linux-kernel - Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

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Message-ID: <4c806435-f32d-1559-9563-ffe3fa69f0d1@daenzer.net>
Date:   Tue, 11 Feb 2020 10:41:48 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     clang-built-linux@...glegroups.com,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2] drm/i915: Disable
 -Wtautological-constant-out-of-range-compare

On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> A recent commit in clang added -Wtautological-compare to -Wall, which is
> enabled for i915 so we see the following warning:
> 
> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> result of comparison of constant 576460752303423487 with expression of
> type 'unsigned int' is always false
> [-Wtautological-constant-out-of-range-compare]
>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> 
> This warning only happens on x86_64 but that check is relevant for
> 32-bit x86 so we cannot remove it.

That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
in both cases, and remain is a 32-bit value in both cases. How can it be
larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer