Re: [RFC] Replicate src dir in build dir

[Tom Tromey <tom@tromey.com>]

>>>>> "Yao" == Yao Qi <qiyaoltc@gmail.com> writes:

Yao> This patch takes the first step toward "Replicate src dir in build dir",
Yao> that is, we create arch/ directory in buildtree, and put amd64.o there
Yao> as an example.

I think this is very nice.  Thanks for doing it.

But, there are a couple of bugs; one serious, one probably not.

Yao>  override COMPILE.post = -c -o $@ -MT $@ -MMD -MP \
Yao> -	-MF $(DEPDIR)/$(basename $(@F)).Tpo
Yao> -override POSTCOMPILE = @mv $(DEPDIR)/$(basename $(@F)).Tpo \
Yao> -	$(DEPDIR)/$(basename $(@F)).Po
Yao> +	-MF $(@D)/$(DEPDIR)/$(@F).Tpo
Yao> +override POSTCOMPILE = @mv $(@D)/$(DEPDIR)/$(@F).Tpo \
Yao> +	$(@D)/$(DEPDIR)/$(@F).Po
Yao>  else
Yao>  override COMPILE.pre = source='$<' object='$@' libtool=no \
Yao>  	DEPDIR=$(DEPDIR) $(DEPMODE) $(depcomp) $(CC)

This hunk removed the $(basename ...) wrapper.  This renames the
dependency files from "basename.Tpo" to "basename.o.Tpo".

However, it didn't change the dependencies include near the end of the
Makefile:

    -include $(patsubst %.o, $(DEPDIR)/%.Po, $(all_object_files))

So, because the names differ, automatic dependency tracking no longer
works.  I was really puzzled today when my rebuilds were so fast! :-)

I think the correct solution is to add the $(basename ...) back.  I
thought at first that the "-include" line could be fixed, but this won't
work in case where "depcomp" is used -- because the name choice also has
to coincide with what depcomp does.

Finally, I think the arch/ change might not work properly with depcomp.
The COMPILE.pre override (quoted in the context of the patch hunk above)
probably needs a tweak to DEPDIR, I would guess DEPDIR=$(@D)/$(DEPDIR).
However, I didn't try this case.  (I think when I wrote this I tested
the depcomp case by hacking the generated Makefile to ensure that the
depcomp code was used; otherwise you need a non-capable compiler.)

Tom