Re: Default NO_CLEAN=yes in 15+

On Tue, Jul 23, 2024 at 03:58:13PM -0400, John Baldwin wrote:
> The buildworld and buildkernel targets include a "clean" step before building
> objects dating back before my time to 'make world' (I haven't looked to see
> how far back it goes).  To permit incremental builds, this step can be skipped
> via NO_CLEAN=yes.  This step is a bit unusual in build systems however.  Most
> build systems have separate commands for building vs cleaning (e.g. 'make all'
> vs 'make clean') and over time FreeBSD's build system has gained dedicated
> clean targets as well (cleanworld and cleankernel).  For myself, I always
> use NO_CLEAN=yes when building worlds and kernels.  If I need a clean build
> I use the dedicated clean targets (e.g. cleanworld) first.  In particular,
> cleanworld/cleankernel are far more efficient since they use a single
> recursive 'rm' whereas the "clean" step involves a full tree walk with
> nested make invocations of the 'cleandir' target.
> 
> A few years ago, Ed Maste added a MK_CLEAN option to src.opts.mk to as a
> WITH/WITHOUT knob for the "clean" step similar to NO_CLEAN=yes.  To preserve
> existing behavior this knob currently defaults to on, but I know Ed's goal
> was to eventually flip the default so that NO_CLEAN builds would be the
> default.  I would like us to do that starting in 15.

+1

We should continue spot cleanups for things like foo.c -> foo.S as
required rather than pointlessly rebuilding everything every time.  It's
much better for a small number of people to spend time tidying things up
than to make expensive cleaning the default.

> Further off, I would suggest that we remove the "clean" step outright,
> perhaps in 16.x.  Regardless, we will need to update documentation to
> prefer the clean targets over WITH_CLEAN=yes if our docs do not do this
> already.

I strongly agree.  It's all weird and unnecessary with things like
git-clean(1) available.  Eventually I'd like to get rid of cleandir and
its weird behavior differences based on .OBJDIR's existence.  This is
tooling for the distant past.

-- Brooks