[clang] [clang] Make LazyOffsetPtr more portable (PR #112927)

[Richard Smith via cfe-commits]

================
@@ -326,45 +328,65 @@ struct LazyOffsetPtr {
   ///
   /// If the low bit is clear, a pointer to the AST node. If the low
   /// bit is set, the upper 63 bits are the offset.
-  mutable uint64_t Ptr = 0;
+  static constexpr size_t DataSize = std::max(sizeof(uint64_t), sizeof(T *));
+  alignas(uint64_t) alignas(T *) mutable unsigned char Data[DataSize] = {};
+
+  unsigned char GetLSB() const {
+    return Data[llvm::sys::IsBigEndianHost ? DataSize - 1 : 0];
+  }
+
+  template <typename U> U &As(bool New) const {
+    unsigned char *Obj =
+        Data + (llvm::sys::IsBigEndianHost ? DataSize - sizeof(U) : 0);
+    if (New)
+      return *new (Obj) U;
+    return *std::launder(reinterpret_cast<U *>(Obj));
+  }
+
+  T *&GetPtr() const { return As<T *>(false); }
+  uint64_t &GetU64() const { return As<uint64_t>(false); }
+  void SetPtr(T *Ptr) const { As<T *>(true) = Ptr; }
+  void SetU64(uint64_t U64) const { As<uint64_t>(true) = U64; }
 
 public:
   LazyOffsetPtr() = default;
-  explicit LazyOffsetPtr(T *Ptr) : Ptr(reinterpret_cast<uint64_t>(Ptr)) {}
+  explicit LazyOffsetPtr(T *Ptr) : Data() { SetPtr(Ptr); }
 
-  explicit LazyOffsetPtr(uint64_t Offset) : Ptr((Offset << 1) | 0x01) {
+  explicit LazyOffsetPtr(uint64_t Offset) : Data() {
     assert((Offset << 1 >> 1) == Offset && "Offsets must require < 63 bits");
     if (Offset == 0)
-      Ptr = 0;
+      SetPtr(NULL);
----------------
zygoloid wrote:

Please use `nullptr` here and below

https://github.com/llvm/llvm-project/pull/112927