[PATCH] D83047: [LiveDebugValues] 2/4 Add instruction-referencing LiveDebugValues implementation

[Jeremy Morse via Phabricator via llvm-commits]

jmorse added a comment.

On the topic of this implementation in general: the more I look at the vlocJoin method, the more I'm convinced it's incorrect. While it does produce an identical binary when stage2 building clang, I think it's only correctly dealing with the kind of control flow that C/C++ produces. If you expose it to weirder control flow, like these MIR files:

  https://gist.github.com/jmorse/7e199c289b2a3c94f73a6f62b515b05f
  https://gist.github.com/jmorse/1c9daca31cd5d6c0d4ba727a5bdfb016

It produces the wrong output (details in a comment on each gist). For the latter, the problem is caused by insufficient number of iterations of vlocDataflow, not sure about the former. One major problem is that by allowing "empty" variable values (represented by no entry in the live-in map), changes that should lead to a variable-value live-out change and thus trigger another dataflow iteration, instead terminates early.

I'm pretty confident that everything else in this pass implementation is correct, and that the variable-value situation can be fixed. If anyone's currently reviewing that portion of code though, it's probably best to stop as I'm going to mess with it further.


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  https://reviews.llvm.org/D83047/new/

https://reviews.llvm.org/D83047