[Bug middle-end/115527] -ftrivial-auto-var-init appears to clobber explicit initializer

[qinzhao at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115527

--- Comment #4 from qinzhao at gcc dot gnu.org ---
when checking the expanded IR for __builtin_clear_padding in the below:

the builtin 

__builtin_clear_padding (&o, 0B);

was expanded as the following sequence 

      D.4430 = &o + 0;
      _2 = D.4430 + 72;
      goto <D.4432>;
      <D.4431>:
      MEM <char[7]> [(struct outer *)D.4430 + 9B] = {};
      D.4430 = D.4430 + 24;
      <D.4432>:
      if (D.4430 != _2) goto <D.4431>; else goto <D.4433>;
      <D.4433>:
      MEM <char[4]> [(struct outer *)&o + 4B] = {};

When examining the above sequence, I locate the bug is at the last statement:

      MEM <char[4]> [(struct outer *)&o + 4B] = {};

this statement incorrectly zeroed the 5th to 8th bytes of the structure "o".
Instead, it should zeroed the &o + 72 + 4B as:

      MEM <char[4]> [(struct outer *)&o + 72 + 4B] = {};

I will check why the expanding of __builtin_clear_padding is wrong.