Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -861,6 +861,8 @@
 def ProfileInstrOutOfDate : DiagGroup<"profile-instr-out-of-date">;
 def ProfileInstrUnprofiled : DiagGroup<"profile-instr-unprofiled">;
 
+def AtomicLibcall : DiagGroup<"atomic-libcall">;
+
 // AddressSanitizer frontent instrumentation remarks.
 def SanitizeAddressRemarks : DiagGroup<"sanitize-address">;
 
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -5120,6 +5120,8 @@
   "_Atomic cannot be applied to "
   "%select{incomplete |array |function |reference |atomic |qualified |}0type "
   "%1 %select{||||||which is not trivially copyable}0">;
+def warn_atomic_builtin_expands_to_libcall : Warning<
+  "atomic builtin expands to library call">, InGroup<AtomicLibcall>, DefaultIgnore;
 
 // Expressions.
 def ext_sizeof_alignof_function_type : Extension<
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -3035,6 +3035,14 @@
 
   ASTContext& Context = this->getASTContext();
 
+  // Warn if the atomic will expand into a library call if requested.
+  TypeInfo ValueTI = Context.getTypeInfo(ValType);
+  uint64_t Size = ValueTI.Width;
+  uint64_t Align = ValueTI.Align;
+  if (!Context.getTargetInfo().hasBuiltinAtomic(Size, Align)) {
+    Diag(DRE->getLocStart(), diag::warn_atomic_builtin_expands_to_libcall);
+  }
+
   // Create a new DeclRefExpr to refer to the new decl.
   DeclRefExpr* NewDRE = DeclRefExpr::Create(
       Context,
Index: test/Sema/atomic-libcall.c
===================================================================
--- /dev/null
+++ test/Sema/atomic-libcall.c
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -verify -fsyntax-only -triple=mips-mti-linux-gnu -Watomic-libcall
+
+// Test that larger than word size atomics are warned about.
+
+long long var;
+
+void foo(long long a) {
+  __sync_fetch_and_add(&var, 0, a); // expected-warning {{atomic builtin expands to library call}}
+}