[clang][bytecode] Initialize bases when bitcasting

When bitcasting into a composite type, we might be initialzing all the
fields of a base, but we still need to mark the base pointer itself
as initialized.

I have a feeling we need to do the same with composite fields, too.

Author: tbaederr

clang/lib/AST/ByteCode/BitcastBuffer.h

@@ -28,6 +28,7 @@ struct Bits {
   size_t getOffsetInByte() const { return N % 8; }
   bool isFullByte() const { return N % 8 == 0; }
   bool nonZero() const { return N != 0; }
+  bool isZero() const { return N == 0; }
 
   Bits operator-(Bits Other) { return Bits(N - Other.N); }
   Bits operator+(Bits Other) { return Bits(N + Other.N); }

clang/lib/AST/ByteCode/InterpBuiltinBitCast.cpp

@@ -134,6 +134,10 @@ static bool enumerateData(const Pointer &P, const Context &Ctx, Bits Offset,
           Layout.getBaseClassOffset(cast<CXXRecordDecl>(B.Decl));
       Bits BitOffset = Offset + Bits(Ctx.getASTContext().toBits(ByteOffset));
       Ok = Ok && enumerateData(Elem, Ctx, BitOffset, F);
+      // FIXME: We should only (need to) do this when bitcasting OUT of the
+      // buffer, not when copying data into it.
+      if (Ok)
+        Elem.initialize();
     }
 
     return Ok;
@@ -225,19 +229,32 @@ static bool readPointerToBuffer(const Context &Ctx, const Pointer &FromPtr,
       FromPtr, Ctx,
       [&](const Pointer &P, PrimType T, Bits BitOffset,
           bool PackedBools) -> bool {
-        // if (!P.isInitialized()) {
-        // assert(false && "Implement uninitialized value tracking");
-        // return ReturnOnUninit;
-        // }
+        CharUnits ObjectReprChars = ASTCtx.getTypeSizeInChars(P.getType());
+        Bits BitWidth = Bits(ASTCtx.toBits(ObjectReprChars));
+        Bits FullBitWidth = BitWidth;
+
+        if (const FieldDecl *FD = P.getField(); FD && FD->isBitField()) {
+          // Unnamed bitfields should be skipped altogether.
+          if (FD->isUnnamedBitField())
+            return true;
+          BitWidth = Bits(std::min(FD->getBitWidthValue(ASTCtx),
+                                   (unsigned)FullBitWidth.getQuantity()));
+        } else if (T == PT_Bool && PackedBools)
+          BitWidth = Bits(1);
 
-        // assert(P.isInitialized());
+        if (BitWidth.isZero())
+          return true;
+
+        if (!P.isInitialized()) {
+          assert(false && "Implement uninitialized value tracking");
+          return ReturnOnUninit;
+        }
+
+        assert(P.isInitialized());
         // nullptr_t is a PT_Ptr for us, but it's still not std::is_pointer_v.
         if (T == PT_Ptr)
           assert(false && "Implement casting to pointer types");
 
-        CharUnits ObjectReprChars = ASTCtx.getTypeSizeInChars(P.getType());
-        Bits BitWidth = Bits(ASTCtx.toBits(ObjectReprChars));
-        Bits FullBitWidth = BitWidth;
         auto Buff =
             std::make_unique<std::byte[]>(ObjectReprChars.getQuantity());
         // Work around floating point types that contain unused padding bytes.
@@ -256,12 +273,6 @@ static bool readPointerToBuffer(const Context &Ctx, const Pointer &FromPtr,
             swapBytes(Buff.get(), NumBits.roundToBytes());
 
         } else {
-          if (const FieldDecl *FD = P.getField(); FD && FD->isBitField())
-            BitWidth = Bits(std::min(FD->getBitWidthValue(ASTCtx),
-                                     (unsigned)FullBitWidth.getQuantity()));
-          else if (T == PT_Bool && PackedBools)
-            BitWidth = Bits(1);
-
           BITCAST_TYPE_SWITCH(T, { P.deref<T>().bitcastToMemory(Buff.get()); });
 
           if (llvm::sys::IsBigEndianHost)

clang/test/AST/ByteCode/builtin-bit-cast.cpp

@@ -335,9 +335,8 @@ void test_record() {
   static_assert(t4 == tuple4{1, 2, 3, 4});
   static_assert(check_round_trip<tuple4>(b));
 
-  /// FIXME: We need to initialize the base pointers in the pointer we're bitcasting to.
-//  constexpr auto b2 = bit_cast<bases>(t4);
-//  static_assert(t4 == b2);
+  constexpr auto b2 = bit_cast<bases>(t4);
+  static_assert(t4 == b2);
 }
 
 void test_partially_initialized() {