From: Nathan Chancellor <email@example.com> To: "Michel Dänzer" <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 10:07:34 -0700 [thread overview] Message-ID: <20200212170734.GA16396@ubuntu-m2-xlarge-x86> (raw) In-Reply-To: <firstname.lastname@example.org> 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. > > > Either of these should be better than completely disabling the warning > for the whole file. Normally, I would agree but I am currently planning to leave -Wtautological-constant-out-of-range-compare disabled when I turn on -Wtautological-compare for the whole kernel because there are plenty of locations in the kernel where these kind of checks depend on various kernel configuration options and the general attitude of kernel developers is that this particular warning is not really helpful for that reason. I'll see if there is a general consensus before moving further since I know i915 turns on a bunch of extra warnings from the rest of the kernel (hence why we are in this situation). Cheers, Nathan
next prev parent reply other threads:[~2020-02-12 17:07 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 [this message] 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 2020-02-13 23:27 ` Nathan Chancellor
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