[Bug tree-optimization/93891] CSE where clobber writes the same value

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93891

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
While the last_vuse thing was originally added for PRE it's also useful for
FRE as the following testcase shows:

int foo(int *p, int b, float *q)
{
  int tem;
  if (b)
    {
      *q = 0;
      tem = *p;
    }
  else
    {
      *q = 1;
      tem = *p;
    }
  return *p - tem;
}

with last_vuse fre1 manages to optimize this to return 0 while w/o it doesn't
(the key is value-numbering loads from both if arms the same).