[libcxx-commits] [PATCH] D119567: [libcxx] String format class marked as packed

[Louis Dionne via Phabricator via libcxx-commits]

ldionne accepted this revision.
ldionne added a comment.
This revision is now accepted and ready to land.

LGTM since this looks good to @Mordante, who wrote this code in the first place. I'm curious to better understand what @joerg 's comment is, though.



================
Comment at: libcxx/include/__format/parser_std_format_spec.h:111
   _Type __type{_Type::__default};
-};
+} __attribute__((packed));
 
----------------
joerg wrote:
> ldionne wrote:
> > Mordante wrote:
> > > hubert.reinterpretcast wrote:
> > > > Mordante wrote:
> > > > > hubert.reinterpretcast wrote:
> > > > > > Use double-underscore form to avoid macro problems.
> > > > > > 
> > > > > > Actually, are we assuming GNU attribute support in general "everywhere"?
> > > > > Is there something like godbolt where I can check the layout on AIX online?
> > > > > I've been working on some changes regarding this class. In that new version unconditionally pack the struct on all platforms might lead to sub-optimal code. For the current code I don't see an issue with the packing.
> > > > > 
> > > > > > Actually, are we assuming GNU attribute support in general "everywhere"?
> > > > > 
> > > > > I theory yes. The pre-commit CI tests all supported platforms. So when the CI passes it works. (The ASAN errors are unrelated.) But I think it would be cleaner to make the packing conditional.
> > > > > 
> > > > > 
> > > > > Is there something like godbolt where I can check the layout on AIX online?
> > > > 
> > > > Yes: godbolt
> > > > 
> > > > https://godbolt.org/z/9j49baqsM
> > > Thanks I wasn't aware godbolt also has AIX support. Good to know for the future.
> > Can't we put that attribute at the beginning of the class declaration, i.e. `class _LIBCPP_TYPE_VIS __attribute__((__packed__)) _Flags`?
> > 
> > This whole time I thought the attribute applied to something different because of the (IMO) unusual placement.
> No, we are consciously not using naked GNU attributes in the headers. I'd also ask for an explicit alignment, even if it is 1.
Can you elaborate?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D119567/new/

https://reviews.llvm.org/D119567