Re: [PATCH 1/9] build: use if_changed more consistently (and correctly) for prelink*.o

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.09.2020 13:56, Roger Pau Monné wrote:
> On Mon, Sep 14, 2020 at 12:15:39PM +0200, Jan Beulich wrote:
>> Switch to $(call if_changed,ld) where possible; presumably not doing so
>> in e321576f4047 ("xen/build: start using if_changed") right away was an
>> oversight, as it did for Arm in (just) one case. It failed to add
>> prelink.o to $(targets), though, causing - judging from the observed
>> behavior on x86 - undue rebuilds of the final binary (because of
>> prelink.o getting rebuild for $(cmd_prelink.o) being empty, in turn
>> because of .prelink.o.cmd not getting read) during "make install-xen".
> 
> I'm not sure I follow why prelink.o needs to be added to targets, does
> this offer some kind of protection against rebuilds when doing make
> install?

In a way, but (as I view it) not really. It is the use of ...

> The switch to if_changed LGTM.

... if_changed which requires this. .*.cmd files will only be loaded
for anything explicitly or implicitly listed as a target. While .o
coming from $(obj-y) get added there automatically, prelink.o is not
something that could be recognized as needing adding, hence the
"manual" insertion.

Without .prelink.o.cmd loaded, $(if_changed ) will always arrange
for it to get re-built, because it then will consider the command
used to build the file to have changed (as the stored one appears to
be empty).

Jan