From: "Michel Dänzer" <email@example.com> To: Nathan Chancellor <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, Rodrigo Vivi <email@example.com> Subject: Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare Date: Wed, 12 Feb 2020 18:17:04 +0100 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <20200212170734.GA16396@ubuntu-m2-xlarge-x86> 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. -- Earthling Michel Dänzer | https://redhat.com Libre software enthusiast | Mesa and X developer
next prev parent reply other threads:[~2020-02-12 17:17 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 [this message] 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 2020-02-13 23:27 ` Nathan Chancellor
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