[PATCH] IR: Add specialized debug info metadata nodes

[Frédéric Riss]

> On Feb 6, 2015, at 1:24 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Fri, Feb 6, 2015 at 1:20 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> 
> > On 2015-Feb-06, at 10:36, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
> >
> >
> >
> > On Wed, Feb 4, 2015 at 4:17 PM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> >
> >> On Feb 4, 2015, at 3:53 PM, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
> >>
> >>
> >>> On Feb 4, 2015, at 3:46 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> >>>
> >>>
> >>>> On 2015-Feb-04, at 15:37, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
> >>>>
> >>>>>>
> >>>>>> - The word 'context' is overloaded: `MDNode::getContext()` already
> >>>>>> exists, and returns an `LLVMContext&`; `DIDescriptor` uses 'context'
> >>>>>> to mean "the node that this one is defined inside".  I chose the
> >>>>>> word 'parent' instead of 'context' here.  Is this word okay?  If
> >>>>>> not, what about 'scope'?  This will be reflected in the assembly
> >>>>>> changes to come (I'd like the C++ names to match the assembly names,
> >>>>>> although technically it's not necessary).
> >>>>>>
> >>>>>> I'd /probably/ go with scope (we already have scope in the MDLocations, so that seems consistent), but fairly on-the-fence.
> >>>>>>
> >>>>>
> >>>>> Weirdly, I didn't even notice that :).  In that case I like 'scope'
> >>>>> better too.  I'll update to that before commit.
> >>>>
> >>>> Seems most natural. Can the futur getScope() return something that doesn’t derive from MDScope?
> >>>
> >>> I don't think so.
> >>
> >> That was my impression also, and it makes it even more appropriate IMHO.
> >>
> >
> > Just a few general remarks to throw into the discussion:
> >
> > - Would it make sense to use something like tablegen to generate the repetitive parts? I’m slightly worried about copy&paste bugs, moderately worried about refactoring it in the future.
> >
> > <classllvm_1_1DIScope__inherit__graph.png>
> >
> > As for scopes, there are several things that bug me about the current class hierarchy that we could fix now:
> > - DIFile should not be a scope (the concept of files is IMO orthogonal to scoping and there is always something more appropriate to put a node into: compile unit, module, namespace)
> >
> > IIRC we do this when # line directives change the file-name part-way through. But I could be wrong... only vague idea.
> >
> > - DIBasicType should not be scope
> >
> > Part of this is the ability to treat all types the same - multiple inheritance might be an alternative, but without that we want to treat all types equally in some codepaths, but then only some types are valid scopes, etc. - so there's a few different axes on which we want to refer to these things. It's tricky. But certainly worth thinking about.
> 
> LLVM-style RTTI doesn't handle multiple inheritance, which restricts
> us somewhat here.
> 
> *nod* I seemed to recall something like that
>  
> I imagine having a hierarchy like:
> 
>   DIScopeBase
>    -> DIScope
>        -> DINamespace
>        -> DISubprogram
>    -> DIType
>        -> DIBasicType
>        -> DIDerivedType
>        -> DITypeScope
>            -> DICompositeType
> 
> would get us most of the way there.
> 
> Pointers can be `DIScopeBase`.  Call-sites that care about scopes
> can check:
> 
>     assert(isa<DIScope>(N) || isa<DITypeScope>(N));
> 
> Yep, if we're willing to pay the cost of having scopes not have a common ancestor to use. That might be the right tradeoff, I'm not sure.
>  
> (We can add `DIScopeBase::isValidScope()` or something to do this.)
> 
> Or maybe we don't even need a `DIScopeBase`.
> 
> Yeah, if we go that way I don't think DIScopeBase would add anything, but not 100% sure.

When I first discovered the type hierarchy, the every-type-is-a-scope thing got me a bit perplexed. I really think we should get rid of it. Would it be possible to have the Composite types point to a real scope instead of being one?

Fred

> 
> >
> > - It’s questionable whether a DICompositetype should be a DIDerivedType
> > - A DISubroutineType should be neither a DIDerivedType nor DIScope
> > - Using DIDerivedTypes for CV qualifiers is a bit wasteful but it does map nicely to DWARF
> >
> > otherwise, thanks for doing this!
> > -- adrian

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