[LLVMdev] llvm dwarf emission

[Nick Lewycky]

I wanted to check that I'm headed in the right direction before I work more
on LLVM debug info. What I'd like to do is update DIBuilder to expose
exactly the facilities represented in modern DWARF, and to store that in
the bitcode. In LLVM we would have a DwarfOpts that specifies the major
version of DWARF we're targetting and some additional compatibility flags
to work around debugger deficiencies.

So for example, DIBuilder's interface has no createPtrToMemberType() which
is odd since it's been there since DWARF 2, but gcc didn't emit that until
very recently so neither does llvm. Instead clang lowers it to a difference
of two other pointer types, which effectively throws away information. My
plan is to always emit the bitcode with the DWARF 4 representation of a
pointer to member, then in codegen, look at DwarfOpts.PtrToMember to decide
whether to emit it as DW_TAG_ptr_to_member_type or the way gcc does for
compatibility with older GDB's, etc.

My premise is that storing the DWARF 4 equivalent data in the bitcode
retains more information, so we can safely lower to another format later.

Does this sound sensible?

Nick
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