combine: Fix ICE with substitution of CONST_INT into PRE_DEC argument [PR104446]

combine: Fix ICE with substitution of CONST_INT into PRE_DEC argument [PR104446]

The following testcase ICEs, because combine substitutes
(insn 10 9 11 2 (set (reg/v:SI 7 sp [ a ])
        (const_int 0 [0])) "pr104446.c":9:5 81 {*movsi_internal}
     (nil))
(insn 13 11 14 2 (set (mem/f:SI (pre_dec:SI (reg/f:SI 7 sp)) [0  S4 A32])
        (reg:SI 85)) "pr104446.c":10:3 56 {*pushsi2}
     (expr_list:REG_DEAD (reg:SI 85)
        (expr_list:REG_ARGS_SIZE (const_int 16 [0x10])
            (nil))))
forming
(insn 13 11 14 2 (set (mem/f:SI (pre_dec:SI (const_int 0 [0])) [0  S4 A32])
        (reg:SI 85)) "pr104446.c":10:3 56 {*pushsi2}
     (expr_list:REG_DEAD (reg:SI 85)
        (expr_list:REG_ARGS_SIZE (const_int 16 [0x10])
            (nil))))
which is invalid RTL (pre_dec's argument must be a REG).
I know substitution creates various forms of invalid RTL and hopes that
invalid RTL just won't recog.
But unfortunately in this case we ICE before we get to recog, as
try_combine does:
  if (n_auto_inc)
    {
      int new_n_auto_inc = 0;
      for_each_inc_dec (newpat, count_auto_inc, &new_n_auto_inc);

      if (n_auto_inc != new_n_auto_inc)
        {
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file, "Number of auto_inc expressions changed\n");
          undo_all ();
          return 0;
        }
    }
and for_each_inc_dec under the hood will do e.g. for the PRE_DEC case:
    case PRE_DEC:
    case POST_DEC:
      {
        poly_int64 size = GET_MODE_SIZE (GET_MODE (mem));
        rtx r1 = XEXP (x, 0);
        rtx c = gen_int_mode (-size, GET_MODE (r1));
        return fn (mem, x, r1, r1, c, data);
      }
and that code rightfully expects that the PRE_DEC operand has non-VOIDmode
(as it needs to be a REG) - gen_int_mode for VOIDmode results in ICE.
I think it is better not to emit the clearly invalid RTL during substitution
like we do for other cases, than to adding workarounds for invalid IL
created by combine to rtlanal.cc and perhaps elsewhere.
As for the testcase, of course it is UB at runtime to modify sp that way,
but if such code is never reached, we must compile it, not to ICE on it.
And I don't see why on other targets which use the autoinc rtxes much more
it couldn't happen with other registers.

2022-02-11  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/104446
	* combine.cc (subst): Don't substitute CONST_INTs into RTX_AUTOINC
	operands.

	* gcc.target/i386/pr104446.c: New test.