[Bug tree-optimization/81110] New: tree-vrp optimize unsigned comparison to signed

["qian.liu at ingenic dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81110

            Bug ID: 81110
           Summary: tree-vrp optimize unsigned comparison to signed
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qian.liu at ingenic dot com
  Target Milestone: ---
            Target: mips-linux-gnu mipsel-linux-gnu

Created attachment 41567
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41567&action=edit
reduced testcase

The following testcase is miscompiled at -O2 on mips .  An unsigned comparison
operation is optimized to a singed comparion instruciton. When d[0] and d[1]
large enough, will get error result.

unsigned short d[2];
int DiffInt ()
{
  if ((unsigned long)(65536*d[1]+d[0]) < (1<<28)){
    return -1;
  }
}

Currently assembly:
        li      $3,268435456                    # 0x10000000
        slt     $2,$2,$3      # error
        beq     $2,$0,$L4

But expected：
        li      $3,268435456                    # 0x10000000
        sltu    $2,$2,$3     # unsigned 
        beq     $2,$0,$L4

In tree-vrp dump file test.c.081t.vrp1, unsigned _8 is simplifed to singned _7.
But this is incorrect.

  int _7;
  long unsigned int _8;
  .....
  _8 = (long unsigned int) _7;

 -  if (_8 <= 268435455)
 +  if (_7 <= 268435455)