Subject: Re: [PATCH 00/38] Fixes related to incorrect usage of unsigned types
To: David Howells <dhowells@redhat.com>
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Cc: Andrzej Hajda <a.hajda@samsung.com>,
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From: Andrzej Hajda <a.hajda@samsung.com>
Message-id: <56011BB9.5030004@samsung.com> (sfid-20150922_111414_699788_3BA1DF44)
Date: Tue, 22 Sep 2015 11:13:29 +0200
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On 09/21/2015 03:42 PM, David Howells wrote:
> Andrzej Hajda <a.hajda-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org> wrote:
> 
>> Semantic patch finds comparisons of types:
>>     unsigned < 0
>>     unsigned >= 0
>> The former is always false, the latter is always true.
>> Such comparisons are useless, so theoretically they could be
>> safely removed, but their presence quite often indicates bugs.
> 
> Or someone has left them in because they don't matter and there's the
> possibility that the type being tested might be or become signed under some
> circumstances.  If the comparison is useless, I'd expect the compiler to just
> discard it - for such cases your patch is pointless.
> 
> If I have, for example:
> 
> 	unsigned x;
> 
> 	if (x == 0 || x > 27)
> 		give_a_range_error();
> 
> I will write this as:
> 
> 	unsigned x;
> 
> 	if (x <= 0 || x > 27)
> 		give_a_range_error();
> 
> because it that gives a way to handle x being changed to signed at some point
> in the future for no cost.  In which case, your changing the <= to an ==
> "because the < part of the case is useless" is arguably wrong.

This is why I have not checked for such cases - I have skipped checks of type
	unsigned <= 0
exactly for the reasons above.

However I have left two other checks as they seems to me more suspicious - they
are always true or false. But as Dmitry and Andrew pointed out Linus have quite
strong opinion against removing range checks in such cases as he finds it
clearer. I think it applies to patches 29-36. I am not sure about patches 26-28,37.

Regards
Andrzej

> 
> David
> --
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