Re: [PATCH] DWARFv5 DW_TAG_aligned_type.

[Mark Wielaard <mjw@redhat.com>]

On Fri, 2014-07-11 at 09:23 -0600, Tom Tromey wrote:
> Mark> +      if (TYPE_USER_ALIGN (domain) != 0)
> Mark> +	fprintf_filtered (stream, " _Alignas (%u)", TYPE_USER_ALIGN (domain));
> 
> I think the preferred spelling in C++ is "alignas", not "_Alignas".
> It would be a bit prettier if this were language-dependent.
> What do you think?  It affects a few spots.

Yes, C++ uses a keyword for this with slightly different, but almost
equal, semantics than the C _Alignas qualifier (C11 does define a header
stdalign.h that has #define alignas _Alignas so that you can use alignas
almost like in C++).

How does the c-typeprint.c code determine the current language of the
type?

> Mark> +/* Add the given user alignment to the element type of the array.  GCC
> Mark> +   outputs DWARF type qualifiers that apply to an array, not the
> Mark> +   element type.  But GDB relies on the array element type to carry
> Mark> +   the cv-qualifiers.  This is mimics section 6.7.3, point 9 of the
> Mark> +   C11 specification (n1570).  */
> Mark> +static struct type *
> Mark> +add_array_cv_aligned_type (struct die_info *die, struct dwarf2_cu *cu,
> Mark> +			   struct type *base_type, unsigned int user_align)
> Mark> +{
> 
> gdb rules put a blank line between the comment and the start of the
> function.

Fixed.

> Mark> +      TYPE_TARGET_TYPE (inner_array) =
> Mark> +	copy_type (TYPE_TARGET_TYPE (inner_array));
> 
> The "=" goes on the start of the next line.

Fixed.

> Mark> +static struct type *
> Mark> +read_tag_aligned_type (struct die_info *die, struct dwarf2_cu *cu)
> Mark> +{
> 
> Needs some kind of intro comment.

Added:

/* Handle DW_TAG_aligned_type and DW_AT_alignment.  */

> Mark> +/* Make a '_Alignas'-qualified version of TYPE (if user_align is
> Mark> +   stricter than the user alignment of TYPE).  */
> Mark> +
> Mark> +struct type *
> Mark> +make_aligned_type (struct type *type, unsigned int user_align)
> Mark> +{
> Mark> +  if (user_align > TYPE_USER_ALIGN (type))
> Mark> +    return make_qualified_aligned_type (type, TYPE_INSTANCE_FLAGS (type),
> Mark> +					user_align, NULL);
> 
> Like Joel I am curious about the need for this.
> 
> I thought maybe it was just following the (C & C++) language standard.
> But would DWARF like this really be emitted by the compiler?
> It wasn't clear to me.

Yeah, it wouldn't actually be emitted by my gcc patch. As I explained to
Joel I just wanted to handle the "nested alignment" case somehow and
assumed C11 semantics. We will define something for this and handle it
properly.

> Mark> +	int new_user_align = TYPE_USER_ALIGN (type);
> 
> Most spots are using unsigned for this.
> 
> Occasionally I'm tempted to adopt the C++ style of ubiquitous typedefs
> to avoid this kind of mismatch.  But I've never really brought that up,
> so I suggest just changing this instance.

Changed to unsigned.

Thanks,

Mark