diff --git a/clang/lib/CodeGen/SwiftCallingConv.cpp b/clang/lib/CodeGen/SwiftCallingConv.cpp
--- a/clang/lib/CodeGen/SwiftCallingConv.cpp
+++ b/clang/lib/CodeGen/SwiftCallingConv.cpp
@@ -93,11 +93,24 @@
     // Just add it all as opaque.
     addOpaqueData(begin, begin + CGM.getContext().getTypeSizeInChars(type));
 
-  // Everything else is scalar and should not convert as an LLVM aggregate.
+    // Atomic types.
+  } else if (const auto *atomicType = type->getAs<AtomicType>()) {
+    auto valueType = atomicType->getValueType();
+    auto atomicSize = CGM.getContext().getTypeSizeInChars(atomicType);
+    auto valueSize = CGM.getContext().getTypeSizeInChars(valueType);
+
+    addTypedData(atomicType->getValueType(), begin);
+
+    // Add atomic padding.
+    auto atomicPadding = atomicSize - valueSize;
+    if (atomicPadding > CharUnits::Zero())
+      addOpaqueData(begin + valueSize, begin + atomicSize);
+
+    // Everything else is scalar and should not convert as an LLVM aggregate.
   } else {
     // We intentionally convert as !ForMem because we want to preserve
     // that a type was an i1.
-    auto llvmType = CGM.getTypes().ConvertType(type);
+    auto *llvmType = CGM.getTypes().ConvertType(type);
     addTypedData(llvmType, begin);
   }
 }
diff --git a/clang/test/CodeGen/64bit-swiftcall.c b/clang/test/CodeGen/64bit-swiftcall.c
--- a/clang/test/CodeGen/64bit-swiftcall.c
+++ b/clang/test/CodeGen/64bit-swiftcall.c
@@ -10,6 +10,9 @@
 #define ERROR __attribute__((swift_error_result))
 #define CONTEXT __attribute__((swift_context))
 
+// CHECK-DAG: %struct.atomic_padded = type { { %struct.packed, [7 x i8] } }
+// CHECK-DAG: %struct.packed = type <{ i64, i8 }>
+//
 // CHECK: [[STRUCT2_RESULT:@.*]] = private {{.*}} constant [[STRUCT2_TYPE:%.*]] { i32 0, i8 0, i8 undef, i8 0, i32 0, i32 0 }
 
 /*****************************************************************************/
@@ -1042,3 +1045,27 @@
   // CHECK-NOT: call void @llvm.lifetime.
   take_int5(return_int5());
 }
+
+typedef struct {
+  unsigned long long a;
+  unsigned long long b;
+} double_word;
+
+typedef struct {
+  _Atomic(double_word) a;
+} atomic_double_word;
+
+// CHECK-LABEL: use_atomic(i64 %0, i64 %1)
+SWIFTCALL void use_atomic(atomic_double_word a) {}
+
+typedef struct {
+  unsigned long long a;
+  unsigned char b;
+} __attribute__((packed)) packed;
+
+typedef struct {
+  _Atomic(packed) a;
+} atomic_padded;
+
+// CHECK-LABEL: use_atomic_padded(i64 %0, i64 %1)
+SWIFTCALL void use_atomic_padded(atomic_padded a) {}