[AIX] SelectionDAGNodes.h: Pack bit-fields that are meant to be packed

Summary:
Certain classes in the subject file are expected to provide different
views of a two-byte field as a collection of various bit-fields. On AIX,
the canonical layout of bit-fields would cause these classes to span
four bytes. Applying the `pack` pragma for compilers that employ the AIX
canonical layout allows these classes to fit within the expected two
bytes. In the future, the pragma would also likely need to be applied
when building with Clang on AIX.

Reviewers: xingxue, sfertile, jasonliu

Reviewed By: xingxue

Subscribers: jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60164

llvm-svn: 357661

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -488,6 +488,17 @@ class SDNode : public FoldingSetNode, public ilist_node<SDNode> {
   // SubclassData.  These are designed to fit within a uint16_t so they pack
   // with NodeType.
 
+#if defined(_AIX) && (!defined(__GNUC__) || defined(__ibmxl__))
+// Except for GCC; by default, AIX compilers store bit-fields in 4-byte words
+// and give the `pack` pragma push semantics.
+#define BEGIN_TWO_BYTE_PACK() _Pragma("pack(2)")
+#define END_TWO_BYTE_PACK() _Pragma("pack(pop)")
+#else
+#define BEGIN_TWO_BYTE_PACK()
+#define END_TWO_BYTE_PACK()
+#endif
+
+BEGIN_TWO_BYTE_PACK()
   class SDNodeBitfields {
     friend class SDNode;
     friend class MemIntrinsicSDNode;
@@ -560,6 +571,9 @@ class SDNode : public FoldingSetNode, public ilist_node<SDNode> {
     LoadSDNodeBitfields LoadSDNodeBits;
     StoreSDNodeBitfields StoreSDNodeBits;
   };
+END_TWO_BYTE_PACK()
+#undef BEGIN_TWO_BYTE_PACK
+#undef END_TWO_BYTE_PACK
 
   // RawSDNodeBits must cover the entirety of the union.  This means that all of
   // the union's members must have size <= RawSDNodeBits.  We write the RHS as