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Re: [PATCH] Fix PR ipa/65908.

> +/* Return true if DECL_ARGUMENT types are valid to be merged.  */
Perhaps bettter as

Perform additional check needed to match types function parameters that are
used.  Unlike for normal parameters it matters if type is TYPE_RESTRICT and we
make an assumption that REFERENCE_TYPE parameters are always non-NULL.

> +
> +bool
> +sem_function::compatible_parm_types_p ()
> +{
> +  tree parm1, parm2;
> +  unsigned i = 0;
> +
> +  for (parm1 = DECL_ARGUMENTS (decl),
> +       parm2 = DECL_ARGUMENTS (m_compared_func->decl);
> +       parm1 && parm2;
> +       parm1 = DECL_CHAIN (parm1), parm2 = DECL_CHAIN (parm2), i++)

I think this is still not right.  What you wan to to do is to have

1) comparible_parm_types_p (t1,t2, index) that returns true if T1 and T2 are
   matching with checks bellow:
> +  {
> +    if (!param_used_p (i))
> +      continue;
> +
> +    if (POINTER_TYPE_P (parm1)
> +	&& (TYPE_RESTRICT (parm1) != TYPE_RESTRICT (parm2)))
> +      return return_false_with_msg ("argument restrict flag mismatch");
> +    /* nonnull_arg_p implies non-zero range to REFERENCE types.  */
> +    if (POINTER_TYPE_P (parm1)
> +	&& TREE_CODE (parm1) != TREE_CODE (parm2)
> +	&& opt_for_fn (decl, flag_delete_null_pointer_checks))
> +      return return_false_with_msg ("pointer wrt reference mismatch");
> +  }
   withtout actually walking the chain.

2) make equals_wpa to walk TYPE_ARG_TYPES of the function type and match them.
   This is because DECL_ARGUMENTS are part of function body and before you
   read it into memory, these are NULL

   Walking DECL_ARGUMENTS here may cause ipa-icf to give up in case one body is
   read (and thus have some arguments) and other is not.

3) make equals_private walk DECL_ARGUMENTS and match them
   (well at the time you populate the map.)
   You probalby can skip matching PARM_DECLS that are !parm_used_p (i)
   for anything else than types_compatible_p.

   We only care they are passed the same way by ABI. Everything else is not


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