[Bug tree-optimization/100922] CSE leads to fully redundant (back to back) zero-extending loads of the same thing in a loop, or a register copy

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100922

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-06-05
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Pre takes:
  <bb 5> [local count: 1073741824]:
  # str_9 = PHI <str_27(4), str_16(8)>
  # out_10 = PHI <str_27(4), out_20(8)>
  str_16 = str_9 + 1;
  c_17 = *str_9;
  _5 = (sizetype) c_17;
  _6 = keep_lut_14(D) + _5;
  inc_18 = *_6;
  *out_10 = c_17;
  _7 = (sizetype) inc_18;
  out_20 = out_10 + _7;
  if (c_17 != 0)
    goto <bb 8>; [89.00%]
  else
    goto <bb 6>; [11.00%]

  <bb 8> [local count: 955630224]:
  goto <bb 5>; [100.00%]

Changes it to:
  <bb 5> [local count: 1073741824]:
  # str_9 = PHI <str_27(4), str_16(6)>
  # out_10 = PHI <str_27(4), out_20(6)>
  # prephitmp_60 = PHI <prephitmp_54(4), pretmp_59(6)>
  str_16 = str_9 + 1;
  c_17 = *str_9;
  _5 = (sizetype) c_17;
  _6 = keep_lut_14(D) + _5;
  *out_10 = c_17;
  _7 = (sizetype) prephitmp_60;
  out_20 = out_10 + _7;
  if (c_17 != 0)
    goto <bb 6>; [89.00%]
  else
    goto <bb 7>; [11.00%]

  <bb 6> [local count: 955630224]:
  pretmp_55 = MEM[(char *)str_9 + 1B];
  _57 = (sizetype) pretmp_55;
  _58 = keep_lut_14(D) + _57;
  pretmp_59 = *_58;
  goto <bb 5>; [100.00%]

And then ch_vect changes it to (basically undoing the PRE):
  <bb 14> [local count: 850510900]:

  <bb 6> [local count: 955630224]:
  # str_42 = PHI <str_41(14), str_27(13)>
  # str_41 = PHI <str_16(14), str_47(13)>
  # out_38 = PHI <out_20(14), out_43(13)>
  pretmp_55 = MEM[(char *)str_42 + 1B];
  _57 = (sizetype) pretmp_55;
  _58 = keep_lut_14(D) + _57;
  pretmp_59 = *_58;
  str_16 = str_41 + 1;
  c_17 = *str_41;
  *out_38 = c_17;
  _7 = (sizetype) pretmp_59;
  out_20 = out_38 + _7;
  if (c_17 != 0)
    goto <bb 14>; [89.00%]
  else
    goto <bb 7>; [11.00%]

And then IVOPTS comes around and does:
  <bb 14> [local count: 850510900]:

  <bb 6> [local count: 955630224]:
  # str_41 = PHI <str_16(14), str_47(13)>
  # out_38 = PHI <out_20(14), out_43(13)>
  pretmp_55 = MEM[(char *)str_41];
  _57 = (sizetype) pretmp_55;
  _58 = keep_lut_14(D) + _57;
  pretmp_59 = *_58;
  str_16 = str_41 + 1;
  c_17 = MEM[(char *)str_16 + -1B];
  *out_38 = c_17;
  _7 = (sizetype) pretmp_59;
  out_20 = out_38 + _7;
  if (c_17 != 0)
    goto <bb 14>; [89.00%]
  else
    goto <bb 7>; [11.00%]

Which is fine except FRE (and DOM), don't recognize the MEM[(char *)str_16 +
-1B] and MEM[(char *)str_41] being the same.  For FRE, it almost looks like
TARGET_MEM_REF is not handled ....