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SubjectRe: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate_vma
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On Fri, 14 Feb 2020, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> Quoting Jani Nikula (2020-02-14 06:36:15)
>> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.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@daenzer.net>
>> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.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

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