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

[Krzysztof Parzyszek via llvm-dev]

Are there any instructions that aren't terminators now, but will become 
terminators with this change?  I'm wondering if this is going to affect 
reading old bitcode, and if so, how it will be handled.

-Krzysztof

On 5/17/2018 4:03 AM, Chandler Carruth via llvm-dev wrote:
> 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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