Re: [Qemu-devel] Proper use of unnest-vars

[Markus Armbruster]

Paolo Bonzini <address@hidden> writes:

> On 15/02/19 08:53, Markus Armbruster wrote:
>> This time, $(obj-y) is very much not blank, and...
>> 
>>     @qapi/ final qapi-introspect.o qapi-types-target.o qapi-types.o 
>> qapi-visit-target.o qapi-visit.o qapi-events-target.o qapi-events.o 
>> qapi-commands-target.o qapi-commands.o
>>     [Trailing make output elided]
>> 
>> ... qapi/Makefile.obj-y *does* clobber it.  Oww.
>> 
>> How come this works anyway?
>
> It works because at this point obj-y is not used anymore, it is assigned
> to all-obj-y a couple lines before:
>
>       all-obj-y := $(obj-y)

Confirms my findings.

> As an aside, target-obj-y seems unnecessary to me.

I have no idea :)

$ git-grep target-obj-y
Makefile.objs:target-obj-y += trace/
Makefile.target:target-obj-y :=
Makefile.target:dummy := $(call unnest-vars,,target-obj-y)
Makefile.target:target-obj-y-save := $(target-obj-y)
Makefile.target:target-obj-y := $(target-obj-y-save)
Makefile.target:all-obj-y += $(target-obj-y)
trace/Makefile.objs:target-obj-y += generated-helpers.o
trace/Makefile.objs:target-obj-y += control-target.o

>> Perhaps unnest-vars could be more hygienic.
>
> Macro hygiene and Make in the same sentence? (well, not sentence but
> still...).

You're right.  My "perhaps could" should be read like "Perhaps we could
make pigs fly, if we apply enough force, say with a trebuchet".

>> But that's not my immediate
>> concern.  All I want to know right now is whether I should refrain from
>> = and := in Makefile.objs.  Paolo, Fam?
>
> No, there is no need for that.

Thanks!

> Really the answer is that we are kind of pushing Makefiles to the limit
> here.  We do get good expressiveness, but at the cost of hiding things
> behind black magic.  In the end I think it's a net benefit, but the cost
> does exist.

Concur.

It's actually not bad most of the time.  Once in a great while, I need
to do something more fancy, and then I'm prone to waste a few hours on
(re-)learning how to work with the magic.