[PATCH] D132224: [Assignment Tracking][5/*] Add core infrastructure for instruction reference

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

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The Assignment Tracking debug-info feature is outlined in this RFC <https://discourse.llvm.org/t/rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir/62367>. This first series of patches adds documentation, the changes necessary to start emitting and using the new metadata, and updates clang with an option to enable the feature. Working with the new metadata in the middle and back end will come later. There are still a few rough edges but I'm putting these patches up now hoping to get feedback on the design and implementation from the upstream community.

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Overview
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It's possible to find intrinsics linked to an instruction by looking at the `MetadataAsValue` uses of the attached `DIAssignID`. That covers `instruction -> intrinsic(s)` lookup. Add a global `DIAssignID -> instruction(s)` map which gives us the ability to perform `intrinsic -> instruction(s)` lookup. Add plumbing to keep the map up to date through optimisations and add utility functions including two that perform those lookups. Finally, add a unittest.

Details / patch tour
--------------------

In **llvm/lib/IR/LLVMContextImpl.h** add `AssignmentIDToInstrs` which maps `DIAssignID *` attachments to `Instruction *`s. Because the `DIAssignID *` is the key we can't use a `TrackingMDNodeRef` for it, and therefore cannot easily update the mapping when a temporary DIAssignID is replaced.

Temporary `DIAssignID`'s are only used in IR parsing to deal with metadata forward references. Update **llvm/lib/AsmParser/LLParser.cpp** to avoid using temporary `DIAssignID`'s for attachments.

In **llvm/lib/IR/Metadata.cpp** add `Instruction::updateDIAssignIDMapping` which is called to remove or add an entry (or both) to `AssignmentIDToInstrs`. Call this from ` Instruction::setMetadata` and add a call to `setMetadata` in `Intruction`'s dtor that explicitly unsets the DIAssignID so that the mappging gets updated.

In **llvm/lib/IR/DebugInfo.cpp** and **DebugInfo.h** add utility functions:

  getAssignmentInsts(const DbgAssignIntrinsic *DAI)
  getAssignmentMarkers(const Instruction *Inst)
  RAUW(DIAssignID *Old, DIAssignID *New)
  deleteAll(Function *F)
   

These core utils are tested in `llvm/unittests/IR/DebugInfoTest.cpp`.

Notes / observations
--------------------

This all needs to be looked at closely from a performance perspective once it's up and running. This mapping is obviously quite intrusive, but I'm not sure we can get around that.


https://reviews.llvm.org/D132224

Files:
  llvm/include/llvm/AsmParser/LLParser.h
  llvm/include/llvm/IR/DebugInfo.h
  llvm/include/llvm/IR/Instruction.h
  llvm/lib/AsmParser/LLParser.cpp
  llvm/lib/IR/DebugInfo.cpp
  llvm/lib/IR/Instruction.cpp
  llvm/lib/IR/LLVMContextImpl.h
  llvm/lib/IR/Metadata.cpp
  llvm/lib/IR/Verifier.cpp
  llvm/unittests/IR/DebugInfoTest.cpp

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