Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -78,6 +78,12 @@
   standard-layout if all base classes and the first data member (or bit-field)
   can be laid out at offset zero.
 
+- Clang's handling of the GCC ``packed`` class attribute in C++ has been fixed
+  to apply only to non-static data members and not to base classes. This fixes
+  an ABI difference between Clang and GCC, but creates an ABI difference between
+  Clang 7 and earlier versions. The old behavior can be restored by setting
+  ``-fclang-abi-compat`` to ``6`` or earlier.
+
 - ...
 
 New Compiler Flags
Index: lib/AST/RecordLayoutBuilder.cpp
===================================================================
--- lib/AST/RecordLayoutBuilder.cpp
+++ lib/AST/RecordLayoutBuilder.cpp
@@ -967,7 +967,7 @@
 
 void ItaniumRecordLayoutBuilder::EnsureVTablePointerAlignment(
     CharUnits UnpackedBaseAlign) {
-  CharUnits BaseAlign = (Packed) ? CharUnits::One() : UnpackedBaseAlign;
+  CharUnits BaseAlign = Packed ? CharUnits::One() : UnpackedBaseAlign;
 
   // The maximum field alignment overrides base align.
   if (!MaxFieldAlignment.isZero()) {
@@ -1175,9 +1175,14 @@
       HasExternalLayout = External.getExternalVBaseOffset(Base->Class, Offset);
   }
   
+  // Clang <= 6 incorrectly applied the 'packed' attribute to base classes.
+  // Per GCC's documentation, it only applies to non-static data members.
   CharUnits UnpackedBaseAlign = Layout.getNonVirtualAlignment();
-  CharUnits BaseAlign = (Packed) ? CharUnits::One() : UnpackedBaseAlign;
- 
+  CharUnits BaseAlign = (Packed && Context.getLangOpts().getClangABICompat() <=
+                                       LangOptions::ClangABI::Ver6)
+                            ? CharUnits::One()
+                            : UnpackedBaseAlign;
+
   // If we have an empty base class, try to place it at offset 0.
   if (Base->Class->isEmpty() &&
       (!HasExternalLayout || Offset == CharUnits::Zero()) &&
Index: test/CodeGenCXX/alignment.cpp
===================================================================
--- test/CodeGenCXX/alignment.cpp
+++ test/CodeGenCXX/alignment.cpp
@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 %s -emit-llvm -o - -triple=x86_64-apple-darwin10 | FileCheck %s
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple=x86_64-apple-darwin10 | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-NOCOMPAT
+// RUN: %clang_cc1 %s -emit-llvm -o - -triple=x86_64-apple-darwin10 -fclang-abi-compat=6.0 | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-V6COMPAT
 
 extern int int_source();
 extern void int_sink(int x);
@@ -54,11 +55,13 @@
     // CHECK: [[B_P:%.*]] = bitcast i8* [[T1]] to [[B]]*
     // CHECK: [[FIELD_P:%.*]] = bitcast [[B]]* [[B_P]] to i8*
     // CHECK: [[TRUNC:%.*]] = trunc i32 [[CALL]] to i8
-    // CHECK: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 4
     // CHECK: [[T0:%.*]] = and i8 [[TRUNC]], 3
     // CHECK: [[T1:%.*]] = and i8 [[OLD_VALUE]], -4
     // CHECK: [[T2:%.*]] = or i8 [[T1]], [[T0]]
-    // CHECK: store i8 [[T2]], i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: store i8 [[T2]], i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: store i8 [[T2]], i8* [[FIELD_P]], align 4
     c.onebit = int_source();
 
     // CHECK: [[C_P:%.*]] = load [[C]]*, [[C]]**
@@ -66,7 +69,8 @@
     // CHECK: [[T1:%.*]] = getelementptr inbounds i8, i8* [[T0]], i64 8
     // CHECK: [[B_P:%.*]] = bitcast i8* [[T1]] to [[B]]*
     // CHECK: [[FIELD_P:%.*]] = bitcast [[B]]* [[B_P]] to i8*
-    // CHECK: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 4
     // CHECK: [[T0:%.*]] = shl i8 [[VALUE]], 6
     // CHECK: [[T1:%.*]] = ashr i8 [[T0]], 6
     // CHECK: [[T2:%.*]] = sext i8 [[T1]] to i32
@@ -83,11 +87,13 @@
     // CHECK: [[B_P:%.*]] = bitcast i8* [[T1]] to [[B]]*
     // CHECK: [[FIELD_P:%.*]] = bitcast [[B]]* [[B_P]] to i8*
     // CHECK: [[TRUNC:%.*]] = trunc i32 [[CALL]] to i8
-    // CHECK: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 4
     // CHECK: [[T0:%.*]] = and i8 [[TRUNC]], 3
     // CHECK: [[T1:%.*]] = and i8 [[OLD_VALUE]], -4
     // CHECK: [[T2:%.*]] = or i8 [[T1]], [[T0]]
-    // CHECK: store i8 [[T2]], i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: store i8 [[T2]], i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: store i8 [[T2]], i8* [[FIELD_P]], align 4
     c->onebit = int_source();
 
     // CHECK: [[C_P:%.*]] = load [[C:%.*]]*, [[C]]**
@@ -95,7 +101,8 @@
     // CHECK: [[T1:%.*]] = getelementptr inbounds i8, i8* [[T0]], i64 8
     // CHECK: [[B_P:%.*]] = bitcast i8* [[T1]] to [[B:%.*]]*
     // CHECK: [[FIELD_P:%.*]] = bitcast [[B]]* [[B_P]] to i8*
-    // CHECK: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 4
     // CHECK: [[T0:%.*]] = shl i8 [[VALUE]], 6
     // CHECK: [[T1:%.*]] = ashr i8 [[T0]], 6
     // CHECK: [[T2:%.*]] = sext i8 [[T1]] to i32
@@ -107,7 +114,8 @@
   // in an alignment-2 variable.
   // CHECK-LABEL: @_ZN5test01dEv
   void d() {
-    // CHECK: [[C_P:%.*]] = alloca [[C:%.*]], align 2
+    // CHECK-V6COMPAT: [[C_P:%.*]] = alloca [[C:%.*]], align 2
+    // CHECK-NOCOMPAT: [[C_P:%.*]] = alloca [[C:%.*]], align 4
     C c;
 
     // CHECK: [[CALL:%.*]] = call i32 @_Z10int_sourcev()
@@ -116,18 +124,21 @@
     // CHECK: [[B_P:%.*]] = bitcast i8* [[T1]] to [[B]]*
     // CHECK: [[FIELD_P:%.*]] = bitcast [[B]]* [[B_P]] to i8*
     // CHECK: [[TRUNC:%.*]] = trunc i32 [[CALL]] to i8
-    // CHECK: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: [[OLD_VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 4
     // CHECK: [[T0:%.*]] = and i8 [[TRUNC]], 3
     // CHECK: [[T1:%.*]] = and i8 [[OLD_VALUE]], -4
     // CHECK: [[T2:%.*]] = or i8 [[T1]], [[T0]]
-    // CHECK: store i8 [[T2]], i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: store i8 [[T2]], i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: store i8 [[T2]], i8* [[FIELD_P]], align 4
     c.onebit = int_source();
 
     // CHECK: [[T0:%.*]] = bitcast [[C]]* [[C_P]] to i8*
     // CHECK: [[T1:%.*]] = getelementptr inbounds i8, i8* [[T0]], i64 8
     // CHECK: [[B_P:%.*]] = bitcast i8* [[T1]] to [[B:%.*]]*
     // CHECK: [[FIELD_P:%.*]] = bitcast [[B]]* [[B_P]] to i8*
-    // CHECK: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-V6COMPAT: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 2
+    // CHECK-NOCOMPAT: [[VALUE:%.*]] = load i8, i8* [[FIELD_P]], align 4
     // CHECK: [[T0:%.*]] = shl i8 [[VALUE]], 6
     // CHECK: [[T1:%.*]] = ashr i8 [[T0]], 6
     // CHECK: [[T2:%.*]] = sext i8 [[T1]] to i32
Index: test/SemaCXX/class-layout.cpp
===================================================================
--- test/SemaCXX/class-layout.cpp
+++ test/SemaCXX/class-layout.cpp
@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++98 -Wno-inaccessible-base
 // RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -fsyntax-only -verify -std=c++11 -Wno-inaccessible-base -fclang-abi-compat=6 -DCLANG_ABI_COMPAT=6
 // expected-no-diagnostics
 
 #define SA(n, p) int a##n[(p) ? 1 : -1]
@@ -570,3 +571,19 @@
   SA(0, sizeof(might_use_tail_padding) == 80);
 }
 } // namespace PR16537
+
+namespace PR37275 {
+  struct X { char c; };
+
+  struct A { int n; };
+  _Static_assert(_Alignof(A) == _Alignof(int), "");
+
+  struct __attribute__((packed)) B : X, A {};
+#if defined(CLANG_ABI_COMPAT) && CLANG_ABI_COMPAT <= 6
+  _Static_assert(_Alignof(B) == 1, "");
+  _Static_assert(__builtin_offsetof(B, n) == 1, "");
+#else
+  _Static_assert(_Alignof(B) == _Alignof(int), "");
+  _Static_assert(__builtin_offsetof(B, n) == 4, "");
+#endif
+}