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

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Message-ID: <20200213214812.GA7980@ubuntu-m2-xlarge-x86>
Date:   Thu, 13 Feb 2020 14:48:12 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Michel Dänzer <michel@...nzer.net>,
        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 Thu, Feb 13, 2020 at 04:37:15PM +0200, Jani Nikula wrote:
> 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...

Would you prefer a patch that adds that cast rather than silencing the
warning outright? It does appear to work for clang.

Cheers,
Nathan