[clang] [clang] LazyOffsetPtr: Use native pointer width (PR #111995)

[John McCall via cfe-commits]

================
@@ -326,25 +326,25 @@ 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;
+  mutable uintptr_t Ptr = 0;
 
 public:
   LazyOffsetPtr() = default;
-  explicit LazyOffsetPtr(T *Ptr) : Ptr(reinterpret_cast<uint64_t>(Ptr)) {}
+  explicit LazyOffsetPtr(T *Ptr) : Ptr(reinterpret_cast<uintptr_t>(Ptr)) {}
 
-  explicit LazyOffsetPtr(uint64_t Offset) : Ptr((Offset << 1) | 0x01) {
-    assert((Offset << 1 >> 1) == Offset && "Offsets must require < 63 bits");
+  explicit LazyOffsetPtr(uintptr_t Offset) : Ptr((Offset << 1) | 0x01) {
+    assert((Offset << 1 >> 1) == Offset && "Offsets must fit in addressable bits");
     if (Offset == 0)
       Ptr = 0;
   }
 
   LazyOffsetPtr &operator=(T *Ptr) {
-    this->Ptr = reinterpret_cast<uint64_t>(Ptr);
+    this->Ptr = reinterpret_cast<uintptr_t>(Ptr);
     return *this;
   }
 
-  LazyOffsetPtr &operator=(uint64_t Offset) {
-    assert((Offset << 1 >> 1) == Offset && "Offsets must require < 63 bits");
+  LazyOffsetPtr &operator=(uintptr_t Offset) {
----------------
rjmccall wrote:

I think the right solution is to store a `uint64_t` and cast it to a `uintptr_t` before casting back and forth to a pointer type.  I'd like to not hard-code the assumption that AST file sizes can't be out of the range of `uintptr_t` here.  I believe you're right that we do `mmap` the entire AST file right now, but there are other implementation approaches that would support AST files larger than the address space.

https://github.com/llvm/llvm-project/pull/111995