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

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v1 01/40] trace: add build framework for merging trace-events files
Date: Tue, 14 Jun 2016 15:53:48 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

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.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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