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

[hubert.reinterpretcast]

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.

[xingxue]

LGTM.

include/llvm/CodeGen/SelectionDAGNodes.h

private:
   /// The operation that this node performs.
   int16_t NodeType;
 
 protected:
   // We define a set of mini-helper classes to help us interpret the bits in our
   // 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;
     friend class MemSDNode;
     friend class SelectionDAG;
 
     uint16_t HasDebugValue : 1;
     uint16_t IsMemIntrinsic : 1;
union {
     char RawSDNodeBits[sizeof(uint16_t)];
     SDNodeBitfields SDNodeBits;
     ConstantSDNodeBitfields ConstantSDNodeBits;
     MemSDNodeBitfields MemSDNodeBits;
     LSBaseSDNodeBitfields LSBaseSDNodeBits;
     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
   // "2" instead of sizeof(RawSDNodeBits) because MSVC can't handle the latter.
   static_assert(sizeof(SDNodeBitfields) <= 2, "field too wide");
   static_assert(sizeof(ConstantSDNodeBitfields) <= 2, "field too wide");
   static_assert(sizeof(MemSDNodeBitfields) <= 2, "field too wide");
   static_assert(sizeof(LSBaseSDNodeBitfields) <= 2, "field too wide");