linux-kernel - Re: 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 21:11:29 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Roland Dreier" <rdreier@...co.com>
Cc:	openib-general@...nib.org, linux-kernel@...r.kernel.org
Subject: Re: dealing with gcc 'comparison is always false' warnings (was: [PATCH] drivers/infiniband: fix comparsion between unsigned and negative)

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.

However, gcc is _just as correct_. It is only crying about seeing a condition
that the programmer could have written with some purpose in mind but which
is being completely compiled away by it when generating the code because
of it being a tautology / contradiction ...

> 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?

No, shutting gcc up wouldn't be the right thing, IMHO. These warnings are
a good reminder to the programmer to go and see if there is a real bug
somewhere and if something really needs to be done with the code (could
be simply to change the type of a variable to signed that was mistakenly
declared unsigned, f.e.).

But yes, the kind of "fixes" you pointed out that _remove_ these conditions
are definitely *not* what we would want to do.

Satyam
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