$ cd simple
$ omake
$ strings test | grep happySee happy_x is linked in the executable but not happy_y.
Suppose we have a compilation unit My, i.e. my.ml with the following code:
module M = My_m
where My_m refers another compilation unit my_m.ml, and my.ml is compiled with -no-alias-deps. Then the compilation of M does not have real dependency over My_m. Linking of my.ml does not trigger linking of my_m.ml if neither of My_m or My.M is used. Thus with -no-alias-deps, you can create a big hierarchy of modules keeping unused submodules unlinked in the final executables. Actually, my.ml compiles even without the compilation of my_m.ml. (The compiler complains a warning 49, though.)
How does it work? With the above settings, uses of My.M in application codes are actually replaced by My_m by the compiler. Thus the compilation units gathered by the special alias module M = My_m with -no-alias-deps are accessible as submodules but internally treated as independent toplevel modules. The OCaml linker handles them as toplevel modules: if they are not used at all, they are dropped from the linkage, and the code size reduces as a result.
-no-alias-deps is very subtle to use correctly. Here are some points you have to be careful.
You have to use -as-map option of ocamldep to get better dependency. This is not available 4.02.1.
If my.ml has module MN = My_m.N, it is not this special alias, therefore if My is linked, My_m is always linked together. A special module alias must have a compilation unit in its rhs.
As we have seen, this linker trick with the special module alias utilizes .cma linking to drop unused submodules from executables. This requres submodules exposed to the global name space: submodules must have unique names, for example by prefixed with the library name like my_, to avoid possible collisions with other libraries. This makes -for-pack trick completely obsolete with -no-alias-deps.
macro/OMakefile provides an use example of -no-alias-deps with OMake macros.