linux-kernel - dealing with gcc 'comparison is always false' warnings (was: [PATCH] drivers/infiniband: fix comparsion between unsigned and negative)

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Date:	Wed, 30 May 2007 08:30:00 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	openib-general@...nib.org
Cc:	linux-kernel@...r.kernel.org
Subject: dealing with gcc 'comparison is always false' warnings (was: [PATCH] drivers/infiniband: fix comparsion between unsigned and negative)

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.

Can't we just make gcc shut up about the comparison and generate no
code for it because it knows it can't be true?

 - R.
-
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