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

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Date:   Wed, 12 Feb 2020 09:52:52 +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 2020-02-11 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.


Either of these should be better than completely disabling the warning
for the whole file.


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