[llvm-dev] Issues with DummyCGSCCPass used for IPRA

[Maxime Chevalier-Boisvert via llvm-dev]

Hello,

I'm trying to enable interprocedural register allocation (IPRA). In order to do this, I’ve added the DummyCGSCCPass to our pipeline, as is done in TargetPassConfig::addISelPrepare. My understanding is that this pass wraps the following passes into an SCC pass manager, so that they are run in bottom-up order, which is necessary to enable register allocation info from callees to be available at call sites.

The problem I’m running into is that we have an ImmutablePass which is run early in our pipeline (and needed in later machine code generation stages), and for some reason, this pass gets finalized and re-run multiple times, destroying the information we need in the process. I’m really not sure what is happening. The pass is clearly marked as required by later passes, but somehow it is found to be “dead”, finalized, and then re-run. To make matters more confusing, we have another ImmutablePass which is run early and used late, but this one doesn’t get finalized. My coworkers and I are really not sure what the difference might be.

Could someone more familiar with IPRA or the PassManager provide some hints as to what might be happening?

Regards,

- Maxime