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

[Richard Biener]

On Mon, 10 Apr 2017, Jason Merrill wrote:

> On Mon, Apr 10, 2017 at 8:50 AM, Richard Biener <rguenther@suse.de> wrote:
> > So here's my variant of a fix.  I constrained the new flag
> > TYPE_TYPELESS_STORAGE to arrays and thus hope my localized fix in
> > build_cplus_array_type will suffice (if I have time I'll play with
> > template instantiation).
> 
> Looks good.  I would expect this to just work with templates, but it
> should of course be tested.  In particular, std::aligned_storage needs
> to work.

The libstdc++ testsuite passes... (of course it did before as well).
I'll experiment with some trivial bits tomorrow.

> >         * tree.c (build_cplus_array_type): Set TYPE_TYPELESS_STORAGE
> >         for arrays of unsigned char or std::byte.
> 
> I think it would be good to have a flag to select whether these
> semantics apply to any char variant and std::byte, only unsigned char
> and std::byte, or only std::byte.

Any suggestion?  Not sure we really need it (I'm hesitant to write
all the testcases to verify it actually works).

Meanwhile re-testing with the following hunk to fix some LTO fallout.

Richard.

Index: gcc/tree.h
===================================================================
--- gcc/tree.h  (revision 246808)
+++ gcc/tree.h  (working copy)
@@ -4914,7 +4917,7 @@ inline bool
 canonical_type_used_p (const_tree t)
 {
   return !(POINTER_TYPE_P (t)
-          || TREE_CODE (t) == ARRAY_TYPE
+          || (TREE_CODE (t) == ARRAY_TYPE && ! TYPE_TYPELESS_STORAGE (t))
           || TREE_CODE (t) == VECTOR_TYPE);
 }