[llvm-dev] [RFC] Promoting the RISC-V backend from 'experimental' to 'official' for 9.0

[Alex Bradbury via llvm-dev]

The 9.0 release is currently scheduled to branch in 10 days time, on
the 18th of July, with a final release expected on 28th August. I
would like to propose promoting the RISC-V backend from its current
"experimental" status to "official" prior to this branch. This means
that the RISC-V backend will be built by default and support included
in standard binary distributions of LLVM/Clang.

# Status

RISC-V is a modular ISA with 32- and 64-bit variants, as well as a range of
options specified in "ISA naming strings". The RISC-V backend supports
both variants and all standard extensions. RV32IMAFDC, RV64IMAFDC, and
the ilp32, ilp32f, ilp32d, lp64, lp64f, lp64d ABIs (the clang
hard-float ABI patch will land imminently
<https://reviews.llvm.org/D60456>).

We have a fairly comprehensive out set of in-tree unit tests, multiple
groups have indicated they are using Clang/LLVM for their RISC-V
embedded firmware builds and more recently we have been pushing
forwards on issues related to building Linux/FreeBSD applications. The
GCC torture suite has a 100% pass rate, we're seeing a 98% pass rate
on the LLVM test-suite (failures are almost all related to C++
exception handling, which we hope to resolve soon), and we've been
able to get over 90% of buildroot's over 20000 packages to build for
RISC-V using clang, where most failures are due to build system issues
or GCCisms. We can compile and run meaningful programs (e.g. build a
rootfs with nginx, serve HTTP requests).

Additionally, I understand that LLD support is now roughly feature-complete
with the exception of support for linker relaxation. Fangrui Song has been
most active on RISC-V LLD recently.

There is initial Rust support for bare metal rv32 and rv64, with support for
hard float Linux targets due to start soon.

I believe that we are ready to flip the switch towards an 'official' target.
At lowRISC, we're ready to address any issues that arise, and as noted below
we're delighted that there's a growing community of contributors around this
backend who are equally invested in its success.

## Thanks

I started work on the RISC-V LLVM backend around the end of 2016
through lowRISC, a not-for-profit open source hardware/software
engineering company I co-founded. I'd like to thank everyone who gave
encouragement, helped with funding in order to support this work, or
submitted reviews or patches. As well as growing the toolchain team at
lowRISC (Luís Marques, Sam Elliott, and myself), we've been able to
help grow a community of contributors around this work. There are far
too many names to mention, but engineers from organisations such as
Qualcomm, AndesTech, Embecosm, Google, and the University of Cambridge
have all made notable contributions.

## End

Although becoming an official backend would be a huge milestone, of course it's
far from the end of the road. We'll be continuing to work on code size and
generated code performance improvements, better testing, working with language
communities like Rust/Swift/Julia/..., support for additional LLVM features
and RISC-V instruction set extensions, etc.

Any questions / concerns / feedback?

--
Alex Bradbury,
CTO and Co-Founder, lowRISC CIC