Hi,
We had a problem with AssertingValueHandles exploding in opt for certain
sequences of passes and registered TR
27050<https://llvm.org/bugs/show_bug.cgi?id=27050> for this. No takers, so
I decided to dig a bit deeper into it and found the cause: The CallGraph
construction is run twice, and as the first instance is left lying around even
after the point at which its analysis is determined to be not
"preserved"! This means the use-list of a function will have
AssertingValueHandles from both CallGraphs. Then the AssertingValueHandles
associated with the first CallGraph blows up when a function is replaced by the
pass argpromotion since only the second CallGraph's AssertingValueHandles
are removed when a function is deleted.
My take is that this would be a bug in LegacyPassManager. Would anyone care to
agree or disagree?
If agreed, then the question is what to do about this. At first glance one would
believe that LegacyPassManager::removeNotPreservedAnalysis() would actually
destroy the CallGraph when it finds the analysis not preserved. However, it does
not. I wonder if this is an error, but when I insert removal in that function,
other stuff breaks, so I'm led to believe that the
removeNotPreservedAnalysis() isn't meant to go that deep, and instead just
clean up some map. Perhaps the CallGraph should be removed a bit later in
removeDeadPasses() instead? But it is not in the relevant lastuse-map. Perhaps
that's the bug? Or something else entirely.
If find the PassManagers a bit hard to get a good grip of, so any help and
insights would be greatly appreciated!
BR, Jesper
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