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NNTP-Posting-Date: Sat, 22 Apr 2017 05:37:05 -0500
From: Gareth Owen <gwowen@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: gcc failure (optimised/non-optimised)
References: <odf2pf$dd0$1@dont-email.me> <odf5er$m60$1@flame.srcf.net> <odf8vf$1v1$1@dont-email.me>
Date: Sat, 22 Apr 2017 11:37:05 +0100
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Xref: csiph.com comp.lang.c:108334

James Harris <james.harris.1@gmail.com> writes:

>> Overflow of signed integers is UB.  Therefore "i < i + 1" can
>> legitimately be simplified to true.
>>
>> Use -fwrapv if you want GCC/Clang to strictly consider signed integer
>> overflow to have defined twos-compliment behaviour.
>>
>> http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html is
>> an excellent set of articles which everyone should read.
>
> Thanks. That's very interesting.
>
> What gets me is that the compiler is (potentially) altering the
> behaviour of the code without issuing a warning. That's true even with
> -Wall -Wextra.

The code does not have (defined) behaviour.  Code whose behaviour is not
defined, does not have to consistent modes of misbehaviour.