The compiler for x64-based processors supports whole-program optimization (WPO). Essentially, this allows the compiler to view all object modules at once before generating code. This also allows the compiler to do several optimizations, such as inlining. The compiler accomplishes this by compiling source modules to a temporary intermediate language and waiting for the link phase for code generation. At that time, all modules compiled for WPO become visible to the compiler.

To use WPO, set the /GL compiler switch. This causes the compiler to generate the intermediate language object files rather than generate code for the object files. Use the /LTCG (link time-code generation) switch to invoke the linker. This indicates to the linker that code generation occurs during this phase. The linker can use WPO without the /LTCG switch, but you might experience extra performance loss during build time. For one-step compiles (builds that do not use the /c compiler switch), the linker is automatically invoked with the /LTCG switch.

You can use WPO to generate both libraries (intermediate file objects) and executable images. You do not need to compile all files with /GL for WPO to work. For example, WPO can be used with typically compiled external libraries. WPO is only performed on those files compiled with the /GL flag.

If you are going to build using WPO, the resultant libraries and objects should not be redistributed, because they have dependencies on the specific version of the compiler used.

Note that WPO builds take longer to compile and require more memory than typical builds.

The linker switches in the following table perform the indicated actions.