fix: changed the empty struct padding handling to take into account structs with required alignments

Author: Théo De Magalhaes

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -3231,6 +3231,11 @@ void MicrosoftRecordLayoutBuilder::finalizeLayout(const RecordDecl *RD) {
   uint64_t UnpaddedSizeInBits = Context.toBits(DataSize);
   UnpaddedSizeInBits -= RemainingBitsInField;
 
+  // MS ABI allocates 1 byte for empty class
+  // (not padding)
+  if (Size.isZero())
+    UnpaddedSizeInBits += 8;
+
   // Respect required alignment.  Note that in 32-bit mode Required alignment
   // may be 0 and cause size not to be updated.
   DataSize = Size;
@@ -3262,9 +3267,10 @@ void MicrosoftRecordLayoutBuilder::finalizeLayout(const RecordDecl *RD) {
     return;
   }
   unsigned CharBitNum = Context.getTargetInfo().getCharWidth();
-  uint64_t DataSizeInBits = Context.toBits(DataSize);
-  if (DataSizeInBits > UnpaddedSizeInBits) {
-    unsigned int PadSize = DataSizeInBits - UnpaddedSizeInBits;
+  uint64_t SizeInBits = Context.toBits(Size);
+
+  if (SizeInBits > UnpaddedSizeInBits) {
+    unsigned int PadSize = SizeInBits - UnpaddedSizeInBits;
     bool InBits = true;
     if (PadSize % CharBitNum == 0) {
       PadSize = PadSize / CharBitNum;

clang/test/SemaCXX/windows-Wpadded.cpp

@@ -32,14 +32,26 @@ struct __attribute__((ms_struct)) StructWithUnion { // expected-warning {{paddin
   short i;
 };
 
-struct __attribute__((ms_struct)) EmptyStruct {
+struct __attribute__((ms_struct)) EmptyStruct {};
 
+struct __attribute__((ms_struct)) AlignedMemberStruct { // expected-warning {{padding size of 'AlignedMemberStruct' with 28 bytes to alignment boundary}}
+    alignas(32) int x;
 };
 
+struct alignas(32) __attribute__((ms_struct)) AlignedNonEmptyStruct { // expected-warning {{padding size of 'AlignedNonEmptyStruct' with 28 bytes to alignment boundary}}
+    int x;
+};
+
+
+struct alignas(16) __attribute__((ms_struct)) AlignedEmptyStruct {}; // expected-warning {{padding size of 'AlignedEmptyStruct' with 15 bytes to alignment boundary}}
+
 int main() {
   Foo f;
   AlignedStruct a;
   Derived d;
   StructWithUnion swu;
   EmptyStruct e;
+  AlignedNonEmptyStruct anes;
+  AlignedMemberStruct ams;
+  AlignedEmptyStruct aes;
 }