[Bug c/18806] New: generate wrong code with hi optimization

[paolo dot abeni at tilab dot com]

compiling the following fragment:

typedef struct pseudoheader       /* pseudo header for TCP checksum calculations */
{
    unsigned int sip, dip;
    unsigned char  zero;
    unsigned char  protocol;
    unsigned short tcplen;
} ph_struct;
                                                                               
                                                       
static inline unsigned short in_chksum_tcp(  unsigned short *h)
{
   unsigned int cksum;
                                                                               
                                                       
   /* PseudoHeader must have 12 bytes */
   cksum  = h[0];
   cksum += h[1]; /* [1] */
   cksum += h[2];
   cksum += h[3];
   cksum += h[4];
   cksum += h[5];
   return cksum;
}
                                                                               
                                                       
int main()
{
    unsigned short a, b;
                                                                               
                                                       
    ph_struct ph;
    union pseudoheader_union{
                                                                               
                                                       
        ph_struct ph;
        unsigned short a[6];
    } p;
                                                                               
                                                       
    ph.sip = 0xa0a44d4; /* [2] */
    ph.dip = 0xf50ac610;
    ph.zero = 0;
    ph.protocol = 0x06;
    ph.tcplen = 1216;
    a = in_chksum_tcp(&ph);
                                                                               
                                                       
    p.ph.sip = 0xa0a44d4;
    p.ph.dip = 0xf50ac610;
    p.ph.zero = 0;
    p.ph.protocol = 0x06;
    p.ph.tcplen = 1216;
    b = in_chksum_tcp(p.a);
    printf("CHKSUM: %hx %hx %d\n", a, b, sizeof(ph));
    return 0;
}

with:

gcc -O2 

the value calulated in the first invocation of in_chksum_tcp function is wrong

this is NOT related to what is described in
http://gcc.gnu.org/bugs.html#nonbugs_c for breaking aliasing rules.

It seems that the compiler, while generating the assembly code, inverts an
assignment operation (marked with [2] in the c code fragment) with the
sum operation (marked with [1] in the c code fragment).

-- 
           Summary: generate wrong code with hi optimization
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P1
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: paolo dot abeni at tilab dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i586-madrake-linux-gnu
  GCC host triplet: i586-madrake-linux-gnu
GCC target triplet: i586-madrake-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18806