Re: [Qemu-devel] [PATCH v1 01/40] trace: add build framework for merging trace-events files

[Stefan Hajnoczi]

On Tue, Jun 14, 2016 at 03:53:48PM -0600, Eric Blake wrote:
> On 06/14/2016 06:26 AM, Stefan Hajnoczi wrote:
> > On Thu, Jun 09, 2016 at 05:57:55PM +0100, Daniel P. Berrange wrote:
> >> +$(BUILD_DIR)/trace-events-all: $(trace-events-y:%=$(SRC_PATH)/%)
> >> +  $(call quiet-command,cat $^ > $@)
> > 
> > $^ needs to be a stable ordering across make invocations and across
> > machines to avoid thrashing ccache and spuriously renumbering trace
> > event IDs.
> > 
> > I guess it is stable but just in case anyone knows of a case where $^
> > would change order without any changes to the actual files (e.g.
> > readdir(2) returning things in a different order, make parallel jobs,
> > etc)...
> 
> As far as I can tell from 'info make', $^ is the list of prerequisites,
> _in prerequisite order_, minus duplicates.  So the only way the order
> will change is if you edit Makefile, and thus it looks stable to me.

Thanks for confirming.

Stefan

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