Methods, systems, and apparatus, including computer programs encoded on computer storage media, for selecting native code instructions. One of the methods includes receiving an initial machine language instruction for execution by a processor in a first execution mode; determining that a portion of the initial machine language instruction, when executed by the processor in a second execution mode, satisfies one or more risk criteria; generating one or more alternative machine language instructions to replace the initial machine language instruction for execution by the processor in the first execution mode, wherein the one or more alternative machine language instructions, when executed by the processor in the second execution mode, mitigate the one or more risk criteria; and providing the one or more alternative machine language instructions.