compiler: do not allocate slotes for unused "_" vars

pkg/compiler/analysis.go

@@ -112,7 +112,11 @@ func countGlobals(f ast.Node) (i int) {
 		case *ast.GenDecl:
 			if n.Tok == token.VAR {
 				for _, s := range n.Specs {
-					i += len(s.(*ast.ValueSpec).Names)
+					for _, id := range s.(*ast.ValueSpec).Names {
+						if id.Name != "_" {
+							i++
+						}
+					}
 				}
 			}
 			return false

pkg/compiler/codegen.go

@@ -434,6 +434,7 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			switch t := spec.(type) {
 			case *ast.ValueSpec:
 				for _, id := range t.Names {
+					if id.Name != "_" {
 						if c.scope == nil {
 							// it is a global declaration
 							c.newGlobal("", id.Name)
@@ -442,6 +443,7 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 						}
 						c.registerDebugVariable(id.Name, t.Type)
 					}
+				}
 				for i := range t.Names {
 					if len(t.Values) != 0 {
 						ast.Walk(c, t.Values[i])

pkg/compiler/global_test.go

@@ -255,3 +255,19 @@ func TestConstDontUseSlots(t *testing.T) {
 	src := buf.String()
 	eval(t, src, big.NewInt(count))
 }
+
+func TestUnderscoreVarsDontUseSlots(t *testing.T) {
+	const count = 128
+	buf := bytes.NewBufferString("package foo\n")
+	for i := 0; i < count; i++ {
+		buf.WriteString(fmt.Sprintf("var _, n%d = 1, 1\n", i))
+	}
+	buf.WriteString("func Main() int { sum := 0\n")
+	for i := 0; i < count; i++ {
+		buf.WriteString(fmt.Sprintf("sum += n%d\n", i))
+	}
+	buf.WriteString("return sum }")
+
+	src := buf.String()
+	eval(t, src, big.NewInt(count))
+}