[PATCH] D100572: [DebugInfo] Ensure DIArgList in bitcode has no null or forward metadata refs

[Stephen Tozer via Phabricator via llvm-commits]

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This solves an issue seen in a Chromium build <https://bugs.chromium.org/p/chromium/issues/detail?id=1198356#c1>, which resulted from incorrect bitcode read/write handling for `DIArgList`. The specific cause of the issue is that previously, when writing `DIArgList` to bitcode, there was no guarantee that all of its arguments had been written to bitcode beforehand. `DIArgList` is strict about its arguments, and will only accept `ValueAsMetadata*`, so forward MD references (temporary `MDNode*`s) are not valid when constructing a `DIArgList` object. It also meant that if a `ConstantAsMetadata` was only referenced in a `DIArgList`, it would not be written at all, resulting in a null record.

This issue was further obfuscated by the fact that `DIArgList` was not being stringent enough when reading and writing bitcode records; although `DIArgList` does not support null values for its arguments, it was allowing the argument records to be null.

This has been fixed by adding a new method to the `ValueEnumerator` to handle `DIArgList` that enumerates any `ConstantAsMetadata` arguments and contains a number of `DIArgList`-specific assertions. The read/write code has also been modified to appropriately assert that its arguments are non-null.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D100572

Files:
  llvm/lib/Bitcode/Reader/MetadataLoader.cpp
  llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
  llvm/lib/Bitcode/Writer/ValueEnumerator.cpp
  llvm/lib/Bitcode/Writer/ValueEnumerator.h

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