[Qemu-devel] Re: [PATCH 2/2] trace: avoid unnecessary recompilation if nothing changed

[Paolo Bonzini]

On 09/27/2010 10:32 AM, Markus Armbruster wrote:
> Why the conditional?  cmp -s fails fine when argument files don't exist.
> 
> Note that common versions of make including GNU Make do not stat a
> rule's target to check whether the rule changed it.  Instead, they
> assume it changed, and remake everything depending on it.
> 
>      address@hidden:~/tmp$ cat Makefile
>      foo: bar
>              echo "Remaking foo"
> 
>      bar:
>              [ -f $@ ] || touch $@&&  echo "Touched bar"
>      address@hidden:~/tmp$ rm -f foo
>      address@hidden:~/tmp$ make
>      [ -f bar ] || touch bar&&  echo "Touched bar"
>      Touched bar
>      echo "Remaking foo"
>      Remaking foo
>      address@hidden:~/tmp$ make
>      echo "Remaking foo"
>      Remaking foo
> 
> I doubt your patch avoids churn as advertized with such makes.

Indeed, see how it's done for config-*.h.

# Uses generic rule in rules.mak
trace.h: trace.h-timestamp
trace.h-timestamp: $(SRC_PATH)/trace-events config-host.mak
        $(call quiet-command,sh $(SRC_PATH)/tracetool --$(TRACE_BACKEND) -h < 
$< > $@,"  GEN  trace.h)
        @cmp $@ trace.h >/dev/null 2>&1 || cp $@ trace.h

(untested).

Paolo