[LLVMdev] request for tutorial

[Sean Silva]

(Sorry about the wall of text, it ended up as a brain dump of a bunch of
backend-related documentation that I know about/have bookmarked in the
past. Hopefully there's something useful in there.)

If you haven't stumbled across them already, these might be helpful:

http://llvm.org/devmtg/2009-10/Korobeynikov_BackendTutorial.pdf
http://jonathan2251.github.io/lbd/
http://eli.thegreenplace.net/2013/02/25/a-deeper-look-into-the-llvm-code-generator-part-1/
http://eli.thegreenplace.net/2012/11/24/life-of-an-instruction-in-llvm/

Another good source of documentation is simply to look at the commits that
introduce new backends, e.g. the initial commits for SystemZ and AArch64
come to mind. If you use git, something like this might be handy:

    git log -p -n1 $(git log --pretty='format:%H' lib/Target/AArch64/ |
tail -n1) >initial.patch

Generally the most consistent source of good documentation is the
devmeetings, so definitely browse what's available at <
http://llvm.org/devmtg/> (even quite old ones can still be really useful,
such as Anton's slides). Also, as I'm sure you've heard, a lot of that
backend stuff is slated for change (getting rid of SelectionDAG,
global-isel, separating the MI layer, etc.), although the process will
probably take a long time.

There's also a lot of backend-related knowledge encoded (trapped?) inside
of the TableGen backends, and AFAIK the only way to really puzzle out the
deep details there is to look at the source code of the TableGen backends
in utils/TableGen/ and correlating that with the generated .inc files (look
in your build dir) and where those .inc files are included inside
lib/Target/$ARCH/; see utils/TableGen/TableGenBackends.h for an overview of
what's there. There is some rough documentation about them in <
http://llvm.org/docs/WritingAnLLVMBackend.html> and <
http://llvm.org/docs/CodeGenerator.html>.

The actual TableGen "language" is documented fairly poorly but actually has
real dedicated documentation available (<
http://llvm.org/docs/TableGenFundamentals.html>, <
http://llvm.org/docs/TableGen/LangRef.html>). There's also tests/TableGen/
which might be useful. To learn about the in-code data structures that are
produced by TableGen and manipulated by the TableGen backends, you
basically need to look at include/llvm/TableGen/Record.h and lib/TableGen.
Beware: the TableGen code is not quite up to par with the rest of the
codebase. Also, don't assume that there's some coherent "underlying
structure" to the TableGen language; it's mostly a product of long periods
of neglect punctuated by quick hacks when a developer gets frustrated by
clunky existing ways to express something (or inability to express
something).

The only documentation (besids the code itself) we have about the MI layer
is the short section <
http://llvm.org/docs/CodeGenerator.html#machine-code-description-classes>.
You can get an idea of the scope of the MI layer from the plans laid out in
<http://thread.gmane.org/gmane.comp.compilers.llvm.devel/65434>. Other than
that you'll probably have to go digging in the source or ask on the mailing
lists.

There's a blog post about MC <
http://blog.llvm.org/2010/04/intro-to-llvm-mc-project.html> and an old dev
meeting talk by Daniel Dunba, along with a section here <
http://llvm.org/docs/CodeGenerator.html#the-mc-layer>r. There's also a
quite thorough tutorial specifically about implementing an integrated
assembler <
http://www.embecosm.com/appnotes/ean10/ean10-howto-llvmas-1.0.html>. Other
than that, I don't know very much material to recommend about MC.

If you gain any insight about a specific thing in the course of your
backend project, please feel free to share that knowledge by documenting <
http://llvm.org/docs/SphinxQuickstartTemplate.html>.

-- Sean Silva


On Mon, Sep 23, 2013 at 8:29 PM, Preston Briggs <preston.briggs at gmail.com>wrote:

> When I registered for dev conference, there was a field asking what I was
> particularly interested in learning. I didn't fill it out then , but it
> occurs to me now that I'd really enjoy a tutorial on how to develop a new
> back end.
>
> I spent some time recently reviewing existing material (documentation and
> code) and not making a lot of progress. Indeed, under some time pressure,
> I'm writing my own back end.
>
> A chance to work through the material with some guidance would be greatly
> appreciated.
>
> Thanks,
> Preston
>
>
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