On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@xxxxxxxxx> wrote: > On Thu, Feb 13, 2020 at 04:37:15PM +0200, Jani Nikula wrote: >> On Wed, 12 Feb 2020, Michel Dänzer <michel@xxxxxxxxxxx> wrote: >> > On 2020-02-12 6:07 p.m., Nathan Chancellor wrote: >> >> On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote: >> >>> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote: >> >>>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote: >> >>>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote: >> >>>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is >> >>>>>> enabled for i915 so we see the following warning: >> >>>>>> >> >>>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: 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))) >> >>>>>> ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~ >> >>>>>> >> >>>>>> This warning only happens on x86_64 but that check is relevant for >> >>>>>> 32-bit x86 so we cannot remove it. >> >>>>> >> >>>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value >> >>>>> in both cases, and remain is a 32-bit value in both cases. How can it be >> >>>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)? >> >>>>> >> >>>> >> >>>> Hi Michel, >> >>>> >> >>>> Can't this condition be true when UINT_MAX == ULONG_MAX? >> >>> >> >>> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit. >> >>> >> >>> >> >>> Anyway, this suggests a possible better solution: >> >>> >> >>> #if UINT_MAX == ULONG_MAX >> >>> if (unlikely(remain > N_RELOC(ULONG_MAX))) >> >>> return -EINVAL; >> >>> #endif >> >>> >> >>> >> >>> Or if that can't be used for some reason, something like >> >>> >> >>> if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX))) >> >>> return -EINVAL; >> >>> >> >>> should silence the warning. >> >> >> >> I do like this one better than the former. >> > >> > FWIW, one downside of this one compared to all alternatives (presumably) >> > is that it might end up generating actual code even on 64-bit, which >> > always ends up skipping the return. >> >> I like this better than the UINT_MAX == ULONG_MAX comparison because >> that creates a dependency on the type of remain. >> >> Then again, a sufficiently clever compiler could see through the cast, >> and flag the warning anyway... > > Would you prefer a patch that adds that cast rather than silencing the > warning outright? It does appear to work for clang. I'd take the cast. If that fails for whatever reason, per-file CFLAGS_gem/i915_gem_execbuffer.o = $(call cc-disable-warning, tautological-constant-out-of-range-compare) over subdir-ccflags-y would be preferrable I think. BR, Jani. > > Cheers, > Nathan -- Jani Nikula, Intel Open Source Graphics Center _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
