These are potentially computed from inside each subdir, and in addition
due to what appears to be a regression in GNU make 4.4, where it is
reevaluating variables that contain $(shell) functions, many times (in
the order of thousands, this was slowing down the build, were on the
Debian amd64 build daemons it went from 5m with GNU make 4.3 to 2h40m
with GNU make 4.4. Although the bulk of the slow down has been fixed
with previous commits, the remaining optimizations are only to avoid
this potentially happening again in the future, and to reduce useless
duplicate work.
Instead of trying to cache the values from within make itself, where
programming this there is extremely painful, and does not seem to be
able to greatly reduce the number of calls, because the build system
is going to be called multiple times for different targets. Simply
externalize the generation into several shell scripts, that we call
to generate a make fragment that then we include from the various
Makefiles.
For a Debian build with GNU make 4.3, this reduces the amount of total
pkg-config calls from around ~1600 to 128, for dpkg-buildflags from
~1100 down to 6, and for dpkg-parsechangelog from ~56 to 17, but the
slow down is not as significant there anyway.
For a Debian build with GNU make 4.4, this reduces the amount of total
pkg-config calls from around ~2600 to 128, for dpkg-buildflags from
~2800 down to 6, and for dpkg-parsechangelog from ~350 to 21.
For a Debian build with GNU make 4.4, this reduces the build time
on this system from 2m10s to ~ 1m30s.
Change-Id: I427d0ea5106dc6ed1ff9e664ccdba2fa0725b7d0
Guillem Jover
7dbe24e4e5