[AIX] Pack BasicBlockBits

[daltenty]

Summary

D51664 introduces a new structure BasicBlockBits which it expects to be
packed a certain way. This change is very similar to D60164, and we apply the
same fix:

"On AIX, the canonical layout of bit-fields would cause
these ... to span four bytes. Applying the pack pragma for compilers that
employ the AIX canonical layout allows these ... 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."

[sfertile]

Fix the formatting that Lint called out, but otherwise LGTM for fixing building natively on AIX.

Would it make sense to move the logic that defines BEGIN_TWO_BYTE_PACK and END_TWO_BYTE_PACK into cmake now that its done in more then one place?

[daltenty]

• Fix formating

[daltenty]

In D74911#1885002, @sfertile wrote:

Would it make sense to move the logic that defines BEGIN_TWO_BYTE_PACK and END_TWO_BYTE_PACK into cmake now that its done in more then one place?

I agree, I will create that as a follow on patch.

llvm/include/llvm/IR/BasicBlock.h

public:
   void renumberInstructions();
 
   /// Returns false if the instruction ordering is incorrect in an debug build.
   /// Always returns true when assertions are disabled. The method does not
   /// assert internally so that we get better location info.
   void validateInstrOrdering() const;
 
 private:
+#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()
   /// Bitfield to help interpret the bits in Value::SubclassData.
   struct BasicBlockBits {
     unsigned short BlockAddressRefCount : 15;
     unsigned short InstrOrderValid : 1;
   };
+  END_TWO_BYTE_PACK()
+
+#undef BEGIN_TWO_BYTE_PACK
+#undef END_TWO_BYTE_PACK
 
   /// Safely reinterpret the subclass data bits to a more useful form.
   BasicBlockBits getBasicBlockBits() const {
     static_assert(sizeof(BasicBlockBits) == sizeof(unsigned short),
                   "too many bits for Value::SubclassData");
     unsigned short ValueData = getSubclassDataFromValue();
     BasicBlockBits AsBits;
     memcpy(&AsBits, &ValueData, sizeof(AsBits));