Index: clang/include/clang/Basic/DiagnosticFrontendKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticFrontendKinds.td
+++ clang/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -270,8 +270,16 @@
   "ifunc resolver function must return a pointer">;
 
 def warn_atomic_op_misaligned : Warning<
-  "%select{large|misaligned}0 atomic operation may incur "
-  "significant performance penalty">, InGroup<DiagGroup<"atomic-alignment">>;
+  "misaligned atomic operation may incur "
+  "significant performance penalty "
+  "(the expected (%0 bytes) exceeds the actual alignment (%1 bytes)">,
+  InGroup<AtomicAlignment>;
+
+def warn_atomic_op_oversized : Warning<
+  "large atomic operation may incur "
+  "significant performance penalty "
+  "(the access size (%0 bytes) exceeds the max lock-free size (%1  bytes))">,
+InGroup<AtomicAlignment>;
 
 def warn_alias_with_section : Warning<
   "%select{alias|ifunc}1 will not be in section '%0' but in the same section "
Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -699,6 +699,7 @@
 def Reorder : DiagGroup<"reorder", [ReorderCtor, ReorderInitList]>;
 def UndeclaredSelector : DiagGroup<"undeclared-selector">;
 def ImplicitAtomic : DiagGroup<"implicit-atomic-properties">;
+def AtomicAlignment : DiagGroup<"atomic-alignment">;
 def CustomAtomic : DiagGroup<"custom-atomic-properties">;
 def AtomicProperties : DiagGroup<"atomic-properties",
                                  [ImplicitAtomic, CustomAtomic]>;
Index: clang/lib/CodeGen/CGAtomic.cpp
===================================================================
--- clang/lib/CodeGen/CGAtomic.cpp
+++ clang/lib/CodeGen/CGAtomic.cpp
@@ -807,10 +807,20 @@
   bool Oversized = getContext().toBits(sizeChars) > MaxInlineWidthInBits;
   bool Misaligned = (Ptr.getAlignment() % sizeChars) != 0;
   bool UseLibcall = Misaligned | Oversized;
+  CharUnits MaxInlineWidth =
+      getContext().toCharUnitsFromBits(MaxInlineWidthInBits);
 
-  if (UseLibcall) {
-    CGM.getDiags().Report(E->getBeginLoc(), diag::warn_atomic_op_misaligned)
-        << !Oversized;
+  DiagnosticsEngine &Diags = CGM.getDiags();
+
+  if (Misaligned) {
+    Diags.Report(E->getBeginLoc(), diag::warn_atomic_op_misaligned)
+        << (int)sizeChars.getQuantity()
+        << (int)Ptr.getAlignment().getQuantity();
+  }
+
+  if (Oversized) {
+    Diags.Report(E->getBeginLoc(), diag::warn_atomic_op_oversized)
+        << (int)sizeChars.getQuantity() << (int)MaxInlineWidth.getQuantity();
   }
 
   llvm::Value *Order = EmitScalarExpr(E->getOrder());