Index: lib/CodeGen/TargetInfo.cpp
===================================================================
--- lib/CodeGen/TargetInfo.cpp
+++ lib/CodeGen/TargetInfo.cpp
@@ -1010,8 +1010,9 @@
   uint64_t Size = Context.getTypeSize(Ty);
 
   // For i386, type must be register sized.
-  // For the MCU ABI, it only needs to be <= 8-byte
-  if ((IsMCUABI && Size > 64) || (!IsMCUABI && !isRegisterSize(Size)))
+  // For the MCU ABI, it only needs to be positive <= 8-byte.
+  if ((IsMCUABI && (Size == 0 || Size > 64)) ||
+      (!IsMCUABI && !isRegisterSize(Size)))
    return false;
 
   if (Ty->isVectorType()) {
Index: test/CodeGen/mcu-struct-return.c
===================================================================
--- test/CodeGen/mcu-struct-return.c
+++ test/CodeGen/mcu-struct-return.c
@@ -0,0 +1,66 @@
+// RUN: %clang_cc1 -triple i386-pc-elfiamcu -emit-llvm %s -o - | FileCheck %s
+
+// Structure that is more than 8 byte.
+struct Big {
+  double a[10];
+};
+
+// Empty union with zero size must be returned via memory.
+union U1 {
+} u1;
+
+// Too large union (80 bytes) must be returned via memory.
+union U2 {
+  struct Big b;
+} u2;
+
+// Aggregate union, must be returned in register.
+union U3 {
+  int x;
+} u3;
+
+// Empty struct with zero size, must be returned via memory.
+struct S1 {
+} s1;
+
+// Aggregate struct, must be returend in register.
+struct S2 {
+  int x;
+} s2;
+
+// CHECK: [[UNION1_TYPE:%.+]] = type {}
+// CHECK: [[UNION2_TYPE:%.+]] = type { [[STRUCT_TYPE:%.+]] }
+// CHECK: [[STRUCT_TYPE]] = type { [10 x double] }
+// CHECK: [[UNION3_TYPE:%.+]] = type { i32 }
+// CHECK: [[STRUCT1_TYPE:%.+]] = type {}
+// CHECK: [[STRUCT2_TYPE:%.+]] = type { i32 }
+
+union U1 foo1() { return u1; }
+union U2 foo2() { return u2; }
+union U3 foo3() { return u3; }
+struct S1 bar1() { return s1; }
+struct S2 bar2() { return s2; }
+// CHECK: define void @foo1([[UNION1_TYPE]]* noalias sret %{{.+}})
+// CHECK: define void @foo2([[UNION2_TYPE]]* noalias sret %{{.+}})
+// CHECK: define i32 @foo3()
+// CHECK: define void @bar1([[STRUCT1_TYPE]]* noalias sret %{{.+}})
+// CHECK: define i32 @bar2()
+
+void run() {
+  union U1 x1 = foo1();
+  union U2 x2 = foo2();
+  union U3 x3 = foo3();
+  struct S1 y1 = bar1();
+  struct S2 y2 = bar2();
+
+  // CHECK: [[X1:%.+]] = alloca [[UNION1_TYPE]]
+  // CHECK: [[X2:%.+]] = alloca [[UNION2_TYPE]]
+  // CHECK: [[X3:%.+]] = alloca [[UNION3_TYPE]]
+  // CHECK: [[Y1:%.+]] = alloca [[STRUCT1_TYPE]]
+  // CHECK: [[Y2:%.+]] = alloca [[STRUCT2_TYPE]]
+  // CHECK: call void @foo1([[UNION1_TYPE]]* sret [[X1]])
+  // CHECK: call void @foo2([[UNION2_TYPE]]* sret [[X2]])
+  // CHECK: {{.+}} = call i32 @foo3()
+  // CHECK: call void @bar1([[STRUCT1_TYPE]]* sret [[Y1]])
+  // CHECK: {{.+}} = call i32 @bar2()
+}