1 files changed, 28 insertions, 1 deletions
diff --git a/lib/AST/RecordLayoutBuilder.cpp b/lib/AST/RecordLayoutBuilder.cpp
index a9d43dfa80c54..de38bc9b64dd2 100644
--- a/lib/AST/RecordLayoutBuilder.cpp
+++ b/lib/AST/RecordLayoutBuilder.cpp
@@ -1751,7 +1751,34 @@ void ItaniumRecordLayoutBuilder::LayoutField(const FieldDecl *D,
       QualType T = Context.getBaseElementType(D->getType());
       if (const BuiltinType *BTy = T->getAs<BuiltinType>()) {
         CharUnits TypeSize = Context.getTypeSizeInChars(BTy);
-        if (TypeSize > FieldAlign)
+
+        if (!llvm::isPowerOf2_64(TypeSize.getQuantity())) {
+          assert(
+              !Context.getTargetInfo().getTriple().isWindowsMSVCEnvironment() &&
+              "Non PowerOf2 size in MSVC mode");
+          // Base types with sizes that aren't a power of two don't work
+          // with the layout rules for MS structs. This isn't an issue in
+          // MSVC itself since there are no such base data types there.
+          // On e.g. x86_32 mingw and linux, long double is 12 bytes though.
+          // Any structs involving that data type obviously can't be ABI
+          // compatible with MSVC regardless of how it is laid out.
+
+          // Since ms_struct can be mass enabled (via a pragma or via the
+          // -mms-bitfields command line parameter), this can trigger for
+          // structs that don't actually need MSVC compatibility, so we
+          // need to be able to sidestep the ms_struct layout for these types.
+
+          // Since the combination of -mms-bitfields together with structs
+          // like max_align_t (which contains a long double) for mingw is
+          // quite comon (and GCC handles it silently), just handle it
+          // silently there. For other targets that have ms_struct enabled
+          // (most probably via a pragma or attribute), trigger a diagnostic
+          // that defaults to an error.
+          if (!Context.getTargetInfo().getTriple().isWindowsGNUEnvironment())
+            Diag(D->getLocation(), diag::warn_npot_ms_struct);
+        }
+        if (TypeSize > FieldAlign &&
+            llvm::isPowerOf2_64(TypeSize.getQuantity()))
           FieldAlign = TypeSize;
       }
     }