linux-kernel - Re: [PATCH 1/1] kbuild: Only enable -Wtautological-constant-out-of-range-compare for W=2

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Message-ID: <20251219201231.GB1404453@ax162>
Date: Fri, 19 Dec 2025 13:12:31 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: david.laight.linux@...il.com
Cc: Arnd Bergmann <arnd@...db.de>, Nicolas Schier <nsc@...nel.org>,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kbuild: Only enable
 -Wtautological-constant-out-of-range-compare for W=2

Hi David,

On Sun, Dec 14, 2025 at 01:15:28PM +0000, david.laight.linux@...il.com wrote:
> From: David Laight <david.laight.linux@...il.com>
> 
> The kernel code style is to use !(expr) rather that (expr) == 0.
> But clang complains that converting some constant expressions
> (eg (0xffffu << 16)) to a boolean always evalutes to true.
> This happens often in the validity checks in #defines.
> Move tautological-constant-out-of-range-compare to W=2 (along with the
> similar type-limits).
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>

I would like Arnd to comment on this before applying because the
reasoning of this change does not feel good enough to disable this
warning. It is not like '== 0' is inherently uncommon in the kernel or
hard to write to avoid the implicit conversion warning. To be honest, I
am a bit surprised -Wtautological-constant-out-of-range-compare fires
for that instead of some sort of -Wconversion warning...

> ---
>  scripts/Makefile.warn | 2 +-I
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/scripts/Makefile.warn b/scripts/Makefile.warn
> index 68e6fafcb80c..e2d467835c5b 100644
> --- a/scripts/Makefile.warn
> +++ b/scripts/Makefile.warn
> @@ -151,7 +151,6 @@ KBUILD_CFLAGS += -Wformat-insufficient-args
>  endif
>  endif
>  KBUILD_CFLAGS += $(call cc-option, -Wno-pointer-to-enum-cast)
> -KBUILD_CFLAGS += -Wno-tautological-constant-out-of-range-compare
>  KBUILD_CFLAGS += $(call cc-option, -Wno-unaligned-access)
>  KBUILD_CFLAGS += -Wno-enum-compare-conditional
>  endif
> @@ -179,6 +178,7 @@ KBUILD_CFLAGS += -Wno-shift-negative-value
>  
>  ifdef CONFIG_CC_IS_CLANG
>  KBUILD_CFLAGS += -Wno-enum-enum-conversion
> +KBUILD_CFLAGS += -Wno-tautological-constant-out-of-range-compare
>  endif
>  
>  ifdef CONFIG_CC_IS_GCC
> -- 
> 2.39.5
>