gopherjs · nevkontakte · Oct 4, 2021 · Oct 3, 2021 · Oct 3, 2021
diff --git a/compiler/astutil/astutil.go b/compiler/astutil/astutil.go
@@ -129,3 +129,21 @@ func FindLoopStmt(stack []ast.Node, branch *ast.BranchStmt, typeInfo *types.Info
 	// This should never happen in a source that passed type checking.
 	panic(fmt.Errorf("continue/break statement %v doesn't have a matching loop statement among ancestors", branch))
 }
+
+// EndsWithReturn returns true if the last effective statement is a "return".
+func EndsWithReturn(stmts []ast.Stmt) bool {
+	if len(stmts) == 0 {
+		return false
+	}
+	last := stmts[len(stmts)-1]
+	switch l := last.(type) {
+	case *ast.ReturnStmt:
+		return true
+	case *ast.LabeledStmt:
+		return EndsWithReturn([]ast.Stmt{l.Stmt})
+	case *ast.BlockStmt:
+		return EndsWithReturn(l.List)
+	default:
+		return false
+	}
+}
diff --git a/compiler/astutil/astutil_test.go b/compiler/astutil/astutil_test.go
@@ -130,6 +130,58 @@ func TestPruneOriginal(t *testing.T) {
 	}
 }
 
+func TestEndsWithReturn(t *testing.T) {
+	tests := []struct {
+		desc string
+		src  string
+		want bool
+	}{
+		{
+			desc: "empty function",
+			src:  `func foo() {}`,
+			want: false,
+		}, {
+			desc: "implicit return",
+			src:  `func foo() { a() }`,
+			want: false,
+		}, {
+			desc: "explicit return",
+			src:  `func foo() { a(); return }`,
+			want: true,
+		}, {
+			desc: "labelled return",
+			src:  `func foo() { Label: return }`,
+			want: true,
+		}, {
+			desc: "labelled call",
+			src:  `func foo() { Label: a() }`,
+			want: false,
+		}, {
+			desc: "return in a block",
+			src:  `func foo() { a(); { b(); return; } }`,
+			want: true,
+		}, {
+			desc: "a block without return",
+			src:  `func foo() { a(); { b(); c(); } }`,
+			want: false,
+		}, {
+			desc: "conditional block",
+			src:  `func foo() { a(); if x { b(); return; } }`,
+			want: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.desc, func(t *testing.T) {
+			fdecl := parseFuncDecl(t, "package testpackage\n"+test.src)
+			got := EndsWithReturn(fdecl.Body.List)
+			if got != test.want {
+				t.Errorf("EndsWithReturn() returned %t, want %t", got, test.want)
+			}
+		})
+	}
+}
+
 func parse(t *testing.T, fset *token.FileSet, src string) *ast.File {
 	t.Helper()
 	f, err := parser.ParseFile(fset, "test.go", src, parser.ParseComments)

diff --git a/compiler/package.go b/compiler/package.go
@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	"github.com/gopherjs/gopherjs/compiler/analysis"
+	"github.com/gopherjs/gopherjs/compiler/astutil"
 	"github.com/neelance/astrewrite"
 	"golang.org/x/tools/go/gcexportdata"
 	"golang.org/x/tools/go/types/typeutil"
@@ -768,7 +769,7 @@ func translateFunction(typ *ast.FuncType, recv *ast.Ident, body *ast.BlockStmt,
 		}
 
 		c.translateStmtList(body.List)
-		if len(c.Flattened) != 0 && !endsWithReturn(body.List) {
+		if len(c.Flattened) != 0 && !astutil.EndsWithReturn(body.List) {
 			c.translateStmt(&ast.ReturnStmt{}, nil)
 		}
 	}))

diff --git a/compiler/statements.go b/compiler/statements.go
@@ -644,7 +644,7 @@ func (fc *funcContext) translateBranchingStmt(caseClauses []*ast.CaseClause, def
 		fc.PrintCond(!flatten, fmt.Sprintf("%sif (%s) {", prefix, condStrs[i]), fmt.Sprintf("case %d:", caseOffset+i))
 		fc.Indent(func() {
 			fc.translateStmtList(clause.Body)
-			if flatten && (i < len(caseClauses)-1 || defaultClause != nil) && !endsWithReturn(clause.Body) {
+			if flatten && (i < len(caseClauses)-1 || defaultClause != nil) && !astutil.EndsWithReturn(clause.Body) {
 				fc.Printf("$s = %d; continue;", endCase)
 			}
 		})
@@ -681,6 +681,7 @@ func (fc *funcContext) translateLoopingStmt(cond func() string, body *ast.BlockS
 	if !flatten && label != nil {
 		fc.Printf("%s:", label.Name())
 	}
+	isTerminated := false
 	fc.PrintCond(!flatten, "while (true) {", fmt.Sprintf("case %d:", data.beginCase))
 	fc.Indent(func() {
 		condStr := cond()
@@ -695,7 +696,6 @@ func (fc *funcContext) translateLoopingStmt(cond func() string, body *ast.BlockS
 			bodyPrefix()
 		}
 		fc.translateStmtList(body.List)
-		isTerminated := false
 		if len(body.List) != 0 {
 			switch body.List[len(body.List)-1].(type) {
 			case *ast.ReturnStmt, *ast.BranchStmt:
@@ -708,7 +708,17 @@ func (fc *funcContext) translateLoopingStmt(cond func() string, body *ast.BlockS
 
 		fc.pkgCtx.escapingVars = prevEV
 	})
-	fc.PrintCond(!flatten, "}", fmt.Sprintf("$s = %d; continue; case %d:", data.beginCase, data.endCase))
+	if flatten {
+		// If the last statement of the loop is a return or unconditional branching
+		// statement, there's no need for an instruction to go back to the beginning
+		// of the loop.
+		if !isTerminated {
+			fc.Printf("$s = %d; continue;", data.beginCase)
+		}
+		fc.Printf("case %d:", data.endCase)
+	} else {
+		fc.Printf("}")
+	}
 }
 
 func (fc *funcContext) translateAssign(lhs, rhs ast.Expr, define bool) string {

diff --git a/compiler/utils.go b/compiler/utils.go
@@ -677,15 +677,6 @@ func rangeCheck(pattern string, constantIndex, array bool) string {
 	return "(" + check + ` ? ($throwRuntimeError("index out of range"), undefined) : ` + pattern + ")"
 }
 
-func endsWithReturn(stmts []ast.Stmt) bool {
-	if len(stmts) > 0 {
-		if _, ok := stmts[len(stmts)-1].(*ast.ReturnStmt); ok {
-			return true
-		}
-	}
-	return false
-}
-
 func encodeIdent(name string) string {
 	return strings.Replace(url.QueryEscape(name), "%", "$", -1)
 }