[llvm-dev] [RFC] Lanai backend

[Chris Lattner via llvm-dev]

> On Feb 9, 2016, at 9:40 AM, Jacques Pienaar via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi all,
> 
> We would like to contribute a new backend for the Lanai processor (derived from the processor described in [1]).

Hi Jacques,

We generally have a low bar for accepting new “experimental” backends, but I think that this is the first proposal to add a target for a hardware that general LLVM contributors can’t have access to.  As such, we’ll have to figure out as a community whether this makes sense.

Here are the tradeoffs I see of accepting the backend:

1) I imagine that there is a big win for you, not having to merge with mainline.  Maintaining an out of tree backend is a pain :-)

2) For the community, this is probably a net loss since changes to common codegen could would be required to update your backend, but no one else in the community would benefit from the target being in mainline.

3) There is probably a small but non-zero benefit to keeping your team working directly on mainline, since you’re more likely to do ancillary work in ToT.  If your development is in mainline, this work is most likely to go into llvm.org instead of into your local branch.

4) There could be an educational benefit of having the backend, particularly if it has unique challenges to overcome.


What do others think about this?  I know that several organizations have not even bothered proposing internal-only targets for inclusion in llvm.org, since they would effectively be contributing dead code that the community would have to maintain.

-Chris