Re: make: dependfile preference list

[David Holland <dholland-tech%netbsd.org@localhost>]

On Thu, Apr 19, 2012 at 09:12:33AM -0700, Simon J. Gerraty wrote:
 > > >      .MAKE.DEPENDFILE_PREFERENCE
 > > >                      The ordered list of makefile names (default
 > > >                      `${.MAKE.DEPENDFILE:T}') that make will look for to
 > > >                      read
 > > >                      dependencies from.  Only the first found will be
 > > >                      read.
 > >
 > >Is there some reason this has to be a builtin and not just a piece of
 > >logic in a make library that wants to do this sort of thing?
 > 
 > Yes, I think so.  Adoption of meta mode, is much easier if it can be
 > seamless.  That is, the 1000's of makefiles that make up the bulk of the
 > tree "just work" either way.

I do not see how this is relevant.

 > I've just finished such a conversion of a very large tree at work with
 > many active developers.  It was a pure cross-build environment.
 > It took over a year and except for the last few months most developers
 > were largely unimpacted (unaware).
 > 
 > We build the entire tree (much bigger than say NetBSD) for over a
 > dozen architectures - in parallel in a single pass.  For a full tree
 > build the 0th bmake reads several thousand makefiles (Makefile.depend*)
 > before starting sub-makes all over the place.  I'm thinking I need to
 > make the job scheduling better - ie. don't start a submake unless you
 > can give it several tokens.
 > The real benefit for most developers though, is that from anywhere
 > in a clean tree, they can run make and build only and exactly what is
 > needed for that directory.
 > 
 > Being able to set .MAKE.DEPENDFILE (default .depend) is a big part of
 > that.

I don't see how.

Somewhere, in some file make reads and processes, you have a depend
rule that populates .depend or Makefile.depend or
Makefile.depend.${MACHINE}. Or maybe it's part of a build rule, but it
isn't actually different that way.

depend:
        frob $(STUFF) > $(DEPENDFILE)

There is no reason to add a new special facility to tell make how to
read $(DEPENDFILE) when all you need to do is add

.-include "$(DEPENDFILE)"

to the same file or some other suitable place.

 > That and thinking about how to make this more palatable to the broader
 > BSD community, prompted the introduction of .MAKE.DEPENDFILE_PREFERENCE.
 > 
 > Yes, it *could* be done with makefiles but given
 > .MAKE.MAKEFILE_PREFERENCE, 
 > .MAKE.DEPENDFILE_PREFERENCE seemed natural and will allow a more elegant
 > solution.  Which is also important for getting people to adopt something
 > new.

I don't see that adding extra unnecessary knobs is elegant.

-- 
David A. Holland
dholland%netbsd.org@localhost