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

[Louis Dionne via Phabricator via llvm-commits]

ldionne added a comment.

You have a typo in your commit message (`becuase`). Generally LGTM with a small style comment. I'd also like to better understand the motivation before shipping this: can someone explain why/how the tests currently depend on the class being packed?



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Comment at: libcxx/include/__format/parser_std_format_spec.h:111
   _Type __type{_Type::__default};
-};
+} __attribute__((packed));
 
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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.


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https://reviews.llvm.org/D119567