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On 7/13/2020 21:30, Thomas Lively via llvm-dev wrote:
\> Part of the problem is that ISel patterns are like their own DSL inside
\> the TableGen DSL, so keywords like "ins", "outs", and "ops" aren't
\> keywords at the TableGen level, but rather at the level of the ISel
\> system implemented with TableGen. Copying existing patterns and reading
\> the comments in Target.td and TargetSelectionDAG.td are the best ways I
\> know of learning how this works. I haven't seen a separate guide,
\> although it would be very cool if one existed.

FWIW, there are also some third-party resources that may be of help:

"Lessons in TableGen"
FOSDEM 2019; Nicolai Hähnle
https://fosdem.org/2019/schedule/event/llvm\_tablegen/
Slides:
https://archive.fosdem.org/2019/schedule/event/llvm\_tablegen/attachments/slides/3304/export/events/attachments/llvm\_tablegen/slides/3304/tablegen.pdf

Series:
\- What has TableGen ever done for us?:
http://nhaehnle.blogspot.com/2018/02/tablegen-1-what-has-tablegen-ever-done.html
\- Functional Programming:
http://nhaehnle.blogspot.com/2018/02/tablegen-2-functional-programming.html
\- Bits: http://nhaehnle.blogspot.com/2018/02/tablegen-3-bits.html
\- Resolving variables:
http://nhaehnle.blogspot.com/2018/03/tablegen-4-resolving-variables.html
\- DAGs: http://nhaehnle.blogspot.com/2018/03/tablegen-5-dags.html

Some of the parts of TableGen used in SelectionDAG are in the backend
docs (e.g., the keywords OP asked about):
https://llvm.org/docs/WritingAnLLVMBackend.html#instruction-set
& https://llvm.org/docs/WritingAnLLVMBackend.html#instruction-selector
(has a simple example of \`PatFrag\` for \`store\`).

There are a few examples of simple .td files an LLVM backend in the
following:

LLVM backend development by example (RISC-V)
2018 LLVM Developers’ Meeting; Alex Bradbury
https://www.youtube.com/watch?v=AFaIP-dF-RA

2014 - Building an LLVM Backend - LLVM Developer's Meeting
https://llvm.org/devmtg/2014-10/#tutorial1
https://llvm.org/devmtg/2014-10/Slides/Cormack-BuildingAnLLVMBackend.pdf
https://llvm.org/devmtg/2014-04/PDFs/Talks/Building%20an%20LLVM%20backend.pdf
http://web.archive.org/http://llvm.org/devmtg/2014-10/Videos/Building%20an%20LLVM%20backend-720.mov
http://llvm.org/devmtg/2014-10/#tutorial1
http://www.inf.ed.ac.uk/teaching/courses/ct/other/LLVMBackend-2015-03-26\_v2.pdf

llvm-leg: LEG Example Backend
LEG Example Backend: a simple example LLVM backend for an ARM-like
architecture: 'LEG'.
https://github.com/frasercrmck/llvm-leg

Best,
Matt