[PATCH] Microsoft Bitfields and VBases

[Warren Hunt]

Hi majnemer, rsmith,

Microsoft adds weird padding before virtual bases if the last field was a bitfield.

http://llvm-reviews.chandlerc.com/D2103

Files:
  lib/AST/RecordLayoutBuilder.cpp
  test/Layout/ms-x86-bitfields-vbases.cpp

Index: lib/AST/RecordLayoutBuilder.cpp
===================================================================
--- lib/AST/RecordLayoutBuilder.cpp
+++ lib/AST/RecordLayoutBuilder.cpp
@@ -2014,6 +2014,10 @@
 //   one.
 // * The last zero size virtual base may be placed at the end of the struct.
 //   and can potentially alias a zero sized type in the next struct.
+// * If the last field is a non-zero length bitfield and we have any virtual
+//   bases then some extra padding is added before the virtual bases for no
+//   obvious reason.
+
 
 namespace {
 struct MicrosoftRecordLayoutBuilder {
@@ -2515,6 +2519,11 @@
   llvm::SmallPtrSet<const CXXRecordDecl *, 2> HasVtordisp =
       computeVtorDispSet(RD);
 
+  // If the last field we laid out was a non-zero length bitfield then add some
+  // extra padding for no obvious reason.
+  if (LastFieldIsNonZeroWidthBitfield)
+    Size += CurrentBitfieldSize;
+
   // Iterate through the virtual bases and lay them out.
   for (CXXRecordDecl::base_class_const_iterator i = RD->vbases_begin(),
                                                 e = RD->vbases_end();
Index: test/Layout/ms-x86-bitfields-vbases.cpp
===================================================================
--- /dev/null
+++ test/Layout/ms-x86-bitfields-vbases.cpp
@@ -0,0 +1,84 @@
+// RUN: %clang_cc1 -fno-rtti -emit-llvm-only -triple i686-pc-win32 -fdump-record-layouts -fsyntax-only -cxx-abi microsoft %s 2>&1 \
+// RUN:            | FileCheck %s
+// RUN: %clang_cc1 -fno-rtti -emit-llvm-only -triple x86_64-pc-win32 -fdump-record-layouts -fsyntax-only -cxx-abi microsoft %s 2>/dev/null \
+// RUN:            | FileCheck %s -check-prefix CHECK-X64
+
+struct B0 { };
+
+struct A : virtual B0 { char a : 1; };
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK:    0 | struct A
+// CHECK:    0 |   (A vbtable pointer)
+// CHECK:    4 |   char a
+// CHECK:    9 |   struct B0 (virtual base) (empty)
+// CHECK:      | [sizeof=9, align=4
+// CHECK:      |  nvsize=8, nvalign=4]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64:    0 | struct A
+// CHECK-X64:    0 |   (A vbtable pointer)
+// CHECK-X64:    8 |   char a
+// CHECK-X64:   17 |   struct B0 (virtual base) (empty)
+// CHECK-X64:      | [sizeof=24, align=8
+// CHECK-X64:      |  nvsize=16, nvalign=8]
+
+struct B : virtual B0 { short a : 1; };
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK:    0 | struct B
+// CHECK:    0 |   (B vbtable pointer)
+// CHECK:    4 |   short a
+// CHECK:   10 |   struct B0 (virtual base) (empty)
+// CHECK:      | [sizeof=10, align=4
+// CHECK:      |  nvsize=8, nvalign=4]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64:    0 | struct B
+// CHECK-X64:    0 |   (B vbtable pointer)
+// CHECK-X64:    8 |   short a
+// CHECK-X64:   18 |   struct B0 (virtual base) (empty)
+// CHECK-X64:      | [sizeof=24, align=8
+// CHECK-X64:      |  nvsize=16, nvalign=8]
+
+struct C : virtual B0 { char a : 1; char : 0; };
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK:    0 | struct C
+// CHECK:    0 |   (C vbtable pointer)
+// CHECK:    4 |   char a
+// CHECK:    5 |   char
+// CHECK:    8 |   struct B0 (virtual base) (empty)
+// CHECK:      | [sizeof=8, align=4
+// CHECK:      |  nvsize=8, nvalign=4]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64:    0 | struct C
+// CHECK-X64:    0 |   (C vbtable pointer)
+// CHECK-X64:    8 |   char a
+// CHECK-X64:    9 |   char
+// CHECK-X64:   16 |   struct B0 (virtual base) (empty)
+// CHECK-X64:      | [sizeof=16, align=8
+// CHECK-X64:      |  nvsize=16, nvalign=8]
+
+struct D : virtual B0 { char a : 1; char b; };
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK:    0 | struct D
+// CHECK:    0 |   (D vbtable pointer)
+// CHECK:    4 |   char a
+// CHECK:    5 |   char b
+// CHECK:    8 |   struct B0 (virtual base) (empty)
+// CHECK:      | [sizeof=8, align=4
+// CHECK:      |  nvsize=8, nvalign=4]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64:    0 | struct D
+// CHECK-X64:    0 |   (D vbtable pointer)
+// CHECK-X64:    8 |   char a
+// CHECK-X64:    9 |   char b
+// CHECK-X64:   16 |   struct B0 (virtual base) (empty)
+// CHECK-X64:      | [sizeof=16, align=8
+// CHECK-X64:      |  nvsize=16, nvalign=8]
+
+int a[
+sizeof(A)+
+sizeof(B)+
+sizeof(C)+
+sizeof(D)];
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