From: Nathan Chancellor <nathan@kernel.org>
To: Vineet Gupta <vineet.gupta@linux.dev>
Cc: Nicolas Schier <nsc@kernel.org>, Borislav Petkov <bp@alien8.de>,
Peter Zijlstra <peterz@infradead.org>,
Jakub Jelinek <jakub@redhat.com>,
Julian Braha <julianbraha@gmail.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Bill Wendling <morbo@google.com>,
Justin Stitt <justinstitt@google.com>,
linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
llvm@lists.linux.dev
Subject: Re: [PATCH v2] kbuild: move gcc's -Wconstant-logical-operand to W=2
Date: Thu, 5 Nov 2026 12:45:41 -0700 [thread overview]
Message-ID: <20261105194541.GA2668266@ax162> (raw)
In-Reply-To: <20261105175748.3057520-1-vineet.gupta@linux.dev>
On Thu, Nov 05, 2026 at 09:57:45AM -0800, Vineet Gupta wrote:
> GCC 17 adds -Wconstant-logical-operand to -Wall. It warns when an operand
> of && or || folds to a constant other than 0 or 1. Unlike clang's version,
> it also checks constant left operands and operators that come from a
> macro expansion. With CONFIG_WERROR=y, an x86 defconfig build fails in
> prepare0:
>
> arch/x86/include/asm/cpufeature.h:72:36: error: use of logical '&&' with constant operand '4194304' [-Werror=constant-logical-operand]
>
> The warning is meant to catch && written for &. Kernel history has no
> such catch. Of the 13 earlier kernel fixes for clang's version, only
> commit dd7fe5d9fd6a ("rtc: max31335: Fix comparison in
> max31335_volatile_reg()") fixed a bug that the warning found. Commit
> 394b52462020 ("drm/i915/gem: fix bitwise and logical AND mixup") fixed a
> & written for &&, which the warning cannot see. The other fixes rewrote
> correct code. In GCC's own tree, the warning found two sites since it
> was added, and both were false positives.
>
> Disable the warning for GCC unless W=2 is given. Keep clang's version,
> which skips macros and constant left operands.
>
> Suggested-by: Borislav Petkov <bp@alien8.de>
> Link: https://lore.kernel.org/all/20261104160722.1637541-1-vineet.gupta@linux.dev/
> Signed-off-by: Vineet Gupta <vineet.gupta@linux.dev>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
This probably wants a 'Cc: stable' so that older kernels can build
warning free with GCC 17.
--
Cheers,
Nathan
prev parent reply other threads:[~2026-11-05 19:45 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-05 17:57 Vineet Gupta
2026-11-05 18:03 ` Nick Desaulniers
2026-11-05 19:45 ` Nathan Chancellor [this message]
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