MS ABI: Correct layout for empty records

Empty records do not always have size equivalent to their alignment.
They only do so when their alignment is at least as large as the minimum
empty struct size: 1 byte in C++ and 4 bytes in C.

llvm-svn: 218661

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -2204,6 +2204,8 @@ public:
   CharUnits CurrentBitfieldSize;
   /// \brief Offset to the virtual base table pointer (if one exists).
   CharUnits VBPtrOffset;
+  /// \brief Minimum record size possible.
+  CharUnits MinEmptyStructSize;
   /// \brief The size and alignment info of a pointer.
   ElementInfo PointerInfo;
   /// \brief The primary base class (if one exists).
@@ -2299,6 +2301,8 @@ MicrosoftRecordLayoutBuilder::getAdjustedElementInfo(
 }
 
 void MicrosoftRecordLayoutBuilder::layout(const RecordDecl *RD) {
+  // For C record layout, zero-sized records always have size 4.
+  MinEmptyStructSize = CharUnits::fromQuantity(4);
   initializeLayout(RD);
   layoutFields(RD);
   DataSize = Size = Size.RoundUpToAlignment(Alignment);
@@ -2308,6 +2312,8 @@ void MicrosoftRecordLayoutBuilder::layout(const RecordDecl *RD) {
 }
 
 void MicrosoftRecordLayoutBuilder::cxxLayout(const CXXRecordDecl *RD) {
+  // The C++ standard says that empty structs have size 1.
+  MinEmptyStructSize = CharUnits::One();
   initializeLayout(RD);
   initializeCXXLayout(RD);
   layoutNonVirtualBases(RD);
@@ -2636,7 +2642,7 @@ void MicrosoftRecordLayoutBuilder::layoutVirtualBases(const CXXRecordDecl *RD) {
 
 void MicrosoftRecordLayoutBuilder::finalizeLayout(const RecordDecl *RD) {
   // Respect required alignment.  Note that in 32-bit mode Required alignment
-  // may be 0 nad cause size not to be updated.
+  // may be 0 and cause size not to be updated.
   DataSize = Size;
   if (!RequiredAlignment.isZero()) {
     Alignment = std::max(Alignment, RequiredAlignment);
@@ -2646,11 +2652,15 @@ void MicrosoftRecordLayoutBuilder::finalizeLayout(const RecordDecl *RD) {
     RoundingAlignment = std::max(RoundingAlignment, RequiredAlignment);
     Size = Size.RoundUpToAlignment(RoundingAlignment);
   }
-  // Zero-sized structures have size equal to their alignment.
   if (Size.isZero()) {
     EndsWithZeroSizedObject = true;
     LeadsWithZeroSizedBase = true;
-    Size = Alignment;
+    // Zero-sized structures have size equal to their alignment if a
+    // __declspec(align) came into play.
+    if (RequiredAlignment >= MinEmptyStructSize)
+      Size = Alignment;
+    else
+      Size = MinEmptyStructSize;
   }
 }
 

clang/test/Layout/ms-x86-basic-layout.cpp

@@ -816,6 +816,36 @@ struct RecordArrayTypedef {
 // CHECK-X64-NEXT:      | [sizeof=16, align=4
 // CHECK-X64-NEXT:      |  nvsize=16, nvalign=4]
 
+struct EmptyIntMemb {
+  int FlexArrayMemb[0];
+};
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:    0 | struct EmptyIntMemb
+// CHECK-NEXT:    0 |   int [0] FlexArrayMemb
+// CHECK-NEXT:      | [sizeof=1, align=4
+// CHECK-NEXT:      |  nvsize=0, nvalign=4]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64-NEXT:    0 | struct EmptyIntMemb
+// CHECK-X64-NEXT:    0 |   int [0] FlexArrayMemb
+// CHECK-X64-NEXT:      | [sizeof=4, align=4
+// CHECK-X64-NEXT:      |  nvsize=0, nvalign=4]
+
+struct EmptyLongLongMemb {
+  long long FlexArrayMemb[0];
+};
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:    0 | struct EmptyLongLongMemb
+// CHECK-NEXT:    0 |   long long [0] FlexArrayMemb
+// CHECK-NEXT:      | [sizeof=1, align=8
+// CHECK-NEXT:      |  nvsize=0, nvalign=8]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64-NEXT:    0 | struct EmptyLongLongMemb
+// CHECK-X64-NEXT:    0 |   long long [0] FlexArrayMemb
+// CHECK-X64-NEXT:      | [sizeof=8, align=8
+// CHECK-X64-NEXT:      |  nvsize=0, nvalign=8]
+
 int a[
 sizeof(TestF0)+
 sizeof(TestF1)+
@@ -840,4 +870,6 @@ sizeof(F6)+
 sizeof(ArrayFieldOfRecords)+
 sizeof(ArrayOfArrayFieldOfRecords)+
 sizeof(RecordArrayTypedef)+
+sizeof(EmptyIntMemb)+
+sizeof(EmptyLongLongMemb)+
 0];

clang/test/Layout/ms-x86-pack-and-align.cpp

@@ -769,6 +769,38 @@ struct QD {
 // CHECK-X64-NEXT:      | [sizeof=8, align=4
 // CHECK-X64-NEXT:      |  nvsize=8, nvalign=4]
 
+struct __declspec(align(4)) EmptyAlignedLongLongMemb {
+  long long FlexArrayMemb[0];
+};
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:    0 | struct EmptyAlignedLongLongMemb
+// CHECK-NEXT:    0 |   long long [0] FlexArrayMemb
+// CHECK-NEXT:      | [sizeof=8, align=8
+// CHECK-NEXT:      |  nvsize=0, nvalign=8]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64-NEXT:    0 | struct EmptyAlignedLongLongMemb
+// CHECK-X64-NEXT:    0 |   long long [0] FlexArrayMemb
+// CHECK-X64-NEXT:      | [sizeof=8, align=8
+// CHECK-X64-NEXT:      |  nvsize=0, nvalign=8]
+
+#pragma pack(1)
+struct __declspec(align(4)) EmptyPackedAlignedLongLongMemb {
+  long long FlexArrayMemb[0];
+};
+#pragma pack()
+
+// CHECK: *** Dumping AST Record Layout
+// CHECK-NEXT:    0 | struct EmptyPackedAlignedLongLongMemb
+// CHECK-NEXT:    0 |   long long [0] FlexArrayMemb
+// CHECK-NEXT:      | [sizeof=4, align=4
+// CHECK-NEXT:      |  nvsize=0, nvalign=4]
+// CHECK-X64: *** Dumping AST Record Layout
+// CHECK-X64-NEXT:    0 | struct EmptyPackedAlignedLongLongMemb
+// CHECK-X64-NEXT:    0 |   long long [0] FlexArrayMemb
+// CHECK-X64-NEXT:      | [sizeof=4, align=4
+// CHECK-X64-NEXT:      |  nvsize=0, nvalign=4]
+
 int a[
 sizeof(X)+
 sizeof(Y)+
@@ -800,4 +832,6 @@ sizeof(PE)+
 sizeof(QB)+
 sizeof(QC)+
 sizeof(QD)+
+sizeof(EmptyAlignedLongLongMemb)+
+sizeof(EmptyPackedAlignedLongLongMemb)+
 0];