Re: Support for out-of-tree build

[Rich Felker <dalias@libc.org>]

On Tue, Nov 17, 2015 at 06:27:11PM -0500, Rich Felker wrote:
> Assuming .o files in arch dirs are ok, does the following work
> (non-VPATH)?
> 
> %.o: $(srcdir)/%.sub
> 	...
> %.o: $(srcdir)/%.s
> 	...
> %.o: $(srcdir)/%.c
> 	...
> 
> (with appropriate commands, of course). If so, I think I like it a
> lot, because it factors the rules for file *types* separately from the
> rules for which files to select (the "tricky" $(OBJS) setting). The
> only big concern is whether this "tricky" setting is too costly in
> runtime performance or complexity.

Here's my idea for the 'tricky' setting of OBJS and it actually looks
very simple and efficient:

ARCH_OBJS = $(wildcard ...)
BASE_OBJS = $(wildcard ...)
OBJS = $(ARCH_OBJS) $(filter-out $(ARCH_OBJS:/$(ARCH)/=/),$(BASE_OBJS))

If the above all works I think we have a pretty good path forward.

It does change where the .o files for arch-specific sources live (one
level deeper in the tree) and requires these dirs to be created in the
output tree, but I think this is actually an improvement since
removing an arch source file will result in the base-level .o file
being built (right now the stale .o file based on the arch asm sticks
around).

Presumably we can also just do:

LOBJS = $(OBJS:.o=.lo)

although it's my intent to remove LOBJS soon anyway and instead have
pic/non-pic objs (and only use pic ones by default). How to name them
will be a fun bikeshed...

Rich