[llvm-dev] RFC: Removing TerminatorInst, simplifying calls

[Chandler Carruth via llvm-dev]

Going to keep this RFC short and to the point:

TerminatorInst doesn't pull its weight in the type system. There is
essentially a single relevant API -- iterating successors. There is no
other interesting aspect shared -- the interface itself just dispatches to
specific instructions to be implemented.

On the flip side, CallInst and InvokeInst have *massive* amounts of code
shared and struggle to be effective due to being unable to share a base
class in the type system. We have CallSite and a host of other complexity
trying to cope with this, and honestly, it isn't doing such a great job.

I propose we make "terminator-ness" a *property* of an instruction and take
it out of the type system. We can build a handful of APIs to dispatch
between instructions with this property and expose successors. This should
be really comparable to the existing code and have nearly no down sides.

Then I propose we restructure the type system to allow CallInst and
InvokeInst to easily share logic:
- Create `CallBase` which is an *abstract* class derived from Instruction
that provides all of the common call logic
- Make `CallInst` derive from this
- Make `InvokeInst` derive from this, extend it for EH aspects and
successors
- Remove `CallSite` and all accompanying code, rewriting it to use
`CallBase`.

The end result will, IMO, be a much simpler IR type system and
implementation. The code interacting with instructions should also be much
more consistent and clear w/o the awkward CallSite "abstraction".

Thoughts? Seem OK at a high level?

Happy to bikeshed the name `CallBase`, but I've discussed this with several
folks, including Reid and Chris and nothing better came up really.
`CallSite` might be nicer, but the confusion with the *existing* type seems
much more problematic.


Assuming folks are happy with this direction, are there any incremental
patches that folks would like to see in pre-commit review? I've only done
some initial investigation of what it takes to cut this through. Provided
folks are positive about the direction, I'll work on what this would
actually look like in practice.

-Chandler
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