[PM/ThinLTO] Port the ThinLTO pipeline (both components) to the new PM.
Based on the original patch by Davide, but I've adjusted the API exposed
to just be different entry points rather than exposing more state
parameters. I've factored all the common logic out so that we don't have
any duplicate pipelines, we just stitch them together in different ways.
I think this makes the build easier to reason about and understand.
This adds a direct method for getting the module simplification pipeline
as well as a method to get the optimization pipeline. While not my
express goal, this seems nice and gives a good place comment about the
restrictions that are imposed on them.
I did make some minor changes to the way the pipelines are structured
here, but hopefully not ones that are significant or controversial:
- I sunk the PGO indirect call promotion to only be run when we have PGO enabled (or as part of the special ThinLTO pipeline).
- I made the extra GlobalOpt run in ThinLTO just happen all the time and at a slightly more powerful place (before we remove available externaly functions). This seems like general goodness and not a big compile time sink, so it didn't make sense to *only* use it in ThinLTO. Fewer differences in the pipeline makes everything simpler IMO.
- I hoisted the ThinLTO stop point pre-link above the the RPO function attr inference. The RPO inference won't infer anything terribly meaningful pre-link (recursiveness?) so it didn't make a lot of sense. But if the placement of RPO inference starts to matter, we should move it to the canonicalization phase anyways which seems like a better place for it (and there is a FIXME to this effect!). But that seemed a bridge too far for this patch.
If we ever need to parameterize these pipelines more heavily, we can
always sink the logic to helper functions with parameters to keep those
parameters out of the public API. But the changes above seemed minor
that we could possible get away without the parameters entirely.
I added support for parsing 'thinlto' and 'thinlto-pre-link' names in
pass pipelines to make it easy to test these routines and play with them
in larger pipelines. I also added a really basic manifest of passes test
that will show exactly how the pipelines behave and work as well as
making updates to them clear.
Lastly, this factoring does introduce a nesting layer of module pass
managers in the default pipeline. I don't think this is a big deal and
the flexibility of decoupling the pipelines seems easily worth it.
Differential Revision: https://reviews.llvm.org/D33540