Re: Add VIEW_CONVERT_EXPR to operand_equal_p

[Jan Hubicka <hubicka@ucw.cz>]

> 
> Adding back the mode check is fine if all types with the same TYPE_CANONICAL have the same mode.  Otherwise we'd regress here.  I thought we do for
> 
> Struct x { int i; };
> Typedef y x __attribute__((packed));
> 
> And then doing
> 
> X x;
> Y y;
> X = y;

Do you have any idea how to turn this into a testcase? I don't think we could
add packed attribute to typedef. Even in
gimple_canonical_types_compatible_p
  /* Can't be the same type if they have different mode.  */
  if (TYPE_MODE (t1) != TYPE_MODE (t2))
    return false;
(which IMO may be wrong WRT -mavx flags where modes of same types may be different
in different TUs)

Therefore I would say that TYPE_CANONICAL determine mode modulo the fact that
incoplete variant of a complete type will have VOIDmode instead of complete
type's mode (during non-LTO).  That is why I allow mode changes for casts from
complete to incomplete.

In longer run I think that every query to useless_type_conversion_p that
contains incomplete types is a confused query.  useless_type_conversion_p is
about operations on the value and there are no operations for incomplete type
(and function types).  I know that ipa-icf-gimple and the following code in
gimplify-stmt checks this frequently:
      /* The FEs may end up building ADDR_EXPRs early on a decl with
         an incomplete type.  Re-build ADDR_EXPRs in canonical form
         here.  */
      if (!types_compatible_p (TREE_TYPE (op0), TREE_TYPE (TREE_TYPE (expr))))
        *expr_p = build_fold_addr_expr (op0);
Taking address of incomplete type or functions, naturally, makes sense.  We may
want to check something else here, like simply
       TREE_TYPE (op0) != TREE_TYPE (TREE_TYPE (expr))
and once ipa-icf is cleanded up start sanity checking in usless_type_conversion
that we use it to force equality only on types that do have values.

We also can trip it when checking TYPE_METHOD_BASETYPE which may be incomplete.
This is in the code checking useless_type_conversion on functions that I think
are confused querries anyway - we need the ABI matcher, I am looking into that.

Honza
> 
> Richard.
> 
> 
> >Honza
> >> 
> >> -- 
> >> Eric Botcazou
>