dblaikie added a subscriber: rsmith.
dblaikie added a comment.
In D68117#1714091 <https://reviews.llvm.org/D68117#1714091>, @SouraVX wrote:
> Hi @probinson @dblaikie @aprantl , I've was investigating and working on
> your inputs regarding the problem with DW_at_defaulted once. I think clang
> has also some issues.
You think clang has bugs, independent of the debug info?
> Though I'm not able to precisely point out. I ranned into some problems in
> CFE while marking out_of_class functions. i.e consider this for instance
> "debug-info-defaulted-out_of_class.cpp" FIXME:. Causing too much trouble and
> possibly can introduce some bugs in clang/llvm.
>
> May be we'll reconsider this in future. Thanks for putting time in reviewing
> and discussing this.
Yeah, I looked at it a bit & asked @rsmith for some thoughts. Eventually
wrapped my head around it.
Clang's codegen occurs during parsing - rather than parsing/AST building
entirely, then code generating. So to take a smaller example:
struct foo {
foo();
// virtual
void f1();
void f2();
};
void foo::f2() { }
foo::foo() = default;
Without the virtual keyword, the first time the debug info needs the type (when
building the description for f1's definition's "this" parameter) the foo::foo()
= default definition hasn't been parsed yet, so there is no definition of
foo::foo() available. (so the out of class defaulting isn't visible)
The reason virtual matters is that the actual class definition of 'foo' is
never built - check the DWARF and metadata, only a declaration of "foo" is
built. And the declarations of the member functions are injected into the
declaration as the definitions are parsed, not before - so they observe the
correct defaulting state.
Another way to show the bug is with -fstandalone-debug, even with a virtual
function it shows the problem.
And moving 'f2' around (before/after 'foo()') also shows the problem - because
having f2 first means the type definition (without virtual, or while using
-fstandalone-debug) is built before any defaulting is seen - so 'foo()' isn't
shown as defaulted.
All of this somewhat goes to my point - out of line defaulting should be tested
and recorded on the out of line definition, not on the declaration, of a
special member.
> Anyway here's the next course of action I'm considering. I'll be marking this
> review as abandoned. I'll raise a fresh new review for DW_AT_deleted feature,
> that;s pretty straight forward, considering C++ spec. If it's deleted, it has
> to be declared/defined{whatever} deleted in it's first declaration. This
> feature also augments the consumer information regarding which spl member
> functions are deleted, hence restrict their calling for user or do something
> useful with this at it's disposal.
>
> @probinson @dblaikie @aprantl Please share your thoughts/comments inclination
> regarding adding this DW_AT_deleted to clang/llvm.
Sure, deleted support seems fine/reasonable/good to me.
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