cmd/compile: use a map to track const switch cases

This is simpler than the sorting technique.
It also allows us to simplify or eliminate
some of the sorting decisions.

Most important, sorting will not work when case clauses
represent ranges of integers: There is no correct
sort order that allows overlap detection by comparing
neighbors. Using a map allows of a cheap, simple
approach to ranges, namely to insert every int
in the map. The equivalent approach for sorting
means juggling temporary Nodes for every int,
which is a lot more expensive.

Change-Id: I84df3cb805992a1b04d14e0e4b2334f943e0ce05
Reviewed-on: https://go-review.googlesource.com/26766
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -89,6 +89,27 @@ func eqval(a, b Val) bool {
 	}
 }
 
+// Interface returns the constant value stored in v as an interface{}.
+// It returns int64s for ints and runes, float64s for floats,
+// complex128s for complex values, and nil for constant nils.
+func (v Val) Interface() interface{} {
+	switch x := v.U.(type) {
+	default:
+		Fatalf("unexpected Interface for %T", v.U)
+		panic("not reached")
+	case *NilVal:
+		return nil
+	case bool, string:
+		return x
+	case *Mpint:
+		return x.Int64()
+	case *Mpflt:
+		return x.Float64()
+	case *Mpcplx:
+		return complex(x.Real.Float64(), x.Imag.Float64())
+	}
+}
+
 type NilVal struct{}
 
 // IntLiteral returns the Node's literal value as an integer.

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

@@ -517,17 +517,59 @@ func (s *exprSwitch) checkDupCases(cc []caseClause) {
 	if len(cc) < 2 {
 		return
 	}
-	sort.Sort(caseClauseByExpr(cc))
-	for i, c1 := range cc[:len(cc)-1] {
-		c2 := cc[i+1]
-		if exprcmp(c1, c2) != 0 {
+	// 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
 		}
-		setlineno(c2.node)
-		Yyerror("duplicate case %v in switch\n\tprevious case at %v", c1.node.Left, c1.node.Line())
+		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())
 	}
-	// put list back in processing order
-	sort.Sort(caseClauseByOrd(cc))
 }
 
 // walk generates an AST that implements sw,
@@ -736,12 +778,6 @@ func (s *typeSwitch) walkCases(cc []caseClause) *Node {
 	return a
 }
 
-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 { return x[i].ordinal < x[j].ordinal }
-
 type caseClauseByExpr []caseClause
 
 func (x caseClauseByExpr) Len() int      { return len(x) }

test/switch5.go

@@ -79,3 +79,14 @@ func f5(a [1]int) {
 	case [1]int{0}: // OK -- see issue 15896
 	}
 }
+
+// Ensure duplicate const bool clauses are accepted.
+func f6() int {
+	switch {
+	case 0 == 0:
+		return 0
+	case 1 == 1: // Intentionally OK, even though a duplicate of the above const true
+		return 1
+	}
+	return 2
+}