__undefined
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:title: Shellcode Compiler Runtime

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__undefined;
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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.