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

[Richard Biener <rguenther@suse.de>]

On April 5, 2017 3:28:32 PM GMT+02:00, Jakub Jelinek <jakub@redhat.com> wrote:
>On Wed, Apr 05, 2017 at 09:46:09AM +0000, Bernd Edlinger wrote:
>> this is related to the P1 codegen bug PR79671:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
>> 
>> Therefore I would like to add this now although
>> the trunk is already at in stage 4.
>> 
>> I propose to add a new type attribute always_alias that
>> works like may_alias but unlike may_alias it should
>> also make instances alias anything.  It is also exposed
>> in C/C++ as __attribute__((always_alias)).
>
>I have just two small comments, for review IMHO want agreement
>between Jason and Richard on this.
>
>One thing is whether it is a good idea to keep this info in the IL
>as an attribute, especially when you add it automatically in the
>C++ FE.  I see e.g. 25 spare bits in tree_type_common.  Don't say we
>need to waste them, but I'd say in C++ unsigned char arrays are fairly
>common and thus not having to create the attribute for them each time
>and look it up might be beneficial.

Agreed.

>Also, wonder if you need to mark all types containing such arrays,
>if you couldn't just set that flag in C++ on unsigned char/std::byte
>arrays (and on anything with that attribute), have that imply alias set
>0 on it and then let the rest of alias machinery handle aggregate types
>containing such fields.

Yes, I expected it to work like this (didn't look at the patch yet).

Richard.

>	Jakub