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

[Yury Norov]

On Wed, Jan 21, 2026 at 02:57:19PM +0000, david.laight.linux@xxxxxxxxx wrote:
> From: David Laight <david.laight.linux@xxxxxxxxx>
> 
> 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@xxxxxxxxx>

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. Can you consider flipping the 'W=c'
behavior?

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? 

What does this 'c' stands for?

Thanks,
Yury

> ---
>  scripts/Makefile.warn | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/Makefile.warn b/scripts/Makefile.warn
> index 68e6fafcb80c..e8a799850973 100644
> --- a/scripts/Makefile.warn
> +++ b/scripts/Makefile.warn
> @@ -2,8 +2,9 @@
>  # ==========================================================================
>  # make W=... settings
>  #
> -# There are four warning groups enabled by W=1, W=2, W=3, and W=e
> -# They are independent, and can be combined like W=12 or W=123e.
> +# There are five warning groups enabled by W=c, W=1, W=2, W=3, and W=e
> +# They are independent, and can be combined like W=12 or W=123e,
> +# except that W=1 implies W=c.
>  # ==========================================================================
>  
>  # Default set of warnings, always enabled
> @@ -109,6 +110,13 @@ KBUILD_CFLAGS += $(call cc-option,-Wenum-conversion)
>  
>  KBUILD_CFLAGS += -Wunused
>  
> +#
> +# W=c - Expensive compile-time checks, implied by W=1
> +#
> +ifneq ($(findstring c, $(KBUILD_EXTRA_WARN))$(findstring 1, $(KBUILD_EXTRA_WARN)),)
> +KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARNc
> +endif
> +
>  #
>  # W=1 - warnings which may be relevant and do not occur too often
>  #
> -- 
> 2.39.5