As IPC mechanisms become faster, stub-code efficiency becomes a performance issue for local client/server RPCs and inter-component communication. Inefficient and unnecessary complex marshalling code can almost double communication costs. We have developed an experimental new IDL compiler that produces near-optimal stub code for gcc and the L4 microkernel. The current experimental IDL4 compiler cooperates with the gcc compiler and its x86 code generator. Other compilers or target machines would require different optimizations. In most cases, the generated stub code is approximately 3 times faster (and shorter) than the code generated by a commonly used portable IDL compiler. Benchmarks have shown that efficient stubs can increase application performance by more than 10 percent. The results are applied within IBM’s SawMill project that aims at technology for constructing multi-server operating systems.