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

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Message-ID: <3ead6685-a5d4-4113-923d-84bf8aee49b3@kernel.org>
Date: Fri, 19 Dec 2025 23:21:51 +0100
From: Vincent Mailhol <mailhol@...nel.org>
To: Dan Carpenter <dan.carpenter@...aro.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 18/12/2025 at 23:31, Vincent Mailhol wrote:

(...)

> Concerning clang, here are the statistics:
> 
> 	$ make -s LLVM=1 CFLAGS_KERNEL="-Wtype-limits" 2>&1 | grep -o '\[-W\S*\]' | sort | uniq -c
> 	      2 [-Wtautological-type-limit-compare]
> 	     15 [-Wtautological-unsigned-enum-zero-compare]$ make -s LLVM=1 CFLAGS_KERNEL="-Wtype-limits"
> 
> (done on a linux v6.19-rc1 defconfig with clang v20.1.8)
> 
> Not so many warnings, at least, less than what I would have thought!
> 
> -Wtautological-unsigned-char-zero-compare and
> -Wtautological-unsigned-zero-compare gave zero findings. So those two
> can be enabled, I guess? I am still surprised that
> -Wtautological-unsigned-zero-compare gives nothing. I would have
> expected some kind of false positives on that one. No sure if I missed
> something here.

I was a bit worried of that -Wtautological-unsigned-zero-compare got
zero findings so I reran a build but this time on an allyesconfig
(minus CONFIG_WERROR):

	$ make -j8 -s LLVM=1 CFLAGS_KERNEL="-Wtype-limits" 2>&1 | grep -o '\[-W\S*\]' | sort | uniq -c
	     29 [-Wtautological-type-limit-compare]
	     55 [-Wtautological-unsigned-enum-zero-compare]
	     76 [-Wtautological-unsigned-zero-compare]

This is closer than expected. And looking at the findings,
-Wtautological-unsigned-zero-compare also warns on some sane code
which is just doing some range checks.

(...)

> In conclusion, I agree that we could enable three of clang's
> -Wtype-limits sub-warning. But this is not the scope of that series. I
> would rather prefer to have this as a separate series.

With this, I want to amend my conclusion. both
-Wtautological-unsigned-enum-zero-compare and
-Wtautological-unsigned-zero-compare should be kept disabled. The only
candidates are -Wtautological-type-limit-compare and
-Wtautological-unsigned-char-zero-compare.

-Wtautological-unsigned-char-zero-compare would need another study. It
seems that this warning is only triggered on platforms where char is
unsigned which explains why I did not see it when building on x86_64.

Well, I think I will stop this clang's -Wtype-limits study for the
moment. If anyone wants to continue the work please go ahead.


Yours sincerely,
Vincent Mailhol