Re: [Qemu-devel] [PATCH] Add CONFIG_QEMU_TIMER to handle qemu-timer-common.o dep

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 08/30/2011 07:02 AM, Lluís wrote:
> Stefan Hajnoczi writes:
>
>> On Mon, Aug 29, 2011 at 8:27 PM, Michael Roth<mdroth@linux.vnet.ibm.com>  wrote:
>>> @@ -380,7 +381,6 @@ else
>>>   trace-obj-y = trace.o
>>>   ifeq ($(TRACE_BACKEND),simple)
>>>   trace-obj-y += simpletrace.o
>>> -user-obj-y += qemu-timer-common.o
>>>   endif
>>>   endif
>
>> Now that we have a concrete patch to look at I think this approach is
>> problematic.  There are several subsystems in QEMU which might be
>> built outside the main qemu binary for qemu-io, qemu-img, qemu-ga,
>> etc.
> [...]
>> If QEMU is split up into libraries then having an explicit list of
>> dependencies for each subsystem will be very useful, whereas the
>> CONFIG_* approach doesn't collect that information in one place.
>
>> So I think explicit subsys-obj-y += qemu-timer-common.o together with
>> $(sort) during the link stage actually allows for a cleaner build
>> system.  I prefer that approach.
>
> I couldn't agree more. The only problem I see with '$(sort)' is that it
> will invariably change the order of object files, which can influence
> code placement.
>
> Whether or not the spatial locality among compilation units is
> important, I don't know. Although I believe it won't have much of a
> performance penalty.
>
> In any case, I tried to find a straightforward way of filtering-out
> repeated words in a list with make, but couldn't find any solution other
> than '$(sort)' or calling an external command with '$(shell)'.

Hmm, looking again I'm confused why we need to do this in the first 
place...the rule is:

LINK = $(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o 
$@ $(1) $(LIBS),"  LINK  $(TARGET_DIR)$@")

%$(EXESUF): %.o
     $(call LINK,$^)

According to the documentation $^ should remove duplicate dependencies, 
so I'm not getting why we need to de-dupe them once they get passed to LINK:

http://www.gnu.org/software/make/manual/make.html#index-g_t_0024_005e-948

Is there some distinction between duplicate dependencies and duplicate 
words in the dependency list?

>
>
> Lluis
>