[Bug rtl-optimization/70687] New: Undefined shift in change_zero_ext in combine.c

["hjl.tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70687

            Bug ID: 70687
           Summary: Undefined shift in change_zero_ext in combine.c
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---

change_zero_ext in combine.c has

     else if (GET_CODE (x) == ZERO_EXTEND
               && GET_CODE (XEXP (x, 0)) == SUBREG
               && GET_MODE (SUBREG_REG (XEXP (x, 0))) == mode
               && subreg_lowpart_p (XEXP (x, 0)))
        {
          size = GET_MODE_PRECISION (GET_MODE (XEXP (x, 0))); 
          x = SUBREG_REG (XEXP (x, 0));
        }
      else  
        continue;

      unsigned HOST_WIDE_INT mask = 1;
      mask <<= size; 
      mask--;

      x = gen_rtx_AND (mode, x, GEN_INT (mask));

For

(gdb) call debug_rtx (*src)
(zero_extend:TI (subreg:DI (reg/v:TI 91 [ ixi ]) 0))

we got

(gdb) call debug_rtx (*src)
(zero_extend:TI (subreg:DI (reg/v:TI 91 [ ixi ]) 0))
(gdb) p size
$9 = 64
(gdb) p sizeof (mask)
$10 = 8

mask <<= size;

is wrong here, which leads to

(and:TI (reg/v:TI 91 [ ixi ])
    (const_int 0 [0]))