Re: [PATCH next 02/14] kbuild: Add W=c for additional compile time checks

[David Laight <david.laight.linux@gmail.com>]

On Mon, 2 Feb 2026 21:47:26 -0700
Nathan Chancellor <nathan@kernel.org> wrote:

> On Mon, Feb 02, 2026 at 08:07:43PM +0000, David Laight wrote:
> > On Mon, 2 Feb 2026 13:33:22 -0500
> > Yury Norov <ynorov@nvidia.com> wrote:
> >   
> > > On Wed, Jan 21, 2026 at 02:57:19PM +0000, david.laight.linux@gmail.com wrote:  
> > > > From: David Laight <david.laight.linux@gmail.com>
> > > > 
> > > > Some compile time checks significantly bloat the pre-processor output
> > > > (particularly when the get nested).
> > > > Since the checks aren't really needed on every compilation enable with
> > > > W=c (adds -DKBUILD_EXTRA_WARNc) so the checks can be enabled per-build.
> > > > Make W=1 imply W=c so the build-bot includes the checks.
> > > > 
> > > > As well as reducing the bloat from existing checks (like those in
> > > > GENMASK() and FIELD_PREP()) it lets additional checks be added
> > > > while there are still 'false positives' without breaking normal builds.
> > > > 
> > > > Signed-off-by: David Laight <david.laight.linux@gmail.com>    
> > > 
> > > Honestly I don't understand this. AFAIU, you've outlined a list of
> > > compiler warnings that slow the compilation down, and you group them
> > > under 'W=c' option.
> > > 
> > > What is the use case for it outside of your series. I think, a typical
> > > user would find more value in an option that enables some warnings but
> > > doesn't sacrifices performance.  
> > 
> > All the compile-time warnings slow down the compilation.
> > Even apparently trivial ones (like the check in the generic READ_ONCE()
> > that the size is 1, 2, 4 or 8 bytes) have a measurable effect.    
> 
> What kind of difference are we talking about here across a regular
> build?

IIRC a couple of %.
I repeated a few builds with/without and it was much greater than the
variation between the builds.
That is a pretty trivial test, but there are a lot of READ_ONCE().
Find a few of those and the difference is significant.

> 
> > The code a typical user compiles won't have anything that trips any of
> > the compile-time asserts.
> > They only really happen when compiling new code or adding new checks.
> >   
> > > Can you consider flipping the 'W=c' behavior?  
> > 
> > Why, most of the time you don't want the checks because the code is
> > known to pass them all.
> > 
> > It also means it can be used for new checks before all the bugs (and
> > false positives) have been fixed.
> > I did think of just enabling the checks for W=1 builds, but that makes
> > it far to hard to enable them.
> > As it is you can use W=ce to enable them and stop on warnings and errors.
> > 
> > Also W=xxx can only really add checks (there are some checks for it being
> > non-empty). Documenting that W=x stopped the 'x' checks being done
> > would be painful.  
> 
> Yeah, I don't think we should be overloading the meaning of W=.

I did thing of supporting W=-e so that you can do W=1-e on a config
that enables WERROR, since there isn't much chance of a W=1e build
succeeding any time soon.

> > > Can you please explicitly mention warnings included in W=c vs W=1? Can
> > > you report compilation time for W=0, W=1 and W=c? What if one needs to
> > > enable fast/slow warnings from 2nd or 3rd level? Would W=2c or W=3c
> > > work in this case?  
> > 
> > The W=123 options are all completely independent, my W=c is the same.
> > I'm not sure it is sane to run W=2 rather than W=12, but it is allowed.
> > 
> > I made W=1 imply W=1c so that the build bot would pick up the extra checks.
> > Apart from that all the 'W' flags are independent.
> > W=123 fiddle with the command line -W options and set -DKBUILD_EXTRA_WARN[123]
> > so that files can include extra checks.
> > W=c just sets the equivalent -D option.  
> 
> Why even have a different option then? You mention above placing the
> checks in W=1 makes them too hard to enable but then you say that you
> made W=1 imply W=c?

What I meant was it is hard to see the errors in a W=1 build.
So I wanted the enable the checks without picking up W=1.

> 
> > > What does this 'c' stands for?  
> > 
> > Anything you want it to :-)
> > Discussion session arranged for 2pm-5pm by the bike shed.  
> 
> W=c already exists, it is documented as enabling extra Kconfig checks in
> 'make help'. Maybe W=x for "ex"pensive checks? Or maybe if we really
> want W=c, maybe rename W=c to W=k for Kconfig? I do not really have a
> strong preference.

It was 'c' for Compile time (a bit silly - but they are source 'compile-time'
tests rather then 'run-time' ones).
I'd only looked in makefile.warn for other uses.

> > In some sense it is 'more warnings than default, but not as many as W=1'.
> > Like a lot of the W=1 warnings I wanted to be able to pick up 'code quality'
> > issues without breaking the build for normal people.  
> 
> So something like a W=0.5? :)

That is really what I was looking for.
Perhaps setting -DKBUILD_WARN_LEVEL=level*10 would do?
So W=1 would set 10, W=2 20 (etc) with some scheme for setting intermediate
values (or at least (say) 5).

> > There are definitely some other checks that could be relegated to W=c
> > once it has been added.
> > 
> > I'd also like to add some checks to min_t/max_t/clamp_t to pick up the
> > worst of the dodgy/broken code without having to get all the patches
> > accepted before the test itself is committed.
> > Tests for code like clamp_t(u32, u64_val, 0, ~0u) (yes there are some)
> > tend to get very long and may be problematic if enabled by default
> > (I got burnt by the 10MB expansion of nested min().)  
> 
> I do not have a super strong opinion about this overall but it feels
> like this is a bit of a slippery slope with warning fragmentation. I
> think I would rather see these be treated like regular compiler warnings
> where the majority of instances are cleaned up then it is added to W=1
> to keep the tree clean.

That is slightly different because (mostly) they are warnings from a
new compiler version so are picked up by people running the new compiler
before they break things and then added to W=1.
Adding an extra check to an existing header needs the change committing
to get test coverage somewhat earlier - maybe to force some reluctant
maintainers to actually fix their code.

I definitely needed to conditionally include the checks with W=e but
without W=1, this seemed the best way to do it.

	David

> 
> Cheers,
> Nathan