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

From: Jani Nikula <jani.nikula@linux.intel.com>
To: Nathan Chancellor <natechancellor@gmail.com>
Cc: clang-built-linux@googlegroups.com,
	"Michel Dänzer" <michel@daenzer.net>,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
Date: Fri, 14 Feb 2020 00:05:24 +0200	[thread overview]
Message-ID: <87mu9mw3bv.fsf@intel.com> (raw)
In-Reply-To: <20200213214812.GA7980@ubuntu-m2-xlarge-x86>

On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote:
> On Thu, Feb 13, 2020 at 04:37:15PM +0200, Jani Nikula wrote:
>> On Wed, 12 Feb 2020, Michel Dänzer <michel@daenzer.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...
>
> Would you prefer a patch that adds that cast rather than silencing the
> warning outright? It does appear to work for clang.

I'd take the cast.

If that fails for whatever reason, per-file

CFLAGS_gem/i915_gem_execbuffer.o = $(call cc-disable-warning, tautological-constant-out-of-range-compare)

over subdir-ccflags-y would be preferrable I think.

BR,
Jani.

>
> Cheers,
> Nathan

-- 
Jani Nikula, Intel Open Source Graphics Center
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