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Date:   Fri, 14 Feb 2020 06:46:13 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Michel Dänzer <michel@...nzer.net>
Subject: Re: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate_vma

On Fri, Feb 14, 2020 at 08:32:19AM +0000, Chris Wilson wrote:
> 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

As noted in the commit message, I ran diff <(objdump -Dr) <(objdump -Dr)
on objects files compiled with and without the patch with clang and gcc
for x86_64 and gcc for i386 (i386 does not build with clang) and there
was zero difference aside from the file names.

At the end of the day, I do not really care how the warning get fixed,
just that it does since it is the only one on x86_64 defconfig.

Cheers,
Nathan

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