[Bug c/79174] New: xorswap *x^=*y^(*y=*x) : sometime don't work with optimisation

["leguirec_alain at yahoo dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79174

            Bug ID: 79174
           Summary: xorswap *x^=*y^(*y=*x) : sometime don't work with
                    optimisation
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leguirec_alain at yahoo dot fr
  Target Milestone: ---

this code don't work with optimisation -O1 or -O2

int x=1, y=3;
int *px=&x;
int *py=&y;
*px^=*py^(*py=*px);
printf("x=%d y=%d\n",x,y);

without -O2 -> x=3 y=1
with -O2 -> x=1 y=1

The swap is useless so the optimisation remove it from the code, it's ok, but
the first parameter send to printf is wrong.

400440: ba 01 00 00 00 mov $0x1,%edx
400445: be 01 00 00 00 mov $0x1,%esi
40044a: bf e0 05 40 00 mov $0x4005e0,%edi
40044f: 31 c0 xor %eax,%eax
400451: e9 ba ff ff ff jmpq 400410 <printf@plt>

see on gcc 4.8.5 on centos 7