[PATCH v2 0/2] kbuild: Cache exploratory calls to the compiler

From: Douglas Anderson
Date: Wed Oct 04 2017 - 18:38:31 EST

This two-patch series attempts to speed incremental builds of the
kernel up by a bit. How much of a speedup you get depends a lot on
your environment, specifically the speed of your workstation and how
fast it takes to invoke the compiler.

In the Chrome OS build environment you get a really big win. For an
incremental build (via emerge) I measured a speedup from ~1 minute to
~35 seconds. ...but Chrome OS calls the compiler through a number of
wrapper scripts and also calls the kernel make at least twice for an
emerge (during compile stage and install stage), so it's a bit of a
worst case.

Perhaps a more realistic measure of the speedup others might see is
running "time make help > /dev/null" outside of the Chrome OS build
environment on my system. When I do this I see that it took more than
1.0 seconds before and less than 0.2 seconds after. So presumably
this has the ability to shave ~0.8 seconds off an incremental build
for most folks out there. While 0.8 seconds savings isn't huge, it
does make incremental builds feel a lot snappier.

Caveats from v1 still copied here, though with Masahiro Yamada's
suggestions from v1 this is starting to feel a little more baked and
I've even dropped the RFC from it (though extra testing still

Please note that I make no illusions of being a Makefile expert nor do
I have any belief that I fully understand the Linux kernel build
system. Please take this patch series as the start of a discussion
about whether others feel like this type of speedup is worthwhile and
how to best accomplish it. Specific things to note:

- I'm happy to paint the bikeshed any color that maintainers want. If
you'd like the cache named differently, in a slightly different
format, or you want me to adjust the spacing / names of Makefile
stuff then please just let me know.

- If this is totally the wrong approach and you have a better idea
then let me know. If you want something that's super complicated to
explain then feel free to post a replacement patch and I'm happy to

- This patch definitely needs extra testing. I've tested it on a very
limited build environment and it seems to be working fine, but I
could believe that with some weird compiler options or on certain
architectures you might need some extra escaping here and there.

Changes in v2:
- Abstract at a different level (like shell-cached) per Masahiro Yamada
- Include ld-version, which I missed the first time

Douglas Anderson (2):
kbuild: Add a cache for generated variables
kbuild: Cache a few more calls to the compiler

Documentation/kbuild/makefiles.txt | 21 +++++++++
Makefile | 4 +-
scripts/Kbuild.include | 90 ++++++++++++++++++++++++++++++++------
3 files changed, 99 insertions(+), 16 deletions(-)