[LLVMdev] Proposal for new Legalization framework

[Chris Lattner]

On Apr 24, 2013, at 5:01 PM, Dan Gohman <dan433584 at gmail.com> wrote:
> In the spirit of the (long-term) intent to migrate away from the SelectionDAG framework, it is desirable to implement legalization passes as discrete passes. Attached is a patch which implements the beginning of a new type legalization pass, to help motivate discussion.

This is a great discussion to have.

> Is LLVM IR the right level for this?

IMO, no, definitely not.

> The main alternative approach that's been discussed is to do FastISel to a target-independent opcode set on MachineInstrs, and then do legalization and ultimately the last phase off instruction selection proper after that. The most obvious advantage of using LLVM IR for legalization is that it's (currently) more developer-friendly. The most obvious advantage of using MachineInstrs is that they would make it easier to do low-level manipulations. Also, doing legalization on MachineInstrs would mean avoiding having LLVM-IR-level optimization passes which lower the IR, which has historically been a design goal of LLVM.

I think that you (in the rest of your email) identify a number of specific problems with using LLVM IR for legalization.  These are a lot of specific issues caused by the fact that LLVM IR is intentionally not trying to model machine issues.  I'm sure you *could* try to make this work by introducing a bunch of new intrinsics into LLVM IR which would model the union of the selection dag ISD nodes along with the target specific X86ISD nodes.  However, at this point, you have only modeled the operations and haven't modeled the proper type system.

LLVM IR is just not the right level for this.  You seem to think it is better than MachineInstrs because of developer friendliness, but it isn't clear to me that LLVM IR with the additions you're talking about would actually be friendly anymore :-)

Personally, I think that the right representation for legalization is MachineInstrs supplemented with a type system that allows MVTs as well as register classes.  If you are seriously interested in pushing forward on this, we should probably discuss it in person, or over beer at the next social or something.

-Chris