[Bug c/42173] New: O3 option changes sprintf behavior

["schoth at itp dot physik dot tu-berlin dot de" <gcc-bugzilla at gcc dot gnu dot org>]

When using the -O3 optimizing option (-O2 and -O1 as well), a sprintf call, in
which the target string and the source string are the same, will no longer show
the same behavior. Not sure, whether this is actually a Bug. Here is an
example:

test.c:
#include <stdlib.h>
#include <stdio.h>
char mystring[30];
int main(int argc, char *argv[]){
        sprintf(mystring,"hello");
        printf("%s\n",mystring);
        sprintf(mystring,"%s world",mystring);
        printf("%s\n",mystring);
        return 0;
}

terminal:
me@here ~ $ gcc -o test test.c
me@here ~ $ ./test
hello                 
hello world           
me@here ~ $ gcc -O3 -o test test.c
me@here ~ $ ./test
hello                 
 world                
me@here ~ $ gcc --version
gcc (Gentoo 4.3.4 p1.0, pie-10.1.5) 4.3.4


I'm fully aware that the line
        sprintf(mystring,"%s world",mystring);
is kind of dangerous, as it will fail as soon as %s is not the first object
inside the string or mystring has reached full length, and should be replaced
with a call to strcat or similar. However, under normal conditions it will work
fine, and it's quite intuitive that the optimizing option changes this
behavior.
Might be related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40479


-- 
           Summary: O3 option changes sprintf behavior
           Product: gcc
           Version: 4.3.4
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: schoth at itp dot physik dot tu-berlin dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42173