[Bug middle-end/94313] stores into string literals sometimes silently eliminated

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94313

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is likely because points-to computes the result of the string functions,
for example in g1 it computes

s_5 = { NULL STRING }

and somehow ref_maybe_used_by_stmt_p computes false for *s and f(s).
That's from

      if (pt_solutions_intersect (gimple_call_use_set (call), &pi->pt))
        return true;

where the points-to set for s_5 demotes to

$8 = {anything = 0, nonlocal = 0, escaped = 0, ipa_escaped = 0, null = 1, 
  vars_contains_nonlocal = 0, vars_contains_escaped = 0, 
  vars_contains_escaped_heap = 0, vars_contains_restrict = 0, 
  vars_contains_interposable = 0, vars = 0x7ffff6d7c520}

so we dropped STRING which we do as follows:

          else if (vi->id == string_id)
            /* Nobody cares - STRING_CSTs are read-only entities.  */
            ;

which is of course true, since nothing can modify STRING_CSTs we can
technically ignore uses of appearant modifications.

Now the inconsistency is that we treat

  MEM[(char *)"abc"] = 120;
  f ("abc"); [tail call]

differently - a missed optimization(?).  Note that before one of my
recent fixes we didn't even treat such stmts as stores (now we do).

So I see nothing inherently wrong here.