[llvm-dev] [GlobalISel][RFC] Contract between LLVM IR and the backends for ISel

[Matthias Braun via llvm-dev]

> On Jan 22, 2016, at 2:36 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> I would like your opinions on the contract we have between the LLVM IR and the backends.
> 
> 
> * Context *
> 
> Right now, the backends are supposed to be able to perform instruction selection on any valid LLVM IR.
What about IR using builtins that the target doesn't support? Is that invalid IR?

> Although this is *not* something I want to change for GlobalISel, I thought I brought that up on the mailing list to discuss the implications.
> 
> In particular, in the past, some people mentioned that they wanted to do (some part of) the legalization on LLVM IR. This may impact the contract we have between LLVM IR inputs and the backends and I would like to clarify that.
> 
> 
> * Feedback Needed *
> 
> 1. In your opinion where does a “backend" start?
> 
> For instance, does a backend starts at llc or at ISel?

In practical terms I'd say the "backend" is all the passes added by LLVMTargetMachine::addPassesToEmitFile()/LLVMTargetMachine::addPassesToEmitMC()... So I don't see any "contract" problems here whether we have some IR passes before ISel or not... Obviously you may have a contract inside the backend as to what IR may reach the ISel phase.

- Matthias