Hi,
This patch folds strstr (s, t) eq/ne s to strcmp (s, t) eq/ne 0 if
strlen (t) is known.
One issue I came across was forwprop1 reverses the order of operands
in eq_expr below:

eg test-case:
_Bool f(char *s, int cond)
{
  char *t1 = __builtin_strstr (s, "hello");
  _Bool t2 = (t1 == s);
  return t2;
}

forwprop1 dump:
f (char * s, int cond)
{
  _Bool t2;
  char * t1;

  <bb 2> [0.0%]:
  t1_3 = __builtin_strstr (s_2(D), "hello");
  t2_4 = s_2(D) == t1_3;
  return t2_4;

}

So I had to check if SSA_NAME_DEF_STMT (rhs2) was call to strstr
rather than rhs1.
I suppose that's OK ?

clang unconditionally transforms
strstr (s, t) == s to strncmp (s, t, strlen (t))
However I am not sure what algorithm glibc's strstr uses, so didn't
attempt to transform
if strlen (t) is unknown. Should we do the transform even if strlen
(t) is unknown ?

Thanks,
Prathamesh