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... BR, Jani. -- Jani Nikula, Intel Open Source Graphics Center _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
