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

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Message-ID: <d81a2cfe-79b6-51d4-023e-0960c0593856@daenzer.net>
Date:   Wed, 12 Feb 2020 18:17:04 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        dri-devel@...ts.freedesktop.org,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Subject: Re: [PATCH v2] drm/i915: Disable
 -Wtautological-constant-out-of-range-compare

On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
> On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
>> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
>>> 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?
>>
>> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
>>
>>
>> Anyway, this suggests a possible better solution:
>>
>> #if UINT_MAX == ULONG_MAX
>> 	if (unlikely(remain > N_RELOC(ULONG_MAX)))
>> 		return -EINVAL;
>> #endif
>>
>>
>> Or if that can't be used for some reason, something like
>>
>> 	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
>> 		return -EINVAL;
>>
>> should silence the warning.
> 
> I do like this one better than the former.

FWIW, one downside of this one compared to all alternatives (presumably)
is that it might end up generating actual code even on 64-bit, which
always ends up skipping the return.


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