[MPlayer-dev-eng] recursive make considered harmful (was: Re: The new Makefiles)

[Diego Biurrun]

On Mon, Jun 02, 2008 at 08:06:26PM +0200, Diego Biurrun wrote:
> On Mon, Jun 02, 2008 at 07:07:22PM +0200, Nico Sabbi wrote:
> > 
> > sorry, but I really don't understand the reason for a single and totally flat
> > Makefile: IMO a Makefile per directory is a kind of modularization (that
> > is generally considered "A good thing" ) , clean for its own nature.
> > Can you explain what we gain now? or in what respect a single Makefile
> > is cleaner?
> 
> The single Makefile is shorter than the single-directory Makefiles
> combined, even with common infrastructure factorized into mpcommon.mak.
> Our Makefile is just above 1000 lines.  Given that it consists largely
> of long lists, it is neither complicated nor particularly long.  In such
> a case I prefer having all the information in one central place.
> 
> But this is not the main reason.  The problem is that recursive Make is
> incorrect.  What the Make program does is build a directed acyclic graph
> of all dependencies/targets and then perform all the necessary steps the
> target you request requires, but no more than those necessary steps.
> 
> Recursive uses of make cut this graph into pieces.  Unfortunately this
> process is not lossless because inter-directory dependencies are left
> out.  This results in Make systems that continuously recompile things
> unnecessarily, fail to recompile necessary parts and generally have
> horrible performance.

Note that it is possible to split Makefiles into pieces without the
build process becoming recursive.  You can use the 'include' Makefile
directive to incorporate Makefile snippets from subdirectories into the
top-level Makefile.

It is even possible to build non-recursive build systems with Make where
you can invoke Make from subdirectories, like Mans did for FFmpeg.  But
this does require a considerable amount of Make trickery.

For MPlayer I did not see much added benefit to make up for the
complication.  In FFmpeg it makes sense to build single libraries
separately.  In MPlayer, you are interested in the binaries, which
are built from the top-level.

Diego