linux-kernel - Re: [PATCH] drm/i915: Remove tautological compare in eb_relocate

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Date:   Sat, 23 Nov 2019 20:05:07 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH] drm/i915: Remove tautological compare in eb_relocate_vma

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
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.

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