Re: [RFC] Replicate src dir in build dir

[Yao Qi <qiyaoltc@gmail.com>]

On Sun, Oct 8, 2017 at 4:24 AM, Tom Tromey <tom@tromey.com> wrote:
>>>>>> "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.
>

Hi Tom,
Thanks for pointing them out.  I will look at them after
I am back to office (13th Oct).

> 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

-- 
Yao (齐尧)