Index: cfe/trunk/lib/CodeGen/TargetInfo.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/TargetInfo.cpp
+++ cfe/trunk/lib/CodeGen/TargetInfo.cpp
@@ -364,7 +364,7 @@
 static bool isEmptyRecord(ASTContext &Context, QualType T, bool AllowArrays) {
   const RecordType *RT = T->getAs<RecordType>();
   if (!RT)
-    return 0;
+    return false;
   const RecordDecl *RD = RT->getDecl();
   if (RD->hasFlexibleArrayMember())
     return false;
@@ -1119,6 +1119,10 @@
     if (!IsRetSmallStructInRegABI && !RetTy->isAnyComplexType())
       return getIndirectReturnResult(RetTy, State);
 
+    // Ignore empty structs/unions.
+    if (isEmptyRecord(getContext(), RetTy, true))
+      return ABIArgInfo::getIgnore();
+
     // Small structures which are register sized are generally returned
     // in a register.
     if (shouldReturnTypeInRegister(RetTy, getContext())) {
Index: cfe/trunk/test/CodeGen/mcu-struct-return.c
===================================================================
--- cfe/trunk/test/CodeGen/mcu-struct-return.c
+++ cfe/trunk/test/CodeGen/mcu-struct-return.c
@@ -0,0 +1,70 @@
+// 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 as void.
+union U1 {
+} u1;
+
+// Too large union (80 bytes) must be returned via memory.
+union U2 {
+  struct Big b;
+} u2;
+
+// Must be returned in register.
+union U3 {
+  int x;
+} u3;
+
+// Empty struct with zero size, must be returned as void.
+struct S1 {
+} s1;
+
+// 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; }
+struct S1 bar3(union U1 u) { return s1; }
+// CHECK: define void @foo1()
+// CHECK: define void @foo2([[UNION2_TYPE]]* noalias sret %{{.+}})
+// CHECK: define i32 @foo3()
+// CHECK: define void @bar1()
+// CHECK: define i32 @bar2()
+// CHECK: define void @bar3()
+
+void run() {
+  union U1 x1 = foo1();
+  union U2 x2 = foo2();
+  union U3 x3 = foo3();
+  struct S1 y1 = bar1();
+  struct S2 y2 = bar2();
+  struct S1 y3 = bar3(x1);
+
+  // 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()
+  // CHECK: call void @foo2([[UNION2_TYPE]]* sret [[X2]])
+  // CHECK: {{.+}} = call i32 @foo3()
+  // CHECK: call void @bar1()
+  // CHECK: {{.+}} = call i32 @bar2()
+  // CHECK: call void @bar3()
+}