[patch] Fix dangling references in thunks at -O0

[Eric Botcazou]

> ISTR the tailcall flag is only a hint and RTL expansion can decide to
> not tailcall based on targets.  So to me it looks like a missed
> disqualification on the RTL expansion side.

That's IMO debatable: the GIMPLE tailcall optimization pass e.g. does:

  /* Make sure the tail invocation of this function does not indirectly
     refer to local variables.  (Passing variables directly by value
     is OK.)  */
  FOR_EACH_LOCAL_DECL (cfun, idx, var)
    {
      if (TREE_CODE (var) != PARM_DECL
	  && auto_var_in_fn_p (var, cfun->decl)
	  && may_be_aliased (var)
	  && (ref_maybe_used_by_stmt_p (call, var)
	      || call_may_clobber_ref_p (call, var)))

Do you want to replace it with something at the RTL level too?  I don't think 
that you can disqualify things at the RTL as easily as at the GIMPLE level.

> Or do we, besides from this very single spot, simply never tailcall at -O0
> and thus never hit this latent issue?

Presumably yes, tail calling is an optimization like the others.

> How does this change the debug experience at -O0 when GIMPLE thunks
> are used?

In Ada this doesn't change much since thunks have line debug info.

-- 
Eric Botcazou