From: Nick Desaulniers <firstname.lastname@example.org> To: "Michel Dänzer" <email@example.com> Cc: Nathan Chancellor <firstname.lastname@example.org>, email@example.com, LKML <firstname.lastname@example.org>, clang-built-linux <email@example.com>, dri-devel <firstname.lastname@example.org>, Rodrigo Vivi <email@example.com> Subject: Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare Date: Thu, 13 Feb 2020 14:43:21 -0800 [thread overview] Message-ID: <CAKwvOdm4eS19-D3pEkKsyZw7VjJP9Jeh5gMZaszwgjrJe63yUg@mail.gmail.com> (raw) In-Reply-To: <firstname.lastname@example.org> On Wed, Feb 12, 2020 at 9:17 AM Michel Dänzer <email@example.com> 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. The warning is pointing out that the conditional is always false, which is correct on 64b. The check is only active for 32b. https://godbolt.org/z/oQrgT_ The cast silences the warning for 64b. (Note that GCC and Clang also generate precisely the same instruction sequences in my example, just GCC doesn't warn on such tautologies). -- Thanks, ~Nick Desaulniers
next prev parent reply other threads:[~2020-02-13 22:43 UTC|newest] Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-11 5:08 [PATCH] " Nathan Chancellor 2020-02-11 6:13 ` [PATCH v2] " Nathan Chancellor 2020-02-11 9:41 ` Michel Dänzer 2020-02-11 20:39 ` Nathan Chancellor 2020-02-12 8:52 ` Michel Dänzer 2020-02-12 17:07 ` Nathan Chancellor 2020-02-12 17:17 ` Michel Dänzer 2020-02-13 14:37 ` [Intel-gfx] " Jani Nikula 2020-02-13 21:48 ` Nathan Chancellor 2020-02-13 22:05 ` Jani Nikula 2020-02-13 22:43 ` Nick Desaulniers [this message] 2020-02-13 23:27 ` Nathan Chancellor
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