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

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Message-ID: <87o8u1wfqs.fsf@intel.com>
Date:   Fri, 14 Feb 2020 13:49:31 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>,
        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

On Fri, 14 Feb 2020, Chris Wilson <chris@...is-wilson.co.uk> 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?

It seems that goal and the warning are fundamentally incompatible.

Back to the original patch?

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center