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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 2 Dec 2019 11:18:20 -0800
Message-ID: <CAKwvOdniXqn3xt3-W0Pqi-X1nWjJ2vUVofjCm1O-UPXZ7_4rXw@mail.gmail.com>
Subject: Re: [PATCH] drm/i915: Remove tautological compare in eb_relocate_vma
To:     Chris Wilson <chris@chris-wilson.co.uk>
Cc:     Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        intel-gfx@lists.freedesktop.org,
        dri-devel <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Sat, Nov 23, 2019 at 12:05 PM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>
> Quoting Nathan Chancellor (2019-11-23 19:53:22)
> > -Wtautological-compare was recently added to -Wall in LLVM, which
> > exposed an if statement in i915 that is always false:
> >
> > ../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)))
> >             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> >
> > Since remain is an unsigned int, it can never be larger than UINT_MAX,
> > which is less than ULONG_MAX / sizeof(struct drm_i915_gem_relocation_entry).
> > Remove this statement to fix the warning.
>
> The check should remain as we do want to document the overflow
> calculation, and it should represent the types used -- it's much easier

What do you mean "represent the types used?"  Are you concerned that
the type of drm_i915_gem_exec_object2->relocation_count might change
in the future?

> to review a stub than trying to find a missing overflow check. If the
> overflow cannot happen as the types are wide enough, no problem, the
> compiler can remove the known false branch.

What overflow are you trying to protect against here?

>
> Tautology here has a purpose for conveying information to the reader.

Well leaving a warning unaddressed is also not a solution.  Either
replace it with a comment or turn off the warning for your subdir.

The warning here looks valid to me; you have a guard for something
that's impossible.
-- 
Thanks,
~Nick Desaulniers