[llvm-dev] Status of the AVR backend

[Dylan McKay via llvm-dev]

Hey all,

It’s been a while since the last update in November, so here’s another one
Changes since November Close to Rust support

A few people have been working on integrating the AVR backend into the Rust
<https://www.rust-lang.org/> programming language. This has mostly been
work on supporting LLVM 4.0 <https://github.com/rust-lang/rust/issues/37609>
.

Once the final LLVM 4.0 tag is made, we can update emscripten, integrate it
into Rust, and then add a small amount of code to the compiler to enable
support.

Take a look at Jake Goulding’s avr-rust/arduino
<https://github.com/avr-rust/arduino> project for a really nice proof of
concept.
Clang support

At this point, you can compile some decently-sized programs directly to ELF
object files. It is still necessary to use the GNU AVR linker because we
don’t have any support for AVR inside the compiler-rt support library.

On top of this, we need to add logic to Clang so that it passes the correct
C runtime library (crt0, ctc1, ..) to the linker.

In order to successfully compile the Arduino core library, we need to add
support for the ‘weak’ linkage attribute, and support the ‘progmem’
attribute for data that should reside in program flash memory.
Experimental buildbot

We’ve gained a buildbot here
<http://lab.llvm.org:8014/builders/llvm-avr-linux> on the staging
buildmaster. It does not send emails to anyone other than myself, but it
will help keep the test suite green!
Relax memory operation pass

A pass was added in D27650 <https://reviews.llvm.org/D27650> that allows us
to correctly emit loads/stores to addresses of the form ‘pointer+offset’
where the offset is larger than 63 bytes.

This would stop us from lowering functions which had more than 63 bytes of
data on the stack.
Next on the agenda LLVM linker support

A long time ago we used to have a fork of LLD which added support for AVR
ELF binaries.

The stale repository is here <https://github.com/avr-llvm/lld>.

Now that LLD is somewhat more stable, it would be really cool to see this
picked up again.
Links

   - A list of all AVR-related patches that have been comitted
   <https://reviews.llvm.org/search/query/ZVuiT4mCLrl6/#R>
   - All open bugs for the backend
   <https://llvm.org/bugs/buglist.cgi?product=libraries&component=Backend%3A%20AVR&resolution=---&list_id=111982>

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