ -602,6 +599,8 @@ func (s *typeSwitch) walk(sw *Node) {
 	a = typecheck(a, Etop)
 	cas = append(cas, a)
 
+	cc := clauses.list
+
 	// insert type equality check into each case block
 	for _, c := range cc {
 		n := c.node
@@ -696,7 +695,7 @@ func (s *typeSwitch) typeone(t *Node) *Node {
 }
 
 // walkCases generates an AST implementing the cases in cc.
-func (s *typeSwitch) walkCases(cc []*caseClause) *Node {
+func (s *typeSwitch) walkCases(cc []caseClause) *Node {
 	if len(cc) < binarySearchMin {
 		var cas []*Node
 		for _, c := range cc {
@@ -723,31 +722,13 @@ func (s *typeSwitch) walkCases(cc []*caseClause) *Node {
 	return a
 }
 
-type caseClauseByOrd []*caseClause
+type caseClauseByOrd []caseClause
 
-func (x caseClauseByOrd) Len() int      { return len(x) }
-func (x caseClauseByOrd) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
-func (x caseClauseByOrd) Less(i, j int) bool {
-	c1, c2 := x[i], x[j]
-	switch {
-	// sort default first
-	case c1.typ == caseKindDefault:
-		return true
-	case c2.typ == caseKindDefault:
-		return false
+func (x caseClauseByOrd) Len() int           { return len(x) }
+func (x caseClauseByOrd) Swap(i, j int)      { x[i], x[j] = x[j], x[i] }
+func (x caseClauseByOrd) Less(i, j int) bool { return x[i].ordinal < x[j].ordinal }
 
-	// sort nil second
-	case c1.typ == caseKindTypeNil:
-		return true
-	case c2.typ == caseKindTypeNil:
-		return false
-	}
-
-	// sort by ordinal
-	return c1.ordinal < c2.ordinal
-}
-
-type caseClauseByExpr []*caseClause
+type caseClauseByExpr []caseClause
 
 func (x caseClauseByExpr) Len() int      { return len(x) }
 func (x caseClauseByExpr) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
@@ -755,7 +736,7 @@ func (x caseClauseByExpr) Less(i, j int) bool {
 	return exprcmp(x[i], x[j]) < 0
 }
 
-func exprcmp(c1, c2 *caseClause) int {
+func exprcmp(c1, c2 caseClause) int {
 	// sort non-constants last
 	if c1.typ != caseKindExprConst {
 		return +1
@@ -814,7 +795,7 @@ func exprcmp(c1, c2 *caseClause) int {
 	return 0
 }
 
-type caseClauseByType []*caseClause
+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] }

src/cmd/compile/internal/gc/swt_test.go

@@ -134,7 +134,7 @@ func TestExprcmp(t *testing.T) {
 		},
 	}
 	for i, d := range testdata {
-		got := exprcmp(&d.a, &d.b)
+		got := exprcmp(d.a, d.b)
 		if d.want != got {
 			t.Errorf("%d: exprcmp(a, b) = %d; want %d", i, got, d.want)
 			t.Logf("\ta = caseClause{node: %#v, typ: %#v}", d.a.node, d.a.typ)