Re: Add VIEW_CONVERT_EXPR to operand_equal_p

[Richard Biener <richard.guenther@gmail.com>]

On Sun, Oct 18, 2015 at 7:14 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>
>> 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)

Ok, so the following works:

struct x { int i; };
typedef struct x y __attribute__((aligned(1)));

void foo (void)
{
  struct x X;
  y Y;
  X = Y;
}

but we use SImode for y as well even though it's alignment is just one byte ...

Not sure what happens on strict-align targets for this and not sure how this
cannot be _not_ a problem.  Consider

void bar (struct x);

and

bar (Y);

or using y *Y and X = *Y or bar (*Y).

> 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.

Incomplete have VOIDmode, right?

> 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.

Ok, so given we seem to be fine in practive with TYPE_MODE (type) ==
TYPE_MODE (TYPE_CANONICAL (type))
(whether that's a but or not ...) I'm fine with re-instantiating the
mode check for
aggregate types.  Please do that with

Index: gcc/gimple-expr.c
===================================================================
--- gcc/gimple-expr.c   (revision 228963)
+++ gcc/gimple-expr.c   (working copy)
@@ -89,8 +89,7 @@ useless_type_conversion_p (tree outer_ty

   /* Changes in machine mode are never useless conversions unless we
      deal with aggregate types in which case we defer to later checks.  */
-  if (TYPE_MODE (inner_type) != TYPE_MODE (outer_type)
-      && !AGGREGATE_TYPE_P (inner_type))
+  if (TYPE_MODE (inner_type) != TYPE_MODE (outer_type))
     return false;

   /* If both the inner and outer types are integral types, then the

Can we asses equal sizes when modes are non-BLKmode then?  Thus

@@ -270,10 +269,9 @@ useless_type_conversion_p (tree outer_ty
      use the types in move operations.  */
   else if (AGGREGATE_TYPE_P (inner_type)
           && TREE_CODE (inner_type) == TREE_CODE (outer_type))
-    return (!TYPE_SIZE (outer_type)
-           || (TYPE_SIZE (inner_type)
-               && operand_equal_p (TYPE_SIZE (inner_type),
-                                   TYPE_SIZE (outer_type), 0)));
+    return (TYPE_MODE (outer_type) != BLKmode
+           || operand_equal_p (TYPE_SIZE (inner_type),
+                               TYPE_SIZE (outer_type), 0));

   else if (TREE_CODE (inner_type) == OFFSET_TYPE
           && TREE_CODE (outer_type) == OFFSET_TYPE)

?  Hoping for VOIDmode incomplete case.

Richard.

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