[Bug tree-optimization/123479] missed optimization for redundant conditional computation

[652023330028 at smail dot nju.edu.cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123479

--- Comment #1 from Yi <652023330028 at smail dot nju.edu.cn> ---
The following reduced test case seems exhibit a missed optimization due to a
similar underlying cause.

https://godbolt.org/z/hbj8aTfxj

void d(long* g, long* d) {
  long n = d - g;
  for (long i = 0; i < n; i++)
    g[i] = n;
}

GCC -O3:
  <bb 2> [local count: 118111600]:
  _1 = d_7(D) - g_8(D);
  if (_1 > 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 9>; [11.00%]

  <bb 3> [local count: 105119324]:
  n_9 = _1 /[ex] 8;
  if (_1 <= 8)
    goto <bb 8>; [10.00%]
  else
    goto <bb 4>; [90.00%]

  <bb 4> [local count: 94607391]:
  _14 = _1 != 0;                     #redundant 
  _13 = (unsigned long) n_9;
  niters.5_18 = _14 ? _13 : 1;
  bnd.6_29 = niters.5_18 >> 1;
  _32 = {n_9, n_9};
  ivtmp.15_27 = (unsigned long) g_8(D);
  _2 = bnd.6_29 * 16;
  _4 = _2 + ivtmp.15_27;