[LLVMdev] FYI, cheat sheet for updating out-of-tree targets for TTI changes

[Chandler Carruth]

Just wanted to help folks out, as the structure of the changes I made to
the in-tree targets was somewhat optimized for code review and handling
reverts. Not necessarily the efficient way to update an out-of-tree target.

First, update your TTI:
- Move the class definition into a header
- Rip out the base class, remove all override and const qualifiers from
methods
- Derive from BasicTTIImpl<YourTargetTTIImpl> so you pick up all the
target-independent code generator implementation
- friend the CRTP base class so it can call a couple of private methods in
your TTI
- define getST() and getTLI() private methods that return pointers to your
target's subtarget and TLI respectively. you probably want to cache those
pointers in members and make these methods simple getters. these are called
by BasicTTIImpl
- Provide a boring constructor for your TTI, you'll directly construct it
for a specific function with your TargetMachine available.

Once you have the TTI updated, update your target machine:
- Nuke the addAnalysisPasses override, that method is gone
- add an override for getTargetIRAnalysis that returns a TargetIRAnalysis
object (you'll need to include TargetTransformInfo.h)
- in your implementation of this override, return a TargetIRAnalysis object
constructed from a lambda, the lambda should be callable with a function
reference, and should return a TargetTransformInfo object wrapping your
target's TTIImpl; you can construct your Impl directly and pass it to the
TargetTransformInfo constructor, it will do all the wrapping for you

AArch64TargetTransformInfo.{h,cpp} and AArch64TargetMachine.cpp are good
examples to emulate.

Lemme know if you hit problems.
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