Compiler-guaranteed safety in code-copying VMs

Virtual Machine authors face a difficult choice: to settle for low performance, cheap interpreter, or to write a specialized and costly compiler. One of the methods to bridge the gap between these two distant solutions is to use the existing code-copying technique that reuses chunks of VM’s binary code creating a simple JIT. While simple in principle this technique is not reliable without a compiler that can guarantee that copied chunks are functionally equivalent, which is often not the case due to aggressive optimizations. We present a proof-of-concept, minimal-impact modification of a highly optimizing compiler, GCC. It allows a VM programmer to mark specific chunks of VM source code as copyable. The chunks of native code resulting from compilation of the marked source become addressable and self-contained. Chunks can be safely copied at VM runtime, concatenated and executed together. With minimal impact on compiler maintenance we guarantee the necessary safety and correctness properties of chunks. This allows code-copying VMs to safely achieve performance improvement up to 200%, 67% average, over direct interpretation. ensured thanks to chunks integrity verification. This maintanable enhancement makes the code-copying technique reliable and thus practially usable.