AST: Alignment attrs override #pragma pack when in ms_struct mode

[majnemer]

Summary

MSVC's behavior is to obey __declspec(align), even under the presence of
the pack pragma; this is not compatible with GCC's behavior.

However, it is reasonable to expose such things under #pragma ms_mode.

lib/AST/RecordLayoutBuilder.cpp

void RecordLayoutBuilder::LayoutField(const FieldDecl *D) {
 
   if (FieldPacked)
     FieldAlign = CharUnits::One();
   CharUnits MaxAlignmentInChars = 
     Context.toCharUnitsFromBits(D->getMaxAlignment());
   FieldAlign = std::max(FieldAlign, MaxAlignmentInChars);
   UnpackedFieldAlign = std::max(UnpackedFieldAlign, MaxAlignmentInChars);
 
-  // The maximum field alignment overrides the aligned attribute.
+  // The maximum field alignment overrides the aligned attribute iff we are not
+  // in ms_abi mode, MSVC obeys __declspec(alignment) requests despite pragma
+  // pack.
   if (!MaxFieldAlignment.isZero()) {
     FieldAlign = std::min(FieldAlign, MaxFieldAlignment);
     UnpackedFieldAlign = std::min(UnpackedFieldAlign, MaxFieldAlignment);
+    if (IsMsStruct) {
+      // Respect align attributes.
+      FieldAlign = std::max(FieldAlign, MaxAlignmentInChars);
+      UnpackedFieldAlign = std::max(UnpackedFieldAlign, MaxAlignmentInChars);
+    }
   }
 
   // Round up the current record size to the field's alignment boundary.
   FieldOffset = FieldOffset.RoundUpToAlignment(FieldAlign);
   UnpackedFieldOffset = 
     UnpackedFieldOffset.RoundUpToAlignment(UnpackedFieldAlign);
 
   if (ExternalLayout) {

test/Sema/pragma-ms_struct.c

 } PackOddity;
 
 #pragma ms_struct off
 
 static int arr[sizeof(PackOddity) == 40 ? 1 : -1];
 
 struct __declspec(ms_struct) bad { // expected-warning {{__declspec attribute 'ms_struct' is not supported}}
 };
+
+#pragma ms_struct on
+#pragma pack(push, 1)
+struct S1 { char a; short b; double c; __attribute__((aligned(32))) double d; char e; double f; };
+#pragma pack(pop)
+static int arr2[sizeof(struct S1) == 64 ? 1 : -1];
+#pragma ms_struct off