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

[Adrian Prantl]

> On May 8, 2015, at 1:00 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> 
>> 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.

Note that DIExternalTypeRef also knows the tag.
> 
> 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.

It would make lookups slower, but otherwise it would be harmless.

> (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?

This is a scenario that can only happen in C++. For maximum space efficiency you would want the external type ref to win or you will end up with two copies of the type (which isn’t otherwise harmful). If you are are using dsymutil, it will still unique the two copies.

>  Will these new nodes show up in the
> `retainedType:` list?)

They will not show up in the retained types 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?

There is different backend handling for LLDB and GDB.

> 2. Is this a reasonable target for a 'scope:' field?  (If not, I
>    think they should be distinct.)

We could attach the Skeleton compile unit but there is no real need for it. Making them distinct sounds fine for me.

-- adrian
> 
> 
>> 
>> - 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
>> 
>> EMAIL PREFERENCES
>>  http://reviews.llvm.org/settings/panel/emailpreferences/
>> 
>