[Bug c/58240] New: GCC optimize strncmp when N=1 incorrectly

["shiyan2016 at 126 dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58240

            Bug ID: 58240
           Summary: GCC optimize strncmp when N=1 incorrectly
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shiyan2016 at 126 dot com

Hi,

I have found a weird bug of GCC. I am trying to overwrite the C 'strncmp'
function, but I get different results when N=1 and N>1, see below test case:
/*
    bug of strncmp when N=1
    gcc: 4.7.2
*/
#include <stdio.h>

int strncmp(const char *s1,const char *s2,int n){
        printf("strncmp\n");
        return 0;
}

int main() {
     const char *s1="def";

     int ret;
     printf("test\n");
     ret=strncmp(s1,"def",1);    // bug: "strncmp" is not printed!!!
     printf("test\n");
     ret=strncmp(s1,"def",2);

    return 0;
}

To reproduce:
$gcc --version
gcc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$gcc 1.c
$./a.out 
test
test
strncmp
$
It will print "strncmp" when N=2, but not printed when N=1. From my
overservation, it seems strncmp(*,*,1) is optimized without calling to strncmp
at all.

Please try to check the behaviour and see whether it is a front-end bug.

By the way, the same test case can work as expected with MS compiler and Intel
compiler.