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Message-ID: <6e02fef204a84b1015f8e1973f1236ff17721e4d.camel@mengyan1223.wang>
Subject: Re: gcc warn unreached else {}
From: Xi Ruoyao <xry111@mengyan1223.wang>
Reply-To: gcc-help <gcc-help@gcc.gnu.org>
To: Jonny Grant <jg@jguk.org>
Cc: gcc-help <gcc-help@gcc.gnu.org>
Date: Thu, 09 Jul 2020 23:00:28 +0800
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References: <3a091703-4842-b738-bb38-fab01e787650@jguk.org>
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On 2020-07-09 10:45 +0100, Jonny Grant wrote:
> Hello!
> 
> There is an example below, (my code is not like this example below). I'm
> reporting a possible issue, and asking if there is a way to detect it in code
> bases. If it's a real issue I can file a bug report on Bugzilla.
> 
> Can gcc warn where code will not get to the 'return 3;' below?
> 
> Cheers, Jonny
> 
> 
> int main(void)
> {
>     const int i = 1;
>     if(1 == i)
>     {
>         return 1;
>     }
>     else if(1 != i)
>     {
>         return 2;
>     }
>     else
>     {
>         return 3;
>     }
> }

Generally finding all the branches in a program impossible to be executed is
unsolvable.  Even in a program without any loop it's still NP-hard.

For some "simple" cases maybe we can implement some heuristics, or use a brute
force approach (as a part of -fanalyzer).  But I doubt if these simple cases are
really useful.
-- 
Xi Ruoyao <xry111@mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University