/[NO]SEPARATE_COMPILATION

  Controls whether an individual compilation unit becomes a separate
  module in an object file.

  The default is /NOSEPARATE_COMPILATION, which groups individual
  compilation units as a single module in an object file.

  When creating modules for use in an object library, consider using
  /SEPARATE_COMPILATION to minimize the size of the routines included
  by the linker as it creates the executable image.  In most cases,
  the default setting (/NOSEPARATE_COMPILATION) allows more
  interprocedural optimizations.