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

From: Nathan Chancellor <natechancellor@gmail.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Michel Dänzer" <michel@daenzer.net>,
	intel-gfx@lists.freedesktop.org,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	"Rodrigo Vivi" <rodrigo.vivi@intel.com>
Subject: Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
Date: Thu, 13 Feb 2020 16:27:23 -0700	[thread overview]
Message-ID: <20200213232723.GA26697@ubuntu-m2-xlarge-x86> (raw)
In-Reply-To: <CAKwvOdm4eS19-D3pEkKsyZw7VjJP9Jeh5gMZaszwgjrJe63yUg@mail.gmail.com>

On Thu, Feb 13, 2020 at 02:43:21PM -0800, Nick Desaulniers wrote:
> On Wed, Feb 12, 2020 at 9:17 AM 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.
> 
> The warning is pointing out that the conditional is always false,
> which is correct on 64b.  The check is only active for 32b.
> https://godbolt.org/z/oQrgT_
> The cast silences the warning for 64b.  (Note that GCC and Clang also
> generate precisely the same instruction sequences in my example, just
> GCC doesn't warn on such tautologies).

Thanks for confirming! I'll send a patch to add the cast later tonight.

Cheers,
Nathan