Improve support of PDB as an external layout source

Summary:
This patch improves support of PDB as an external layout source
in the next cases:

- Multiple non-virtual inheritance from packed base classes. When using
  external layout, there's no need to align `NonVirtualSize` of a base class.
  It may cause an overlapping when the next base classes will be layouted
  (but there is a slightly different case in the test because I can't find
  a way to specify a base offset);
- Support of nameless structs and unions. There is no info about nameless child
  structs and unions in Microsoft cl-emitted PDBs. Instead all its fields
  are just treated as outer structure's (union's) fields. This also causes
  a fields overlapping, and makes it possible for unions to have fields located
  at a non-zero offset.

Reviewers: rsmith, zturner, rnk, mstorsjo, majnemer

Reviewed By: rnk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D49871

llvm-svn: 338353

1 files changed, 33 insertions, 0 deletions

diff --git a/clang/test/CodeGenCXX/override-layout-nameless-struct-union.cpp b/clang/test/CodeGenCXX/override-layout-nameless-struct-union.cpp
new file mode 100644
index 00000000000..acc1ff715c6
--- /dev/null
+++ b/clang/test/CodeGenCXX/override-layout-nameless-struct-union.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -w -fdump-record-layouts-simple -foverride-record-layout=%S/Inputs/override-layout-nameless-struct-union.layout %s | FileCheck %s
+
+// CHECK: Type: struct S
+// CHECK:   Size:64
+// CHECK:   Alignment:32
+// CHECK:   FieldOffsets: [0, 32, 32]
+struct S {
+  short _s;
+//union {
+    int _su0;
+    char _su1;
+//};
+};
+
+// CHECK: Type: union U
+// CHECK:   Size:96
+// CHECK:   Alignment:32
+// CHECK:   FieldOffsets: [0, 0, 32, 64, 68, 73]
+union U {
+  short _u;
+//struct {
+    char _us0;
+    int _us1;
+    unsigned _us20 : 4;
+    unsigned _us21 : 5;
+    unsigned _us22 : 6;
+//};
+};
+
+void use_structs() {
+  S ss[sizeof(S)];
+  U us[sizeof(U)];
+}