On 04/05/17 15:28, Jakub Jelinek 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. >
Yes, but it is not clear if it will be available in LTO, for instance we used to stream TYPE_ALIAS_SET == 0 information to let frontends mark types that are opaque for TBAA. This however did not work as intended, because TYPE_ALIAS_SET == 0 was regularly lost in type merging. But streaming an attribute works seamless, and is not lost in type merging AFAICT. > 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. > Actually I set the flag only on C++ code with std::byte and struct/union with embedded unsigned char or std::byte arrays, but not on char arrays for instance, so I hope that it will not pessimize the codegen too much. The attribute does not propagate from normal std::byte to the enclosing structure. With LTO it is impossible to know which -std= option was used and all C and C++ code is merged together. So I was trying hard to fin a way how it can work without breaking LTO. Bernd.
