__undefined

This intrinsic can be used in place of any value. It gives undefined results, usually by omitting code. The runtime result will vary based on the choices of the optimizer and code generator. The compiler will typically leave the register or memory location being targeted in its original state, completely omitting any instructions that would have been required to update it. Any arithmetic performed with __undefined as part of the computation will optimized down to a single usage of the __undefined intrinsic, as the results are by definition undefined.

One of the most common uses of this intrinsic is to exit the program while treating the exit code as a "don’t care" value, using exit(__undefined). A main function that returns void implicitly does this.