Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 04/06/17 09:47, Richard Biener wrote:
> On Wed, 5 Apr 2017, Bernd Edlinger wrote:
>
>> On 04/05/17 19:22, Bernd Edlinger wrote:
>>> On 04/05/17 18:08, Jakub Jelinek wrote:
>>>
>>> Yes, exactly.  I really want to reach the deadline for gcc-7.
>>> Fixing the name is certainly the most important first step,
>>> and if everybody agrees on "typeless_storage", for the name
>>> I can start with adjusting the name, and look into how
>>> to use a spare type-flag that should be a mechanical change.
>>>
>>
>> Jakub, I just renamed the attribute and reworked the patch
>> as you suggested, reg-testing is not yet completed, but
>> it looks good so far.  I also added a few more tests.
>>
>> I have changed the documentation as Richi suggested, but
>> I am not too sure what to say here.
>
> The alias.c changes are not sufficient.  I think what you want is
> sth like
>
> Index: gcc/alias.c
> ===================================================================
> --- gcc/alias.c	(revision 246678)
> +++ gcc/alias.c	(working copy)
> @@ -136,6 +136,9 @@ struct GTY(()) alias_set_entry {
>    bool is_pointer;
>    /* Nonzero if is_pointer or if one of childs have has_pointer set.  */
>    bool has_pointer;
> +  /* Nonzero if we have a child serving as typeless storage (or are
> +     such storage ourselves).  */
> +  bool has_typeless_storage;
>
>    /* The children of the alias set.  These are not just the immediate
>       children, but, in fact, all descendants.  So, if we have:
> @@ -419,7 +422,8 @@ alias_set_subset_of (alias_set_type set1
>    /* Check if set1 is a subset of set2.  */
>    ase2 = get_alias_set_entry (set2);
>    if (ase2 != 0
> -      && (ase2->has_zero_child
> +      && (ase2->has_typeless_storage
> +	  || ase2->has_zero_child
>  	  || (ase2->children && ase2->children->get (set1))))
>      return true;
>

I think get_alias_set(t) will return 0 for typeless_storage
types, and therefore has_zero_child will be set anyway.
I think both mean the same thing in the end, but it depends on
what typeless_storage should actually mean, and we have
not yet the same idea about it.

> @@ -825,6 +829,7 @@ init_alias_set_entry (alias_set_type set
>    ase->has_zero_child = false;
>    ase->is_pointer = false;
>    ase->has_pointer = false;
> +  ase->has_typeless_storage = false;
>    gcc_checking_assert (!get_alias_set_entry (set));
>    (*alias_sets)[set] = ase;
>    return ase;
> @@ -955,6 +960,7 @@ get_alias_set (tree t)
>       Just be pragmatic here and make sure the array and its element
>       type get the same alias set assigned.  */
>    else if (TREE_CODE (t) == ARRAY_TYPE
> +	   && ! TYPE_TYPELESS_STORAGE (t)
>  	   && (!TYPE_NONALIASED_COMPONENT (t)
>  	       || TYPE_STRUCTURAL_EQUALITY_P (t)))
>      set = get_alias_set (TREE_TYPE (t));
> @@ -1094,6 +1100,15 @@ get_alias_set (tree t)
>
>    TYPE_ALIAS_SET (t) = set;
>
> +  if (TREE_CODE (t) == ARRAY_TYPE
> +      && TYPE_TYPELESS_STORAGE (t))
> +    {
> +      alias_set_entry *ase = get_alias_set_entry (set);
> +      if (!ase)
> +	ase = init_alias_set_entry (set);
> +      ase->has_typeless_storage = true;
> +    }
> +
>    /* If this is an aggregate type or a complex type, we must record any
>       component aliasing information.  */
>    if (AGGREGATE_TYPE_P (t) || TREE_CODE (t) == COMPLEX_TYPE)
> @@ -1173,6 +1188,8 @@ record_alias_subset (alias_set_type supe
>  	    superset_entry->has_zero_child = true;
>            if (subset_entry->has_pointer)
>  	    superset_entry->has_pointer = true;
> +	  if (subset_entry->has_typeless_storage)
> +	    superset_entry->has_typeless_storage = true;
>
>  	  if (subset_entry->children)
>  	    {
>
>
> please also restrict TYPE_TYPELESS_STORAGE to ARRAY_TYPEs (otherwise
> more complications will arise).
>
> Index: gcc/cp/class.c
> ===================================================================
> --- gcc/cp/class.c      (revision 246678)
> +++ gcc/cp/class.c      (working copy)
> @@ -2083,7 +2083,8 @@ fixup_attribute_variants (tree t)
>    tree attrs = TYPE_ATTRIBUTES (t);
>    unsigned align = TYPE_ALIGN (t);
>    bool user_align = TYPE_USER_ALIGN (t);
> -  bool may_alias = lookup_attribute ("may_alias", attrs);
> +  bool may_alias = TYPE_TYPELESS_STORAGE (t)
> +                  || lookup_attribute ("may_alias", attrs);
>
>    if (may_alias)
>      fixup_may_alias (t);
> @@ -7345,6 +7348,12 @@ finish_struct_1 (tree t)
>       the class or perform any other required target modifications.  */
>    targetm.cxx.adjust_class_at_definition (t);
>
> +  if (cxx_dialect >= cxx1z && cxx_type_contains_byte_buffer (t))
> +    {
> +      TYPE_TYPELESS_STORAGE (t) = 1;
> +      fixup_attribute_variants (t);
> ...
>
> I don't think you need all this given alias.c only looks at
> TYPE_MAIN_VARIANTs.

I wanted to be able to declare a int __attribute__((typeless_storage))
as in the test case, and the sample in the spec.  And that
information is not in the TYPE_MAIN_VARIANT.  Therefore I look for
typeless_storage before "t = TYPE_MAIN_VARIANT (t)".

>
> Index: gcc/cp/decl.c
> ===================================================================
> --- gcc/cp/decl.c       (revision 246678)
> +++ gcc/cp/decl.c       (working copy)
> @@ -14081,10 +14081,11 @@ start_enum (tree name, tree enumtype, tree
> underly
>           enumtype = pushtag (name, enumtype, /*tag_scope=*/ts_current);
>
>           /* std::byte aliases anything.  */
> -         if (enumtype != error_mark_node
> +         if (cxx_dialect >= cxx1z
> +             && enumtype != error_mark_node
>               && TYPE_CONTEXT (enumtype) == std_node
>               && !strcmp ("byte", TYPE_NAME_STRING (enumtype)))
> -           TYPE_ALIAS_SET (enumtype) = 0;
> +           TYPE_TYPELESS_STORAGE (enumtype) = 1;
>         }
>        else
>
> not needed (but also not sufficient - you need to handle arrays of
> byte somewhere).

See cxx_type_contains_byte_buffer: this function looks recursively into
structures and unions, and returns the information if the beast
contains an array of unsigned char or std::byte.

>
> @@ -1491,6 +1491,7 @@ struct GTY(()) tree_type_common {
>    unsigned needs_constructing_flag : 1;
>    unsigned transparent_aggr_flag : 1;
>    unsigned restrict_flag : 1;
> +  unsigned typeless_storage_flag : 1;
>    unsigned contains_placeholder_bits : 2;
>
>    ENUM_BITFIELD(machine_mode) mode : 8;
>
> bits are grouped in groups of 8 bits, this breaks it.
>

Oh..., does this explain the problems that I had with this version???

> @@ -8041,7 +8041,8 @@ build_pointer_type_for_mode (tree to_type, machine
>
>    /* If the pointed-to type has the may_alias attribute set, force
>       a TYPE_REF_CAN_ALIAS_ALL pointer to be generated.  */
> -  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
> +  if (TYPE_TYPELESS_STORAGE (to_type)
> +      || lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
>      can_alias_all = true;
>
>    /* In some cases, languages will have things that aren't a POINTER_TYPE
> @@ -8110,7 +8111,8 @@ build_reference_type_for_mode (tree to_type, machi
>
>    /* If the pointed-to type has the may_alias attribute set, force
>       a TYPE_REF_CAN_ALIAS_ALL pointer to be generated.  */
> -  if (lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
> +  if (TYPE_TYPELESS_STORAGE (to_type)
> +      || lookup_attribute ("may_alias", TYPE_ATTRIBUTES (to_type)))
>      can_alias_all = true;
>
>    /* In some cases, languages will have things that aren't a
>
> not needed.
>

You mean, because the get_alias_set (to_type) will be 0 anyways,
and can_alias_all wont change the semantic?


Bernd.

> +/* Nonzero if the type should behave like a character type
> +   with respect to aliasing sementics.  */
> +#define TYPE_TYPELESS_STORAGE(NODE) \
> +  (TYPE_CHECK (NODE)->type_common.typeless_storage_flag)
>
> ARRAY_TYPE_CHECK (NODE)->
>
> Richard.
>