linux-kernel - Re: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate

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Message-ID: <158166913989.4660.10674824117292988120@skylake-alporthouse-com>
Date:   Fri, 14 Feb 2020 08:32:19 +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>,
        Michel Dänzer <michel@...nzer.net>
Subject: Re: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate_vma

Quoting Jani Nikula (2020-02-14 06:36:15)
> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@...il.com> wrote:
> > A recent commit in clang added -Wtautological-compare to -Wall, which is
> > enabled for i915 after -Wtautological-compare is disabled for the rest
> > of the kernel so we see the following warning on x86_64:
> >
> >  ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1433:22: 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)))
> >             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> >  ../include/linux/compiler.h:78:42: note: expanded from macro 'unlikely'
> >  # define unlikely(x)    __builtin_expect(!!(x), 0)
> >                                             ^
> >  1 warning generated.
> >
> > It is not wrong in the case where ULONG_MAX > UINT_MAX but it does not
> > account for the case where this file is built for 32-bit x86, where
> > ULONG_MAX == UINT_MAX and this check is still relevant.
> >
> > Cast remain to unsigned long, which keeps the generated code the same
> > (verified with clang-11 on x86_64 and GCC 9.2.0 on x86 and x86_64) and
> > the warning is silenced so we can catch more potential issues in the
> > future.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/778
> > Suggested-by: Michel Dänzer <michel@...nzer.net>
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> 
> Works for me as a workaround,

But the whole point was that the compiler could see that it was
impossible and not emit the code. Doesn't this break that?
-Chris