[Bug target/115406] [15 Regression] wrong code with vector compare at -O0 with -mavx512f since r15-920-gb6c6d5abf0d31c

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115406

--- Comment #3 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
typedef __attribute__((__vector_size__ (1))) char V;

char
foo (V v)
{
  return ((V) v == v)[0];
}

int
main ()
{
  char x = foo ((V) { });
  if (x != -1)
    __builtin_abort ();
}

w/ vcond_mask_qiqi, it's not lowered by veclower, and we get

char foo (V v)
{
  vector(1) signed char D.5142;
  char D.5141;
  vector(1) <signed-boolean:8> _1;
  vector(1) signed char _2;
  char _5;

  <bb 2> :
  _1 = { -1 };
  _2 = VEC_COND_EXPR <_1, { -1 }, { 0 }>;
  D.5142 = _2;
  _5 = VIEW_CONVERT_EXPR<char>(D.5142);

  <bb 3> :
<L0>:
  return _5;
}

But it's further simplified to 

char foo (V v)
{
  vector(1) signed char D.3765;
  char D.3764;
  vector(1) <signed-boolean:8> _1;
  vector(1) signed char _2;
  char _5;

  <bb 2> :
  _1 = { -1 };
  _2 = VIEW_CONVERT_EXPR<vector(1) signed char>(_1);
  D.3765 = _2;
  _5 = VIEW_CONVERT_EXPR<char>(D.3765);

  <bb 3> :
<L0>:
  return _5;

}

by isel

and for _2 = VIEW_CONVERT_EXPR<vector(1) signed char>(_1); we explicitly clear
the upper bits due to PR113576, and then we get 1 hit the abort.

It sound to me 
  _1 = { -1 };
  _2 = VEC_COND_EXPR <_1, { -1 }, { 0 }>;
shouldn't be simplified to 
_2 = VIEW_CONVERT_EXPR<vector(1) signed char>(_1);

when nunits is less than mode precision since the upper bit will be cleared.