[Clang][MS] Remove assertion on BaseOffset can't be smaller than Size.

[zequanwu]

Summary

This assertion triggered when we have two base classes sharing the same offset
and the first base is empty and the second class is non-empty.
Remove it for correctness.

I can't add a test case for this because -foverride-record-layout doesn't read
base class info at all. I can add that support later for testing if needed.

[rnk]

Please do extend the simple record layout dumping format. I think it should be relatively simple to add string dumping and parsing to clang/lib/Frontend/LayoutOverrideSource.cpp.

[zequanwu]

Extend the output and reading of -fdump-record-layouts-simple.
Add a test case.

[rnk]

clang/lib/AST/RecordLayoutBuilder.cpp

3727

Rather than iterating the map and sorting afterwards, IMO it's better to iterate RD->bases() and look each base up in the map and use that offset. It guarantees they'll be in the right order, even if the offsets aren't in ascending order (although due to the ABI, they usually are).

3739

Ditto here, better to iterate over vbases().

clang/lib/Frontend/LayoutOverrideSource.cpp

38

Can you pass in ULL directly here, rather than making a local and copying it out?

230

I think it's better to iterate vbases() directly, and then iterate bases() and skip bases that are virtual.

clang/test/CodeGenCXX/override-layout-ms.cpp

1

Go ahead and add a second RUN line without the "override-record-layout" flag to confirm the layout is correct.

[zequanwu]

Address comments.

[rnk]

lgtm, thanks!

[leonardchan]

Hi. I think this caused the override-layout.cpp test to fail on our windows builder (https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8778279637538184401/+/u/clang/test/stdout?format=raw):

FAIL: Clang :: CodeGenCXX/override-layout.cpp (7816 of 18732)
******************** TEST 'Clang :: CodeGenCXX/override-layout.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe -cc1 -internal-isystem c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include -nostdsysteminc -std=c++14 -w -fdump-record-layouts-simple C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp > C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.layouts
: 'RUN: at line 2';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe -cc1 -internal-isystem c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include -nostdsysteminc -std=c++14 -w -fdump-record-layouts-simple C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp > C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.before
: 'RUN: at line 3';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe -cc1 -internal-isystem c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include -nostdsysteminc -std=c++14 -w -DPACKED= -DALIGNED16= -fdump-record-layouts-simple -foverride-record-layout=C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.layouts C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp > C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
: 'RUN: at line 4';   diff -u C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.before C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
: 'RUN: at line 5';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\filecheck.exe --check-prefixes=CHECK,PRE17 C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp < C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
: 'RUN: at line 7';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe -cc1 -internal-isystem c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include -nostdsysteminc -std=c++17 -w -fdump-record-layouts-simple C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp > C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.layouts
: 'RUN: at line 8';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe -cc1 -internal-isystem c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include -nostdsysteminc -std=c++17 -w -fdump-record-layouts-simple C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp > C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.before
: 'RUN: at line 9';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe -cc1 -internal-isystem c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include -nostdsysteminc -std=c++17 -w -DPACKED= -DALIGNED16= -fdump-record-layouts-simple -foverride-record-layout=C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.layouts C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp > C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
: 'RUN: at line 10';   diff -u C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.before C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
: 'RUN: at line 11';   c:\b\s\w\ir\x\w\staging\llvm_build\bin\filecheck.exe --check-prefixes=CHECK,CXX17 C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp < C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
--
Exit Code: 1

Command Output (stdout):
--
$ ":" "RUN: at line 1"
$ "c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe" "-cc1" "-internal-isystem" "c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include" "-nostdsysteminc" "-std=c++14" "-w" "-fdump-record-layouts-simple" "C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp"
$ ":" "RUN: at line 2"
$ "c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe" "-cc1" "-internal-isystem" "c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include" "-nostdsysteminc" "-std=c++14" "-w" "-fdump-record-layouts-simple" "C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp"
$ ":" "RUN: at line 3"
$ "c:\b\s\w\ir\x\w\staging\llvm_build\bin\clang.exe" "-cc1" "-internal-isystem" "c:\b\s\w\ir\x\w\staging\llvm_build\lib\clang\17\include" "-nostdsysteminc" "-std=c++14" "-w" "-DPACKED=" "-DALIGNED16=" "-fdump-record-layouts-simple" "-foverride-record-layout=C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.layouts" "C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\CodeGenCXX\override-layout.cpp"
$ ":" "RUN: at line 4"
$ "diff" "-u" "C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.before" "C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after"
# command output:
--- C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.before
+++ C:\b\s\w\ir\x\w\staging\llvm_build\tools\clang\test\CodeGenCXX\Output\override-layout.cpp.tmp.after
@@ -116,5 +116,5 @@
   Size:512
   Alignment:128
   BaseOffsets: [0]>
-  VBaseOffsets: [48]>
+  VBaseOffsets: [33]>
   FieldOffsets: [256]>

error: command failed with exit status: 1

--

Would you be able to send out a fix or revert? Thanks.

[thakis]

Also on http://45.33.8.238/win/79874/step_7.txt

Reverted in 9c560350dd57d0dcb849c5915fa7c50b139ce671 for now.

[zequanwu]

In D152472#4423160, @thakis wrote:

Also on http://45.33.8.238/win/79874/step_7.txt

Reverted in 9c560350dd57d0dcb849c5915fa7c50b139ce671 for now.

Thanks for reporting. Relanded at https://reviews.llvm.org/rG879e88693338657aec092db749ddfcb582c65491 with fixes.

[thakis]

This is still breaking check-clang on windows: http://45.33.8.238/win/79908/step_7.txt

Please take a look and revert for now if it takes a while to fix.

[zequanwu]

In D152472#4425713, @thakis wrote:

This is still breaking check-clang on windows: http://45.33.8.238/win/79908/step_7.txt

Please take a look and revert for now if it takes a while to fix.

Sorry, I thought I applied the fix, but that was on another machine. The actually fix is in https://reviews.llvm.org/rG9d910b1073198716f85e5a7e7e9c8fd1f24d4c60.

clang/include/clang/Frontend/LayoutOverrideSource.h

class LayoutOverrideSource : public ExternalASTSource {
     /// The layout of a given record.
     struct Layout {
       /// The size of the record.
       uint64_t Size;
 
       /// The alignment of the record.
       uint64_t Align;
 
+      /// The offsets of non-virtual base classes in the record.
+      SmallVector<CharUnits, 8> BaseOffsets;
+
+      /// The offsets of virtual base classes in the record.
+      SmallVector<CharUnits, 8> VBaseOffsets;
+
       /// The offsets of the fields, in source order.
       SmallVector<uint64_t, 8> FieldOffsets;
     };
 
     /// The set of layouts that will be overridden.
     llvm::StringMap<Layout> Layouts;
 
   public:

clang/lib/AST/RecordLayoutBuilder.cpp

if (PreviousBaseLayout && PreviousBaseLayout->endsWithZeroSizedObject() &&
     Size++;
   ElementInfo Info = getAdjustedElementInfo(BaseLayout);
   CharUnits BaseOffset;
 
   // Respect the external AST source base offset, if present.
   bool FoundBase = false;
   if (UseExternalLayout) {
     FoundBase = External.getExternalNVBaseOffset(BaseDecl, BaseOffset);
-    if (FoundBase) {
-      assert(BaseOffset >= Size && "base offset already allocated");
+    if (BaseOffset > Size) {
       Size = BaseOffset;
     }
   }
 
   if (!FoundBase) {
     if (MDCUsesEBO && BaseDecl->isEmpty()) {
       assert(BaseLayout.getNonVirtualSize() == CharUnits::Zero());
       BaseOffset = CharUnits::Zero();
void ASTContext::DumpRecordLayout(const RecordDecl *RD, raw_ostream &OS,
   OS << "<ASTRecordLayout\n";
   OS << "  Size:" << toBits(Info.getSize()) << "\n";
   if (!isMsLayout(*this))
     OS << "  DataSize:" << toBits(Info.getDataSize()) << "\n";
   OS << "  Alignment:" << toBits(Info.getAlignment()) << "\n";
   if (Target->defaultsToAIXPowerAlignment())
     OS << "  PreferredAlignment:" << toBits(Info.getPreferredAlignment())
        << "\n";
+  if (Info.CXXInfo) {
+    SmallVector<uint64_t, 4> Bases;
+    for (auto Base: Info.CXXInfo->BaseOffsets) {
+      Bases.push_back(Base.second.getQuantity());
+    }
+    llvm::sort(Bases);
+    OS << "  BaseOffsets: [";
+    for (unsigned i = 0, e = Bases.size(); i != e; ++i) {
+      if (i)
+        OS << ", ";
+      OS << Bases[i];
+    }
+    OS << "]>\n";
+    SmallVector<uint64_t, 4> VBases;
+    for (auto VBase: Info.CXXInfo->VBaseOffsets) {
+      VBases.push_back(VBase.second.VBaseOffset.getQuantity());
+    }
+    llvm::sort(VBases);
+    OS << "  VBaseOffsets: [";
+    for (unsigned i = 0, e = VBases.size(); i != e; ++i) {
+      if (i)
+        OS << ", ";
+      OS << VBases[i];
+    }
+    OS << "]>\n";
+  }
   OS << "  FieldOffsets: [";
   for (unsigned i = 0, e = Info.getFieldCount(); i != e; ++i) {
     if (i)
       OS << ", ";
     OS << Info.getFieldOffset(i);
   }
   OS << "]>\n";
 }

clang/lib/Frontend/LayoutOverrideSource.cpp

 //===--- LayoutOverrideSource.cpp --Override Record Layouts ---------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 #include "clang/Frontend/LayoutOverrideSource.h"
 #include "clang/AST/Decl.h"
+#include "clang/AST/DeclCXX.h"
 #include "clang/Basic/CharInfo.h"
 #include "llvm/Support/raw_ostream.h"
 #include <fstream>
 #include <string>
 
 using namespace clang;
 
 /// Parse a simple identifier.
 static std::string parseName(StringRef S) {
   if (S.empty() || !isAsciiIdentifierStart(S[0]))
     return "";
 
   unsigned Offset = 1;
   while (Offset < S.size() && isAsciiIdentifierContinue(S[Offset]))
     ++Offset;
 
   return S.substr(0, Offset).str();
 }
 
+/// Parse an unsigned integer and move S to the next non-digit character.
+static bool parseUnsigned(StringRef &S, unsigned long long &ULL) {
+  if (S.empty() || !isDigit(S[0]))
+    return false;
+  unsigned Idx = 1;
+  while (Idx < S.size() && isDigit(S[Idx]))
+    ++Idx;
+  unsigned long long Offset = 0;
+  (void)S.substr(0, Idx).getAsInteger(10, Offset);
+  S = S.substr(Idx);
+  ULL = Offset;
+  return true;
+}
+
 LayoutOverrideSource::LayoutOverrideSource(StringRef Filename) {
   std::ifstream Input(Filename.str().c_str());
   if (!Input.is_open())
     return;
 
   // Parse the output of -fdump-record-layouts.
   std::string CurrentType;
   Layout CurrentLayout;
while (Input.good()) {
 
     // Check for the size of the type.
     StringRef::size_type Pos = LineStr.find(" Size:");
     if (Pos != StringRef::npos) {
       // Skip past the " Size:" prefix.
       LineStr = LineStr.substr(Pos + strlen(" Size:"));
 
       unsigned long long Size = 0;
-      (void)LineStr.getAsInteger(10, Size);
+      if (parseUnsigned(LineStr, Size))
         CurrentLayout.Size = Size;
       continue;
     }
 
     // Check for the alignment of the type.
     Pos = LineStr.find("Alignment:");
     if (Pos != StringRef::npos) {
       // Skip past the "Alignment:" prefix.
       LineStr = LineStr.substr(Pos + strlen("Alignment:"));
 
       unsigned long long Alignment = 0;
-      (void)LineStr.getAsInteger(10, Alignment);
+      if (parseUnsigned(LineStr, Alignment))
         CurrentLayout.Align = Alignment;
       continue;
     }
 
-    // Check for the size/alignment of the type.
+    // Check for the size/alignment of the type. The number follows "size=" or
+    // "align=" indicates number of bytes.
     Pos = LineStr.find("sizeof=");
     if (Pos != StringRef::npos) {
       /* Skip past the sizeof= prefix. */
       LineStr = LineStr.substr(Pos + strlen("sizeof="));
 
       // Parse size.
       unsigned long long Size = 0;
-      (void)LineStr.getAsInteger(10, Size);
-      CurrentLayout.Size = Size;
+      if (parseUnsigned(LineStr, Size))
+        CurrentLayout.Size = Size * 8;
 
       Pos = LineStr.find("align=");
       if (Pos != StringRef::npos) {
         /* Skip past the align= prefix. */
         LineStr = LineStr.substr(Pos + strlen("align="));
 
         // Parse alignment.
         unsigned long long Alignment = 0;
-        (void)LineStr.getAsInteger(10, Alignment);
-        CurrentLayout.Align = Alignment;
+        if (parseUnsigned(LineStr, Alignment))
+          CurrentLayout.Align = Alignment * 8;
       }
 
       continue;
     }
 
     // Check for the field offsets of the type.
     Pos = LineStr.find("FieldOffsets: [");
-    if (Pos == StringRef::npos)
-      continue;
-
+    if (Pos != StringRef::npos) {
       LineStr = LineStr.substr(Pos + strlen("FieldOffsets: ["));
       while (!LineStr.empty() && isDigit(LineStr[0])) {
-      // Parse this offset.
-      unsigned Idx = 1;
-      while (Idx < LineStr.size() && isDigit(LineStr[Idx]))
-        ++Idx;
+        unsigned long long Offset = 0;
+        if (parseUnsigned(LineStr, Offset))
+          CurrentLayout.FieldOffsets.push_back(Offset);
+
+        // Skip over this offset, the following comma, and any spaces.
+        LineStr = LineStr.substr(1);
+        while (!LineStr.empty() && isWhitespace(LineStr[0]))
+          LineStr = LineStr.substr(1);
+      }
+    }
 
+    // Check for the base offsets.
+    Pos = LineStr.find("BaseOffsets: [");
+    if (Pos != StringRef::npos) {
+      LineStr = LineStr.substr(Pos + strlen("BaseOffsets: ["));
+      while (!LineStr.empty() && isDigit(LineStr[0])) {
         unsigned long long Offset = 0;
-      (void)LineStr.substr(0, Idx).getAsInteger(10, Offset);
+        if (parseUnsigned(LineStr, Offset))
+          CurrentLayout.BaseOffsets.push_back(CharUnits::fromQuantity(Offset));
 
-      CurrentLayout.FieldOffsets.push_back(Offset);
+        // Skip over this offset, the following comma, and any spaces.
+        LineStr = LineStr.substr(1);
+        while (!LineStr.empty() && isWhitespace(LineStr[0]))
+          LineStr = LineStr.substr(1);
+      }
+    }
+
+    // Check for the virtual base offsets.
+    Pos = LineStr.find("VBaseOffsets: [");
+    if (Pos != StringRef::npos) {
+      LineStr = LineStr.substr(Pos + strlen("VBaseOffsets: ["));
+      while (!LineStr.empty() && isDigit(LineStr[0])) {
+        unsigned long long Offset = 0;
+        if (parseUnsigned(LineStr, Offset))
+          CurrentLayout.VBaseOffsets.push_back(CharUnits::fromQuantity(Offset));
 
         // Skip over this offset, the following comma, and any spaces.
-      LineStr = LineStr.substr(Idx + 1);
+        LineStr = LineStr.substr(1);
         while (!LineStr.empty() && isWhitespace(LineStr[0]))
           LineStr = LineStr.substr(1);
       }
     }
+  }
 
   // Flush the last type/layout, if there is one.
   if (!CurrentType.empty())
     Layouts[CurrentType] = CurrentLayout;
 }
 
 bool
 LayoutOverrideSource::layoutRecordType(const RecordDecl *Record,
for (RecordDecl::field_iterator F = Record->field_begin(),
 
     FieldOffsets[*F] = Known->second.FieldOffsets[NumFields];
   }
 
   // Wrong number of fields.
   if (NumFields != Known->second.FieldOffsets.size())
     return false;
 
+  // Provide base offsets.
+  if (const auto *RD = dyn_cast<CXXRecordDecl>(Record)) {
+    unsigned NumNB = 0;
+    unsigned NumVB = 0;
+    for (const auto &I : RD->bases()) {
+      const CXXRecordDecl *Base = I.getType()->getAsCXXRecordDecl();
+      if (I.isVirtual()) {
+        if (NumVB >= Known->second.VBaseOffsets.size())
+          continue;
+        VirtualBaseOffsets[Base] = Known->second.VBaseOffsets[NumVB++];
+      } else {
+        if (NumNB >= Known->second.BaseOffsets.size())
+          continue;
+        BaseOffsets[Base] = Known->second.BaseOffsets[NumNB++];
+      }
+    }
+  }
+
   Size = Known->second.Size;
   Alignment = Known->second.Align;
   return true;
 }
 
 LLVM_DUMP_METHOD void LayoutOverrideSource::dump() {
   raw_ostream &OS = llvm::errs();
   for (llvm::StringMap<Layout>::iterator L = Layouts.begin(),

clang/test/CodeGenCXX/Inputs/override-layout-ms.layout

+*** Dumping AST Record Layout
+Type: struct E1
+
+Layout: <ASTRecordLayout
+  Size:8
+  Alignment:8
+  BaseOffsets: []>
+  VBaseOffsets: []>
+  FieldOffsets: []>
+
+*** Dumping AST Record Layout
+Type: struct Mid
+
+Layout: <ASTRecordLayout
+  Size:64
+  Alignment:64
+  BaseOffsets: []>
+  VBaseOffsets: []>
+  FieldOffsets: [0]>
+
+*** Dumping AST Record Layout
+Type: struct E2
+
+Layout: <ASTRecordLayout
+  Size:8
+  Alignment:8
+  BaseOffsets: []>
+  VBaseOffsets: []>
+  FieldOffsets: []>
+
+*** Dumping AST Record Layout
+Type: struct Combine
+
+Layout: <ASTRecordLayout
+  Size:64
+  Alignment:64
+  BaseOffsets: [0, 0, 0]>
+  VBaseOffsets: []>
+  FieldOffsets: []>

clang/test/CodeGenCXX/override-layout-ms.cpp

+// RUN: %clang_cc1 -w -triple=x86_64-pc-win32 -fms-compatibility -fdump-record-layouts -foverride-record-layout=%S/Inputs/override-layout-ms.layout %s | FileCheck  %s
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK:          0 | struct E1 (empty)
+// CHECK:            | [sizeof=1, align=1,
+// CHECK:            |  nvsize=0, nvalign=1]
+// CHECK: *** Dumping AST Record Layout
+// CHECK:          0 | struct Mid
+// CHECK:          0 |   void * p
+// CHECK:            | [sizeof=8, align=8,
+// CHECK:            |  nvsize=8, nvalign=8]
+// CHECK: *** Dumping AST Record Layout
+// CHECK:          0 | struct E2 (empty)
+// CHECK:            | [sizeof=1, align=1,
+// CHECK:            |  nvsize=0, nvalign=1]
+// CHECK: *** Dumping AST Record Layout
+// CHECK:          0 | struct Combine
+// CHECK:          0 |   struct E1 (base) (empty)
+// CHECK:          0 |   struct Mid (base)
+// CHECK:          0 |     void * p
+// CHECK:          0 |   struct E2 (base) (empty)
+// CHECK:            | [sizeof=8, align=8,
+// CHECK:            |  nvsize=8, nvalign=8]
+
+
+struct E1 {};
+struct E2 {};
+struct Mid {void *p; };
+struct __declspec(empty_bases) Combine : E1, Mid, E2 {};
+Combine g;
+