[clang][bytecode] Add some convenience API to BitcastBuffer (#169516)

So we check the offsets before using them.

Author: tbaederr

clang/lib/AST/ByteCode/BitcastBuffer.h

@@ -89,6 +89,12 @@ struct BitcastBuffer {
     Data = std::make_unique<std::byte[]>(ByteSize);
   }
 
+  /// Returns the byte at the given offset.
+  std::byte *atByte(unsigned Offset) {
+    assert(Offset < FinalBitSize.roundToBytes());
+    return Data.get() + Offset;
+  }
+
   /// Returns the buffer size in bits.
   Bits size() const { return FinalBitSize; }
   Bytes byteSize() const { return FinalBitSize.toBytes(); }
@@ -113,6 +119,13 @@ struct BitcastBuffer {
   std::unique_ptr<std::byte[]> copyBits(Bits BitOffset, Bits BitWidth,
                                         Bits FullBitWidth,
                                         Endian TargetEndianness) const;
+
+  /// Dereferences the value at the given offset.
+  template <typename T> T deref(Bytes Offset) const {
+    assert(Offset.getQuantity() < FinalBitSize.roundToBytes());
+    assert((Offset.getQuantity() + sizeof(T)) <= FinalBitSize.roundToBytes());
+    return *reinterpret_cast<T *>(Data.get() + Offset.getQuantity());
+  }
 };
 
 } // namespace interp

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -1997,8 +1997,8 @@ static bool interp__builtin_memcmp(InterpState &S, CodePtr OpPC,
   for (size_t I = 0; I != CmpSize; I += ElemSize) {
     if (IsWide) {
       INT_TYPE_SWITCH(*S.getContext().classify(ASTCtx.getWCharType()), {
-        T A = *reinterpret_cast<T *>(BufferA.Data.get() + I);
-        T B = *reinterpret_cast<T *>(BufferB.Data.get() + I);
+        T A = *reinterpret_cast<T *>(BufferA.atByte(I));
+        T B = *reinterpret_cast<T *>(BufferB.atByte(I));
         if (A < B) {
           pushInteger(S, -1, Call->getType());
           return true;
@@ -2009,8 +2009,8 @@ static bool interp__builtin_memcmp(InterpState &S, CodePtr OpPC,
         }
       });
     } else {
-      std::byte A = BufferA.Data[I];
-      std::byte B = BufferB.Data[I];
+      std::byte A = BufferA.deref<std::byte>(Bytes(I));
+      std::byte B = BufferB.deref<std::byte>(Bytes(I));
 
       if (A < B) {
         pushInteger(S, -1, Call->getType());