Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -859,6 +859,8 @@
 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"profile-instr-out-of-date"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">;
 
@@ -880,4 +882,4 @@
 
 // A warning group for warnings about code that clang accepts when
 // compiling OpenCL C/C++ but which is not compatible with the SPIR spec.
-def SpirCompat : DiagGroup<"spir-compat">;
\ No newline at end of file
+def SpirCompat : DiagGroup<"spir-compat">;
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -5118,6 +5118,8 @@
   "_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 +3035,15 @@
 
   ASTContext& Context = this->getASTContext();
 
+  // Warn if the requested atomic will expand into a library call, as this can
+  // impact performance significantly if it requires kernel intervention or
+  // some other shared but hidden lock.
+  if (Context.getTypeSizeInChars(ValType) >
+      Context.toCharUnitsFromBits(
+          Context.getTargetInfo().getMaxAtomicInlineWidth())) {
+    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}}
+}