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. Cheers, Nathan _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
