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

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Message-ID: <877e0qy2n8.fsf@intel.com>
Date:   Thu, 13 Feb 2020 16:37:15 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Michel Dänzer <michel@...nzer.net>,
        Nathan Chancellor <natechancellor@...il.com>
Cc:     clang-built-linux@...glegroups.com,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

On Wed, 12 Feb 2020, Michel Dänzer <michel@...nzer.net> wrote:
> 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.

I like this better than the UINT_MAX == ULONG_MAX comparison because
that creates a dependency on the type of remain.

Then again, a sufficiently clever compiler could see through the cast,
and flag the warning anyway...

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center