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From: Nathan Chancellor <natechancellor@gmail.com>
To: "Michel Dänzer" <michel@daenzer.net>
Cc: Jani Nikula <jani.nikula@linux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@intel.com>,
	clang-built-linux@googlegroups.com,
	intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
Date: Tue, 11 Feb 2020 13:39:35 -0700	[thread overview]
Message-ID: <20200211203935.GA16176@ubuntu-m2-xlarge-x86> (raw)
In-Reply-To: <4c806435-f32d-1559-9563-ffe3fa69f0d1@daenzer.net>

On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
> 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)?
> 

Hi Michel,

Can't this condition be true when UINT_MAX == ULONG_MAX? clang does not
warn on a 32-bit x86 build from what I remember. Honestly, my
understanding of overflow is pretty shoddy, this is mostly based on what
I have heard from others.

I sent a patch trying to remove that check but had it rejected:

https://lore.kernel.org/lkml/20191123195321.41305-1-natechancellor@gmail.com/

Cheers,
Nathan

  reply	other threads:[~2020-02-11 20:39 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11  5:08 [PATCH] " Nathan Chancellor
2020-02-11  6:13 ` [PATCH v2] " Nathan Chancellor
2020-02-11  9:41   ` Michel Dänzer
2020-02-11 20:39     ` Nathan Chancellor [this message]
2020-02-12  8:52       ` Michel Dänzer
2020-02-12 17:07         ` Nathan Chancellor
2020-02-12 17:17           ` Michel Dänzer
2020-02-13 14:37             ` [Intel-gfx] " Jani Nikula
2020-02-13 21:48               ` Nathan Chancellor
2020-02-13 22:05                 ` Jani Nikula
2020-02-13 22:43             ` Nick Desaulniers
2020-02-13 23:27               ` Nathan Chancellor

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