linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Nathan Chancellor <natechancellor@gmail.com>
To: Jani Nikula <jani.nikula@linux.intel.com>
Cc: "Michel Dänzer" <michel@daenzer.net>,
	clang-built-linux@googlegroups.com,
	intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
Date: Thu, 13 Feb 2020 14:48:12 -0700	[thread overview]
Message-ID: <20200213214812.GA7980@ubuntu-m2-xlarge-x86> (raw)
In-Reply-To: <877e0qy2n8.fsf@intel.com>

On Thu, Feb 13, 2020 at 04:37:15PM +0200, Jani Nikula wrote:
> On Wed, 12 Feb 2020, Michel Dänzer <michel@daenzer.net> 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

  reply	other threads:[~2020-02-13 21:48 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 [this message]
2020-02-13 22:05                 ` Jani Nikula
2020-02-13 22:43             ` Nick Desaulniers
2020-02-13 23:27               ` Nathan Chancellor

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200213214812.GA7980@ubuntu-m2-xlarge-x86 \
    --to=natechancellor@gmail.com \
    --cc=clang-built-linux@googlegroups.com \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=intel-gfx@lists.freedesktop.org \
    --cc=jani.nikula@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=michel@daenzer.net \
    --subject='Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).