diff --git a/internal/shader/shader.go b/internal/shader/shader.go
index 918413227..986964e9f 100644
--- a/internal/shader/shader.go
+++ b/internal/shader/shader.go
@@ -724,6 +724,15 @@ func (cs *compileState) parseBlock(outer *block, fname string, stmts []ast.Stmt,
 		}
 	}()
 
+	if outer.outer == nil && len(outParams) > 0 && outParams[0].name != "" {
+		for i := range outParams {
+			block.ir.Stmts = append(block.ir.Stmts, shaderir.Stmt{
+				Type:      shaderir.Init,
+				InitIndex: len(inParams) + i,
+			})
+		}
+	}
+
 	for _, stmt := range stmts {
 		ss, ok := cs.parseStmt(block, fname, stmt, inParams, outParams)
 		if !ok {
diff --git a/internal/shader/stmt.go b/internal/shader/stmt.go
index 0e050da5e..ebcc5375d 100644
--- a/internal/shader/stmt.go
+++ b/internal/shader/stmt.go
@@ -379,9 +379,11 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 
 	case *ast.ReturnStmt:
 		if len(stmt.Results) != len(outParams) {
-			// TODO: Implenet multiple-value context.
-			cs.addError(stmt.Pos(), fmt.Sprintf("the number of returning variables must be %d but %d", len(outParams), len(stmt.Results)))
-			return nil, false
+			if !(len(stmt.Results) == 0 && len(outParams) > 0 && outParams[0].name != "") {
+				// TODO: Implenet multiple-value context.
+				cs.addError(stmt.Pos(), fmt.Sprintf("the number of returning variables must be %d but %d", len(outParams), len(stmt.Results)))
+				return nil, false
+			}
 		}
 
 		for i, r := range stmt.Results {
diff --git a/internal/shader/testdata/out.expected.metal b/internal/shader/testdata/out.expected.metal
new file mode 100644
index 000000000..70c3652ff
--- /dev/null
+++ b/internal/shader/testdata/out.expected.metal
@@ -0,0 +1,8 @@
+void F0(thread float& l0, thread array<float, 4>& l1, thread float4& l2);
+
+void F0(thread float& l0, thread array<float, 4>& l1, thread float4& l2) {
+	l0 = float(0);
+	l1 = {};
+	l2 = float4(0);
+	return;
+}
diff --git a/internal/shader/testdata/out.expected.vs b/internal/shader/testdata/out.expected.vs
new file mode 100644
index 000000000..2a13481ba
--- /dev/null
+++ b/internal/shader/testdata/out.expected.vs
@@ -0,0 +1,11 @@
+void F0(out float l0, out float l1[4], out vec4 l2);
+
+void F0(out float l0, out float l1[4], out vec4 l2) {
+	l0 = float(0);
+	l1[0] = float(0);
+	l1[1] = float(0);
+	l1[2] = float(0);
+	l1[3] = float(0);
+	l2 = vec4(0);
+	return;
+}
diff --git a/internal/shader/testdata/out.go b/internal/shader/testdata/out.go
new file mode 100644
index 000000000..1e8a8ba85
--- /dev/null
+++ b/internal/shader/testdata/out.go
@@ -0,0 +1,5 @@
+package main
+
+func Foo() (a float, b [4]float, c vec4) {
+	return
+}
diff --git a/internal/shader/testdata/return.expected.vs b/internal/shader/testdata/return.expected.vs
new file mode 100644
index 000000000..10d6a37bf
--- /dev/null
+++ b/internal/shader/testdata/return.expected.vs
@@ -0,0 +1,8 @@
+void F0(in vec2 l0, out vec3 l1, out vec4 l2);
+
+void F0(in vec2 l0, out vec3 l1, out vec4 l2) {
+	l1 = vec3(0);
+	l2 = vec4(0);
+	l1 = vec3(1.0);
+	return;
+}
diff --git a/internal/shader/testdata/return.go b/internal/shader/testdata/return.go
new file mode 100644
index 000000000..4a78cc926
--- /dev/null
+++ b/internal/shader/testdata/return.go
@@ -0,0 +1,6 @@
+package main
+
+func Foo(a vec2) (b vec3, c vec4) {
+	b = vec3(1)
+	return
+}
diff --git a/internal/shader/testdata/vertex.expected.vs b/internal/shader/testdata/vertex.expected.vs
index da841bca3..7ef00ba3c 100644
--- a/internal/shader/testdata/vertex.expected.vs
+++ b/internal/shader/testdata/vertex.expected.vs
@@ -7,6 +7,9 @@ varying vec4 V1;
 
 void main(void) {
 	mat4 l0 = mat4(0);
+	gl_Position = vec4(0);
+	V0 = vec2(0);
+	V1 = vec4(0);
 	l0 = mat4((2.0) / ((U0).x), 0.0, 0.0, 0.0, 0.0, (2.0) / ((U0).y), 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, -1.0, -1.0, 0.0, 1.0);
 	gl_Position = (l0) * (vec4(A0, 0.0, 1.0));
 	V0 = A1;
diff --git a/internal/shader/testdata/vertex_fragment.expected.vs b/internal/shader/testdata/vertex_fragment.expected.vs
index da841bca3..7ef00ba3c 100644
--- a/internal/shader/testdata/vertex_fragment.expected.vs
+++ b/internal/shader/testdata/vertex_fragment.expected.vs
@@ -7,6 +7,9 @@ varying vec4 V1;
 
 void main(void) {
 	mat4 l0 = mat4(0);
+	gl_Position = vec4(0);
+	V0 = vec2(0);
+	V1 = vec4(0);
 	l0 = mat4((2.0) / ((U0).x), 0.0, 0.0, 0.0, 0.0, (2.0) / ((U0).y), 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, -1.0, -1.0, 0.0, 1.0);
 	gl_Position = (l0) * (vec4(A0, 0.0, 1.0));
 	V0 = A1;
diff --git a/internal/shaderir/glsl/glsl.go b/internal/shaderir/glsl/glsl.go
index bd779d33e..2a998c318 100644
--- a/internal/shaderir/glsl/glsl.go
+++ b/internal/shaderir/glsl/glsl.go
@@ -414,6 +414,8 @@ func (c *compileContext) glslBlock(p *shaderir.Program, topBlock, block *shaderi
 				}
 			}
 			lines = append(lines, fmt.Sprintf("%s%s = %s;", idt, glslExpr(&lhs), glslExpr(&rhs)))
+		case shaderir.Init:
+			lines = append(lines, c.initVariable(p, topBlock, block, s.InitIndex, false, level)...)
 		case shaderir.If:
 			lines = append(lines, fmt.Sprintf("%sif (%s) {", idt, glslExpr(&s.Exprs[0])))
 			lines = append(lines, c.glslBlock(p, topBlock, s.Blocks[0], level+1)...)
diff --git a/internal/shaderir/metal/metal.go b/internal/shaderir/metal/metal.go
index 46cf210dd..69d21826b 100644
--- a/internal/shaderir/metal/metal.go
+++ b/internal/shaderir/metal/metal.go
@@ -313,10 +313,10 @@ func localVariableName(p *shaderir.Program, topBlock *shaderir.Block, idx int) s
 
 func (c *compileContext) initVariable(p *shaderir.Program, topBlock, block *shaderir.Block, index int, decl bool, level int) []string {
 	idt := strings.Repeat("\t", level+1)
+	name := localVariableName(p, topBlock, index)
 	t := p.LocalVariableType(topBlock, block, index)
 
 	var lines []string
-	name := localVariableName(p, topBlock, index)
 	if decl {
 		lines = append(lines, fmt.Sprintf("%s%s = %s;", idt, c.metalVarDecl(p, &t, name, false, false), c.metalVarInit(p, &t)))
 	} else {
@@ -412,6 +412,20 @@ func (c *compileContext) metalBlock(p *shaderir.Program, topBlock, block *shader
 			lines = append(lines, idt+"}")
 		case shaderir.Assign:
 			lines = append(lines, fmt.Sprintf("%s%s = %s;", idt, metalExpr(&s.Exprs[0]), metalExpr(&s.Exprs[1])))
+		case shaderir.Init:
+			init := true
+			if topBlock == p.VertexFunc.Block {
+				// In the vertex function, varying values are the output parameters.
+				// These values are represented as a struct and not needed to be initialized.
+				na := len(p.Attributes)
+				nv := len(p.Varyings)
+				if s.InitIndex < na+nv+1 {
+					init = false
+				}
+			}
+			if init {
+				lines = append(lines, c.initVariable(p, topBlock, block, s.InitIndex, false, level)...)
+			}
 		case shaderir.If:
 			lines = append(lines, fmt.Sprintf("%sif (%s) {", idt, metalExpr(&s.Exprs[0])))
 			lines = append(lines, c.metalBlock(p, topBlock, s.Blocks[0], level+1)...)
diff --git a/internal/shaderir/program.go b/internal/shaderir/program.go
index b30867385..8fe9ec241 100644
--- a/internal/shaderir/program.go
+++ b/internal/shaderir/program.go
@@ -73,6 +73,7 @@ type Stmt struct {
 	ForEnd      constant.Value
 	ForOp       Op
 	ForDelta    constant.Value
+	InitIndex   int
 }
 
 type StmtType int
@@ -81,6 +82,7 @@ const (
 	ExprStmt StmtType = iota
 	BlockStmt
 	Assign
+	Init
 	If
 	For
 	Continue