[PATCH] D68117: [DWARF-5] Support for C++11 defaulted, deleted member functions.

[Paul Robinson via Phabricator via llvm-commits]

probinson added a comment.

In D68117#1710295 <https://reviews.llvm.org/D68117#1710295>, @dblaikie wrote:

> I think the existing feature's perhaps a bit misspecified - because where you put the DEFAULTED_{in_class,out_of_class} would communicate that without needing the two values - if you put it on an out of line function definition, it's defaulted out of line, if you put it on the in-class declaration then it's defaulted inline.
>
> But spec'd the way it is, if we want to implement this at all/if there's some actual user need (admittedly the noreturn attribute has gone in recently without a discussion of usage... so I'm not the only gatekeeper here & other people have other opinions, and that's OK - if someone (@probinson @aprantl etc) want to tell me I'm being unreasonable here & we should just put it in - it's only a bit here or there & not likely to make a huge difference to DWARF size & possibly enable some scenarios we haven't imagined yet and avoid the chicken-and-egg problem for the future implementer of such features, that's OK) - I'd essentially implement it that way. Put DEFAULTED_out_of_class on the member function definition DIE if it's defaulted there, and put DEFAULTED_in_class on the declaration DIE if it's defaulted there.
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> And I'd probably only spend one bit of flags on this, if possible - "not defaulted (0/assumed for all subprograms) or defaulted (1)" and translate it into one of the two values (in_class or out_of_class) in the backend based on which subprogram DIE it's attached to.


That seems reasonable too.  Of course if we're already spending a bit on Deleted, and the same subprogram can't be both Deleted and Defaulted, it costs no extra DISP bits to represent the 4 cases (defaulted-in-class, defaulted-out-of-class, deleted, none-of-the-above).

@SouraVX I would say we should never emit DEFAULTED_no.  If the compiler organized its internal data differently, and had a canned abbreviation for ctors/dtors that included DW_AT_defaulted, then we'd need to fill that in; but that's not how LLVM handles DWARF, it creates abbreviations on demand.  So, doing nothing in the none-of-the-above case would be best here.

(@dblaikie Aside regarding noreturn, the original DWARF proposal was for a debugger wanting to know a given function will not return.  As a flag, in the DWARF it costs an extra abbrev entry but no space in .debug_info.  Cheap enough for me.)


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