llvm dev - Sep 2021 - Accessing an out-of-tree custom AA in LLVM

Hi,

I am still wondering how to access the results of my custom, out-of-tree AA in
other (out-of-tree) passes; using the LLVM provided mechanisms for AA
"enrichment", and using opt as the top-level.  I have attached some
code which works (on LLVM 8, still) for me, but some questions remain:

(1) What is the correct way to hook my call-back into the ExternalAAWrapperPass?
The code I can find uses createExternalAAWrapper pass with an appropriate CB,
but in my case I don't have a pass manager lying around I can hook the
resulting ImmutablePass into.  Instead, my approach is to rely on the
ExternalAAWrapperPass to be there, and then (in my custom_aa_pass::runOnFunction
!) manually hook in the CB.  That feels wrong, but seems to work.  Previously, I
experimented with subclassing ExternalAAWrapperPass, but that was fragile.

(2) Using the LegacyAARGetter in a ModulePass works (after adding
https://reviews.llvm.org/D71348 ), but I manually need to instantiate the
custom_aa_pass via a command line argument to opt; it is somehow not picked up
from AA.addRequired<custom_aa_pass>(), and also manually nudging it with
an additional getAnalysis<custom-aa-pass>(F) in runOnModule does not make
it appear.  Is that intentional?

Overall, the entire (legacy) pass manager code feels like black magic to me, and
it is not quite clear whether what I have come up with is sensible.  Any
comments (other than code style, please; I know it is different ^^) on the
attached would be much appreciated!

Thanks,
  Stephan
--
Stephan Diestelhorst
System Architect
Xilinx - Data Center Group
Cambridge, UK

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