On 2020-02-14 12:49 p.m., Jani Nikula wrote: > On Fri, 14 Feb 2020, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: >> Quoting Jani Nikula (2020-02-14 06:36:15) >>> On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@xxxxxxxxx> 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@xxxxxxxxxxx> >>>> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx> >>> >>> 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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
