It really depends on how complex your code is and what you're targeting. For example, the V8 JavaScript engine has a ton of ifdefs depending on OS (Windows, Mac

It really depends on how complex your code is and what you're targeting. For example, the V8 JavaScript engine has a ton of ifdefs depending on OS (Windows, Mac, Linux, Fushia), bitness (32 or 64), chipset (ARM, ARM64, PPC, x64, etc.) and combinations thereof. For example, see some of the defines in https://github.com/v8/v8/blob/master/src/common/globals.h#L154 https://github.com/v8/v8/blob/master/src/common/globals.h#L1... and their uses throughout the codebase. EDIT: I forgot Android, and all the compiler specific flags. (https://github.com/v8/v8/blob/master/include/v8config.h https://github.com/v8/v8/blob/master/include/v8config.h is probably a better reference).