linux-kernel - Re: dealing with gcc 'comparison is always false' warnings

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Message-ID: <465DC9E9.3040904@imap.cc>
Date:	Wed, 30 May 2007 21:00:57 +0200
From:	Tilman Schmidt <tilman@...p.cc>
To:	Satyam Sharma <satyam.sharma@...il.com>
CC:	Roland Dreier <rdreier@...co.com>, openib-general@...nib.org,
	linux-kernel@...r.kernel.org
Subject: Re: dealing with gcc 'comparison is always false' warnings

Am 30.05.2007 17:41 schrieb Satyam Sharma:
> On 5/30/07, Roland Dreier <rdreier@...co.com> wrote:
>> thanks... I'm wondering if there's a consensus among kernel hackers
>> about changes like:
>>
>>  > -    if (hdr.cmd < 0 || hdr.cmd >= ARRAY_SIZE(ucma_cmd_table))
>>  > +    if (hdr.cmd >= ARRAY_SIZE(ucma_cmd_table))
>>  >              return -EINVAL;
>>
>> I understand that new gcc sees that hdr.cmd is unsigned and hence
>> can't be < 0, and generates a warning for that, and having a build
>> cluttered with warnings hides bugs and so on.  However the code here
>> looks quite sensible to me -- otherwise we end up with missing range
>> checking if hdr.cmd ever changes to a signed type.  This seems like a
>> good way to introduce bugs: delete valid range checking code to shut
>> up a silly gcc warning, and then change the type of a variable.
> 
> You're *absolutely* correct about the issue that these "fixes" that remove
> such conditions end up remove range-checking making the code more
> flakey / less readable.

I disagree. Changing the type of a variable is a significant
modification. If someone does that, he or she *must* check every
use of that variable, at which point he or she will also modify
any range checks accordingly. Having checks that don't fit with
the previous type *distracts* from that job. "Oh, did I modify
that part already? Guess I can skip checking the rest of that
function then." Oops.

Nor is readability a suitable argument. Checking if hdr.cmd is
less than zero gives the misleading impression that it *could*
be less than zero, thus *impairing* readability.

jm2c
T.

-- 
Tilman Schmidt                          E-Mail: tilman@...p.cc
Bonn, Germany
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