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

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Date:   Fri, 14 Feb 2020 16:36:12 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        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, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate_vma

On 2020-02-14 12:49 p.m., Jani Nikula wrote:
> 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.

Not really:

    if (sizeof(remain) >= sizeof(unsigned long) &&
	unlikely(remain > N_RELOC(ULONG_MAX)))
             return -EINVAL;

In contrast to the cast, this doesn't generate any machine code on 64-bit:

https://godbolt.org/z/GmUE4S

but still generates the same code on 32-bit:

https://godbolt.org/z/hAoz8L


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer