From: already5chosen@yahoo.com   
      
   On Thu, 1 Jan 2026 22:54:05 +0100   
   highcrew  wrote:   
      
   > Hello,   
   >   
   > While I consider myself reasonably good as C programmer, I still   
   > have difficulties in understanding undefined behavior.   
   > I wonder if anyone in this NG could help me.   
   >   
   > Let's take an example.  There's plenty here:   
   > https://en.cppreference.com/w/c/language/behavior.html   
   > So let's focus on https://godbolt.org/z/48bn19Tsb   
   >   
   > For the lazy, I report it here:   
   >   
   >    int table[4] = {0};   
   >    int exists_in_table(int v)   
   >    {   
   >        // return true in one of the first 4 iterations   
   >        // or UB due to out-of-bounds access   
   >        for (int i = 0; i <= 4; i++) {   
   >            if (table[i] == v) return 1;   
   >        }   
   >        return 0;   
   >    }   
   >   
   > This is compiled (with no warning whatsoever) into:   
   >   
   >    exists_in_table:   
   >            mov     eax, 1   
   >            ret   
   >    table:   
   >            .zero   16   
   >   
   >   
   > Well, this is *obviously* wrong. And sure, so is the original code,   
   > but I find it hard to think that the compiler isn't able to notice it,   
   > given that it is even "exploiting" it to produce very efficient code.   
   >   
   > I understand the formalism: the resulting assembly is formally   
   > "correct", in that UB implies that anything can happen.   
   > Yet I can't think of any situation where the resulting assembly   
   > could be considered sensible.  The compiled function will   
   > basically return 1 for any input, and the final program will be   
   > buggy.   
   >   
   > Wouldn't it be more sensible to have a compilation error, or   
   > at least a warning?  The compiler will be happy even with -Wall   
   > -Wextra -Werror.   
   >   
   > There's plenty of documentation, articles and presentations that   
   > explain how this can make very efficient code... but nothing   
   > will answer this question: do I really want to be efficiently   
   > wrong?   
   >   
   > I mean, yes I would find the problem, thanks to my 100% coverage   
   > unit testing, but couldn't the compiler give me a hint?   
   >   
   > Could someone drive me into this reasoning? I know there is a lot of   
   > thinking behind it, yet everything seems to me very incorrect!   
   > I'm in deep cognitive dissonance here! :) Help!   
   >   
      
   IMHO, for compiler that eliminated all comparisons (I assume that it was   
   gcc -O2/-O3) an absence of warning is a bug.   
   It's worth reporting.   
      
   And it has nothing to do with C standard and what considered UB by the   
   standard and what not. It's a bug from practical POV and I believe that   
   gcc maintainers will admit it and will fix it. Eventually, that is. Not   
   too quickly.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)