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

[David Majnemer]

Hi rjmccall, whunt, rsmith, rnk, akyrtzi,

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.

http://reviews.llvm.org/D4649

Files:
  lib/AST/RecordLayoutBuilder.cpp
  test/Sema/pragma-ms_struct.c

Index: lib/AST/RecordLayoutBuilder.cpp
===================================================================
--- lib/AST/RecordLayoutBuilder.cpp
+++ lib/AST/RecordLayoutBuilder.cpp
@@ -1710,10 +1710,17 @@
   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.
Index: test/Sema/pragma-ms_struct.c
===================================================================
--- test/Sema/pragma-ms_struct.c
+++ test/Sema/pragma-ms_struct.c
@@ -61,3 +61,10 @@
 
 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
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