[PATCH] Add a DIExternalTypeRef debug metadata node to the IR.

[Duncan P. N. Exon Smith]

> On 2015 May 8, at 15:32, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Fri, May 8, 2015 at 11:18 AM, Adrian Prantl <aprantl at apple.com> wrote:
> Hi dexonsmith, dblaikie, echristo,
> 
> This adds a DIExternalTypeRef debug metadata node to the IR that is meant to be emitted by Clang when referring to a type that is defined in a .pcm file. DIExternalTypeRef is a child of DICompositeTypeBase because it shares common traits such as the unique identifier.
> 
> Could this be an attribute to the DIClassType/StructureType? (to emit the currently needed type unit references, we still should know the correct class/structure type tag) LLVM already knows to hash the ref identifier (mangled name) for the type unit identifier.

I think they should be separate for now.  When we have an
`identifier:` field in `DICompositeType` we use the
string-based `DITypeRef`s instead of direct pointers.  I don't
imagine there's any value in adding that indirection for these.

(Although, this makes me wonder about an LTO scenario.  Imagine you
have two translation units, and they both use a type identified by
(e.g.) "my-type".  One of them pulled the type in from a module
and has a `DIExternalTypeRef`, while the other translation unit
didn't use modules and has a `DICompositeType`.  What should the
debug info look like?  Is it okay if we end up with both?  If not,
which one should win?  Will these new nodes show up in the
`retainedType:` list?)

Another couple of questions to decide whether to make a new node:

 1. Will the backend logic be essentially the same for the two, or
    would we have disjoint handling?
 2. Is this a reasonable target for a 'scope:' field?  (If not, I
    think they should be distinct.)


> 
> - David
>  
> 
> The idea how this is going to be used is that the frontend will discover that a type originates from an AST file and instead of and constructing a full DIType (and thus deserializing the type from the AST) it creates a DIExternalTypeRef with the tag type, mangled name and the .pcm file. The backend then can emit this as a split-DWARF-style forward declaration using DW_FORM_ref_sig8 (GDB) or as a string reference (LLDB) + and accelerator table entry.
> 
> REPOSITORY
>   rL LLVM
> 
> http://reviews.llvm.org/D9612
> 
> Files:
>   include/llvm/Bitcode/LLVMBitCodes.h
>   include/llvm/IR/DIBuilder.h
>   include/llvm/IR/DebugInfoMetadata.h
>   include/llvm/IR/Metadata.def
>   include/llvm/IR/Metadata.h
>   lib/AsmParser/LLParser.cpp
>   lib/Bitcode/Reader/BitcodeReader.cpp
>   lib/Bitcode/Writer/BitcodeWriter.cpp
>   lib/IR/AsmWriter.cpp
>   lib/IR/DIBuilder.cpp
>   lib/IR/DebugInfoMetadata.cpp
>   lib/IR/LLVMContextImpl.h
>   lib/IR/Verifier.cpp
>   test/Assembler/DIExternalTypeRef.ll
> 
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