linux-kernel - Re: [PATCH 1/2] kbuild: remove gcc's -Wtype-limits

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Message-ID: <aURXpAwm-ITVlHMl@stanley.mountain>
Date: Thu, 18 Dec 2025 22:36:04 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Vincent Mailhol <mailhol@...nel.org>
Cc: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nsc@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Chris Mason <clm@...com>, David Sterba <dsterba@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kbuild@...r.kernel.org, linux-sparse@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	dri-devel@...ts.freedesktop.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 1/2] kbuild: remove gcc's -Wtype-limits

On Thu, Dec 18, 2025 at 07:50:01PM +0100, Vincent Mailhol wrote:
> W=2 builds are heavily polluted by the -Wtype-limits warning.
> 
> Here are some W=12 statistics on Linux v6.19-rc1 for an x86_64
> defconfig (with just CONFIG_WERROR set to "n") using gcc 14.3.1:
> 
> 	 Warning name			count	percent
> 	-------------------------------------------------
> 	 -Wlogical-op			    2	  0.00 %
> 	 -Wmaybe-uninitialized		  138	  0.20 %
> 	 -Wunused-macros		  869	  1.24 %
> 	 -Wmissing-field-initializers	 1418	  2.02 %
> 	 -Wshadow			 2234	  3.19 %
> 	 -Wtype-limits			65378	 93.35 %
> 	-------------------------------------------------
> 	 Total				70039	100.00 %
> 
> As we can see, -Wtype-limits represents the vast majority of all
> warnings. The reason behind this is that these warnings appear in
> some common header files, meaning that some unique warnings are
> repeated tens of thousands of times (once per header inclusion).
> 
> Add to this the fact that each warning is coupled with a dozen lines
> detailing some macro expansion. The end result is that the W=2 output
> is just too bloated and painful to use.
> 
> Three years ago, I proposed in [1] modifying one such header to
> silence that noise. Because the code was not faulty, Linus rejected
> the idea and instead suggested simply removing that warning.
> 
> At that time, I could not bring myself to send such a patch because,
> despite its problems, -Wtype-limits would still catch the below bug:
> 
> 	unsigned int ret;
> 
> 	ret = check();
> 	if (ret < 0)
> 		error();
> 
> Meanwhile, based on another suggestion from Linus, I added a new check
> to sparse [2] that would catch the above bug without the useless spam.
> 
> With this, remove gcc's -Wtype-limits. People who still want to catch
> incorrect comparisons between unsigned integers and zero can now use
> sparse instead.
> 
> On a side note, clang also has a -Wtype-limits warning but:
> 
>   * it is not enabled in the kernel at the moment because, contrary to
>     gcc, clang did not include it under -Wextra.
> 
>   * it does not warn if the code results from a macro expansion. So,
>     if activated, it would not cause as much spam as gcc does.
> 
>   * -Wtype-limits is split into four sub-warnings [3] meaning that if
>     it were to be activated, we could select which one to keep.
> 

Sounds good.  I like your Sparse check.

Maybe we should enable the Sparse checking as well because it sounds
like they are doing a lot of things right.  I think Smatch catches the
same bugs that Clang would but it would be good to have multiple
implementations.  The -Wtautological-unsigned-enum-zero-compare trips
people up because they aren't necessarily expecting enums to be
unsigned.

regards,
dan carpenter