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SubjectRe: [PATCH] drm/i915: Cast remain to unsigned long in eb_relocate_vma
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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?
-Chris

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