llvm dev - Jan 2021 - Requirements for getting a new backend mainlined?

Hi, all!

I am currently working on a backend for the Parallax Propeller 2, and 
would like to know what the requirements are for a new backend to get 
mainlined into LLVM, and also get any general advice for writing a 
backend so that I don't have to go back and change a bunch of things 
later when I'm ready to submit a patch.

I'm new to working with LLVM, so any advice outside what I just asked 
for is also appreciated.

Thanks, Braden.

Here's a starting point, at least:
https://llvm.org/docs/DeveloperPolicy.html#adding-a-new-target

On Sat, Jan 23, 2021 at 10:44 AM Braden Nelson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi, all!
>
> I am currently working on a backend for the Parallax Propeller 2, and
> would like to know what the requirements are for a new backend to get
> mainlined into LLVM, and also get any general advice for writing a
> backend so that I don't have to go back and change a bunch of things
> later when I'm ready to submit a patch.
>
> I'm new to working with LLVM, so any advice outside what I just asked
> for is also appreciated.
>
> Thanks, Braden.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20210123/dc375c63/attachment.html>

HI Braden,

I’m also working pretty hard recently on upstreaming Motorola M68k backend to
LLVM [1], and the works are wrapping up so I might can give some advices

I think the first thing you need to do is showing to this community that it is
not a toy backend and many people (users) are relying on the target
architecture.

Regarding the technical and code reviewing part. In addition to the developer
policy page posted by David, I would like to highlight some points:
1.  Usually a new target will enter LLVM as an experimental target first (it
will not be build by default unless designated by the
`LLVM_EXPERIMENTAL_TARGETS_TO_BUILD` cmake variable) before becoming an official
one.

2. Split all changes into multiple patches, in which each of them is (more or
less) grouping similar functionalities. And most important of all, each patch
should be buildable by itself. You can take a look on how I split M68k’s
patches.

3. Following up (2), it would be great to commit all patches at once after all
of them are approved.

4. IIUC, your target architecture (Propeller 2) is a less known ISA. As a
reviewer, personally I would like to see comments in the files (usually .td
files) explaining some of the architectures details (e.g. addressing modes,
special behaviors, uncommon operations) that gives reviewers some gravities of
how this architecture looks like.

5. You probably have known — follow LLVM’s coding style and some programming
guidelines, like avoid using data structure ABC under XYZ scenarios. For the
former, just run clang-format-diff before submitting patches for review. The
latter is in this [2] page.

6. Help reviewing other backends’ patches as well. Since it’s most likely
patches in your target are reviewed by maintainers from other backends.

7. Last but not the least, assign a code owner. Code owner is obligated to
review patches in his/her “mission area”, if it has not been reviewed by others.
Also, targets that are inactively maintained will be removed from LLVM.


Hope these tips help :-)

Best,
-Min


[1] First patch in the series of 8: https://reviews.llvm.org/D88385
<https://reviews.llvm.org/D88385> . Click on “Stack” tab in the “Revision
Contents” section to see rest of the patches

[2] https://llvm.org/docs/CodingStandards.html
<https://llvm.org/docs/CodingStandards.html> Actually this page also
includes coding styles.
> On Jan 23, 2021, at 10:43 AM, Braden Nelson via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi, all!
> 
> I am currently working on a backend for the Parallax Propeller 2, and would
like to know what the requirements are for a new backend to get mainlined into
LLVM, and also get any general advice for writing a backend so that I don't
have to go back and change a bunch of things later when I'm ready to submit
a patch.
> 
> I'm new to working with LLVM, so any advice outside what I just asked
for is also appreciated.
> 
> Thanks, Braden.
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20210123/cf06be8f/attachment.html>