[Bug target/55876] internal compiler error: in gen_rtx_SUBREG, at emit-rtl.c:776

["vries at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55876

--- Comment #1 from vries at gcc dot gnu.org 2013-01-04 15:23:25 UTC ---
while expanding the xor, we try to widen an DImode reg from QImode to SImode:
...
#6  0x085d7da5 in widen_operand (op=0xf7cd2ec0, mode=SImode, oldmode=QImode,
unsignedp=1, no_extend=1) at src/gcc-mainline/gcc/optabs.c:333
333         return gen_rtx_SUBREG (mode, force_reg (GET_MODE (op), op), 0);
(gdb) l
328         return convert_modes (mode, oldmode, op, unsignedp);
329
330       /* If MODE is no wider than a single word, we return a paradoxical
331          SUBREG.  */
332       if (GET_MODE_SIZE (mode) <= UNITS_PER_WORD)
333         return gen_rtx_SUBREG (mode, force_reg (GET_MODE (op), op), 0);
334
335       /* Otherwise, get an object of MODE, clobber it, and set the
low-order
336          part to OP.  */
337
(gdb) call debug_rtx (op)
(reg:DI 222 [ used+-7 ])
...

Just before expand, the representation looks like:
...
foo ()
{
  unsigned charD.13 usedD.1407;
  charD.2 _7;
  unsigned charD.13 _13;
  charD.2 _19;
  charD.2 _28;
  charD.2 _37;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  # VUSE <.MEM_1(D)>
  _19 = MEM[(const charD.2 *)&bufferD.1387];
  used_20 = _19 != 70 ? 1 : 0;
  # VUSE <.MEM_1(D)>
  _28 = MEM[(const charD.2 *)&bufferD.1387 + 1B];
  used_29 = _28 == 70 ? used_20 : 1;
  # VUSE <.MEM_1(D)>
  _37 = MEM[(const charD.2 *)&bufferD.1387 + 2B];
  used_38 = _37 == 70 ? used_29 : 1;
  # VUSE <.MEM_1(D)>
  _7 = MEM[(const charD.2 *)&bufferD.1387 + 3B];
  used_10 = _7 == 70 ? used_38 : 1;
  _13 = used_10 ^ 1;
  # VUSE <.MEM_1(D)>
  return _13;
;;    succ:       EXIT

}
...

The reason that used_10 is in a DImode reg is because
  used_10 = _7 == 70 ? used_38 : 1
and the all the other conditional moves before are promoted by
expand_cond_expr_using_cmove.