Removal of MDString-based type references in LLVM 4

[langston-barrett]

It seems LLVM 4 added a quite different way to refer to types in metadata. I'm not sure whether changes to the AST are necessary to accommodate this change, or if this is a parsing-only issue.

• Potentially relevant mailing list posts:
  • http:https://lists.llvm.org/pipermail/llvm-dev/2016-April/098318.html
  • http:https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160125/327936.html
• Github commit: llvm-mirror/llvm@de74840#diff-9c14d2f1497265054486ae1a3361d534R2389
• LLVM 4 language reference:
  • DICompositeType: https://releases.llvm.org/4.0.1/docs/LangRef.html#dicompositetype
  • DISubprogram: https://releases.llvm.org/4.0.1/docs/LangRef.html#disubprogram

[langston-barrett]

The fix should include the following test case for the bitcode parser:

• .ll with new-style references: https://github.com/llvm-mirror/llvm/blob/de7484036b628b08be6acbfb5feac405d7450300/test/DebugInfo/AArch64/cfi-eof-prologue.ll
• .ll with old-style references (3.8 release): https://github.com/llvm-mirror/llvm/blob/release_38/test/DebugInfo/AArch64/cfi-eof-prologue.ll#L1

When you run

nix-shell --pure -p llvm_39 --run "llvm-as cfi-eof-prologue.new.ll -o new.bc"
nix-shell --pure -p llvm_38 --run "llvm-as cfi-eof-prologue.old.ll -o old.bc"

and then run llvm-disasm on both files, the scope: fields are silently dropped from the new version, whereas they are properly retained in the old version.

[langston-barrett]

This isn't really related to the AST or pretty-printer, it's an issue for the parser.