On Wed, 5 Apr 2017, Jason Merrill wrote: > On Wed, Apr 5, 2017 at 1:41 PM, Bernd Edlinger > <bernd.edlin...@hotmail.de> wrote: > > On 04/05/17 17:20, Richard Biener wrote: > >>> 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). > > My impression is that this is how GCC 6 worked, but GCC 7 decides to > ignore alias set 0 members. Is that right?
Yes, in GCC 6 an access via an aggregate type that had an alias-set zero member was using alias-set zero. So we'd re-instantiate that behavior for aggregates containing a type marked with the proposed new flag. > > I want to allow *only* what the C++ standard requires or what Jason says > > of course :), and not a single bit more, because it suppresses otherwise > > correct optimizations. > > > > So a struct with a std::byte member is not alias_set 0, > > only the std::byte itself is, but an array of std::byte > > is itself typeless_storage, and makes the whole structure > > also typeless_storage > > Well, only the array member, not the whole structure, but it may make > sense for GCC to treat the whole structure as such internally. Yes, it's the easiest way to implement the required behavior. But I wouldn't document it that way but clearly state that only the marked member behaves so but that for the purpose of accesses via a container type the standards access rules apply as if the member is accessed with a character type. Richard.
