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Date: Mon, 27 Sep 2010 18:16:55 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH 2/2] trace: avoid unnecessary recompilation
 if nothing changed
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To: Blue Swirl <blauwirbel@gmail.com>
Cc: Markus Armbruster <armbru@redhat.com>, qemu-devel <qemu-devel@nongnu.org>

On 09/27/2010 06:13 PM, Blue Swirl wrote:
>> 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).
>
> I just copied the rule from %/config-devices.mak. Is also that rule
> then incorrect?

config-devices.mak will cause the Makefile to be reread, but no 
recompilations, so it's fine not to use a timestamp file.  Headers are 
more complicated.

> Perhaps there could be a macro for this, not unlike move-if-change
> script in various GNU packages?

The problem is not really the move-if-change script, but rather the 
timestamping rules.  You can use $(eval) for that but it quickly becomes 
unmanageable.

Paolo