[PATCH] Add a DWOId field to DICompileUnit (so DWARF skeleton CUs can be expression in IR).

[David Blaikie]

And this is the part of the recent debug info schema changes that makes me
a bit sad - the amount of code that has to change to add a field to a debug
info record... that's a lot of files to touch, etc. :/

On Mon, May 4, 2015 at 12:58 PM, Adrian Prantl <aprantl at apple.com> wrote:

> Hi dexonsmith, dblaikie, echristo,
>
> This patch adds a DWOId field to DICompileUnit so DWARF skeleton CUs can
> be expression in IR.
>

We might want to call this ModuleId - normal DWOs won't have a DWOId passed
down from the frontend (because it's computed in the backend by hashing the
contents of the debug_info in the .dwo file). But maybe DWOId is fine - not
sure, just throwing some ideas around.

I suppose this will be used on both sides of the module+fission work -
passed down for a non-trivial CU when building the module's debug info
itself. Then also passed down attached to a trivial CU when building the
skeleton CU to reference the module? (in the latter case, it'll need to be
used for the comdat group identifier too - we'll need some way to
differentiate these two cases, then? as well as including all the special
llvm attributes for module reconstitution (pending where they actually end
up))


> A skeleton CU is a (typically empty) DW_TAG_compile_unit that has a
> DW_AT_(GNU)_dwo_name and a DW_AT_(GNU)_dwo_id attribute. It is used to
> refer to external debug info.
>
> This is a prerequisite for clang module debugging as discussed in
> http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-November/040076.html.
> In order to refer to external types stored in split DWARF (dwo) objects,
> such as clang modules, we need to emit skeleton CUs, which identify the
> dwarf object (i.e., the clang module) by filename (the SplitDebugFilename)
> and a hash value, the dwo_id.
>
> This patch only contains the IR changes. The idea is that a CUs with a
> non-zero dwo_id field will be emitted together with a DW_AT_GNU_dwo_name
> and DW_AT_GNU_dwo_id attribute.
>
> REPOSITORY
>   rL LLVM
>
> http://reviews.llvm.org/D9488
>
> Files:
>   include/llvm/IR/DIBuilder.h
>   include/llvm/IR/DebugInfoMetadata.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
>   test/Assembler/mdcompileunit.ll
>   unittests/IR/MetadataTest.cpp
>
> EMAIL PREFERENCES
>   http://reviews.llvm.org/settings/panel/emailpreferences/
>
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