In C++ (and C for that matter) separate source files are separate compilation units. So it is trivial to parallelize. The compilation process produces object co

In C++ (and C for that matter) separate source files are separate compilation units. So it is trivial to parallelize. The compilation process produces object code, which, generally, has to be linked into an executable to be useful and that is usually not a parallelized task even though nothing prevents you from writing a parallel linker in theory. Unless LTO (link-time optimization) is enabled, the bulk of the linker's time is in I/O so making it parallel would not produce any performance gains till recently. Nowadays we have SSDs that can benefit from deeply queued I/O and, probably, somebody will write a parallel linker eventually.