[PATCH] D118715: [DebugInfo][InstrRef] Fix a tombstone-in-DenseMap crash from D117877

[Jeremy Morse via Phabricator via llvm-commits]

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This is a follow up to D117877 <https://reviews.llvm.org/D117877>: variable assignments of DBG_VALUE $noreg, or DBG_INSTR_REFs where no value can be found, are represented as a `DbgValue` object with Kind "Undef". This explicitly represents "There is no value for this, and never can be". In D117877 <https://reviews.llvm.org/D117877> I added a special-case to make some assignment accounting faster, without considering this scenario. It causes variables to be given the value `ValueIDNum::EmptyValue`, which would fail gracefully and wouldn't produce an illegal location, it'd just be discarded. However, recently that value began being entered into a DenseMap, where it's a tombstone value, causing assertions.

This patch handles that scenario and adds a regression test. The more general fix is to wrap `DbgValue`'s `ID` field in Optional<>, just like the `MachineOperand` field is, but I'm aiming for the minimal fix right now :(


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D118715

Files:
  llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
  llvm/test/DebugInfo/MIR/InstrRef/single-assign-propagation.mir

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