[Bug c/82051] New: [mips]mips emit different results when compiling union var using -O0/-O2

[zwzhangwen.zhang at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82051

            Bug ID: 82051
           Summary: [mips]mips emit different results when compiling union
                    var using -O0/-O2
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zwzhangwen.zhang at huawei dot com
  Target Milestone: ---

When compiling union var using -O0/-O2, elf print different results.
Command line:
mips-linux-gnueabi-gcc  -w -march=34kc -static ./test.c -O0/-O2

Executing results are as follows:
[O0]
g_121.f0=fffffce9
g_121.f1=81
g_121.f2=ce900000
g_121.f3=ce
g_121.f4=ce
[O2]
g_121.f0=fffffce9
g_121.f1=81
g_121.f2=ce900000
g_121.f3=ce
g_121.f4=fc

==> gcc-6.3.0/7.1.0/8.0.0


I have check the results with X86 compiler, the result is as expected:
g_121.f0=fffffce9
g_121.f1=ce9
g_121.f2=ce9
g_121.f3=e9
g_121.f4=e9

==> gcc-4.3.4

ccp2 pass g_121.f4 propagates 252 and I have no deep investigattion.

testcase:
#include <stdio.h>                                                              
union U0 {
   signed f0 : 12;
   volatile long long  f1;
   volatile int f2;
   const volatile unsigned char  f3;
   unsigned char  f4;
};

static union U0 g_121 = {0x549E5CE9L};/* VOLATILE GLOBAL g_121 */


/* ---------------------------------------- */
int main (int argc, char* argv[])
{

    printf ("g_121.f0=%x\n",g_121.f0);
    printf ("g_121.f1=%x\n",g_121.f1);
    printf ("g_121.f2=%x\n",g_121.f2);
    printf ("g_121.f3=%x\n",g_121.f3);
    printf ("g_121.f4=%x\n",g_121.f4);

        return 0;
}