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

[Diego Biurrun]

On Mon, Jun 02, 2008 at 07:07:22PM +0200, Nico Sabbi wrote:
> Il Monday 02 June 2008 17:18:49 Diego Biurrun ha scritto:
> > I did read your patch when you sent it to me.  However, I do not
> > remember every single detail and it does not apply anymore.  By "I'll
> > look at your patch" I just meant to say that I will investigate how it
> > can be used to fix our linking problems, not that I wanted to look at
> > it for the first time.
> 
> 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.

At work I once had the misfortune of having to debug a recursive make
system where everything was split into tiny snippets.  I continuously
lost track of which part got included from where and after staring at it
for a few hours, I rewrote it non-recursively in a fraction of that
time.  The end result was a very simple, single Makefile with about 30%
of the previous total line count.  It was also faster and correct.

So unless you go over a certain threshold I generally prefer 60 lines
over 6 x 10 lines.

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.

For a more in-depth explanation, read Peter Miller's short but
informative paper "Recursive Make Considered Harmful":

http://miller.emu.id.au/pmiller/books/rmch/

I hope this clears it up.  If you have more questions, ask, but it's
really been explained many times in many places, the best I know being
the above-mentioned paper.

Diego