[llvm-dev] What was the IR made for precisely? (original) (raw)

Renato Golin via llvm-dev llvm-dev at lists.llvm.org
Fri Oct 28 12:45:10 PDT 2016

On 28 October 2016 at 20:13, Chris Lattner <clattner at apple.com> wrote:

Is there a specific design that you think would work? One of the major problems with this sort of proposal is that you need the entire clang type system to do this, which means it depends on a huge chunk of the Clang AST. At that point, this isn’t a small library that clang uses, this is a library layered on top of Clang itself.

Hi Chris,

I personally don't think we can (should) make IR completely target independent, but we could reduce the lowering complexity by adding some (more?) intermediate steps between the AST and IR. This was discussed a long time ago and I haven't kept track, so I may be completely out of reality here.

The idea was not to create another representation, but to have some wrapper classes around the IRBuilder that would know about target-specific knowledge and simplify the lowering. The example we used was mostly around argument marshaling (byval, sret), so that front-ends could be simplified a bit.

For example, Clang would lower all function calls using structures with structs and this wrapper would transform into arrays and casts, move the arguments around, return void, etc. to make sure that the PCS is understood by the middle/back-end of choice.

This could start as a GSoC project, but it may have to be split between investigation (ie break everything) and implementation (fix everything back again). But since this is just a wrapper, other front-ends can still use their old ways and migrate to this wrapper later on.

Or this could be done already, and I'm out of ideas. :)

cheers, --renato

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