Re: Gcc silently transforms a finite loop to an infinite one

[David Brown <david at westcontrol dot com>]

On 29/11/2011 15:25, Ian Lance Taylor wrote:
> Nadezhda IvanÐvna Vyukova<niva@niisi.msk.ru>  writes:
>
> > I am involved in support of customers who use GCC.
> > Recently a customer has complaint that gcc-4.1.2
> > generates an infinite loop for the following program:
> >
> >    #include<stdio.h>
> >    extern void f (int);
> >    int main ()
> >    {
> >    char e = 0;
> >       do
> >       {
> >         printf ("--- e = %i\n", e);
> >       }
> >       while (++e);
> >    }
> >
> > When compiled with -O2 or higher the program prints
> > 0, 1, ..., 128, ... 1000, ... etc.
> > (Similar behavior is exposed by gcc-4.6.2).
> >
> > I've explained the customer that by default char is treated
> > as signed char on our platform and therefore this program
> > does not conform ISO C90, as it causes the integer overflow
> > (undefined behavior). But he was not satisfied.
> > He argued that the program compiled with gcc-3.4.6 behaves
> > "correctly" and now the compiler silently produces an
> > incomprehensible code.
>
> Tell your customer to use the -fno-strict-overflow option.
>
> > IMHO it would be better to issue a warning
> > when a finite loop is transformed to an infinite one
> > (as a result of -ftree-vrp).
>
> Tell your customer to use the -Wstrict-overflow option.
>
> This message might have been better directed to gcc-help@gcc.gnu.org.
>
> I would have no objection to adding a default warning when a finite loop
> is converted to an infinite loop.  It's harder than it sounds, though.
> It's not like gcc is looking a loop and deciding to make it infinite.
> It's looking at a branch instruction and deciding that the condition can
> never be true.
>
> Ian

A better method is perhaps to get users to enable warning flags by 
default - but that's also harder than it sounds!

I've always thought that gcc should have two new warning flags that are 
enabled by default - one being the "Warning - no warning flags given on 
command line" flag, and the other being the "Warning - compiler crippled 
by lack of -O flag" flag :-)