[llvm-dev] [RFC] RISC-V backend

[Alex Bradbury via llvm-dev]

Hi all,

I am proposing the integration of a backend targeting the RISC-V ISA.

RISC-V is a free and open instruction set architecture that was originally
developed at UC Berkeley. Future development of the ISA specification will be
handled by the 501(c)(6) non-profit RISC-V Foundation and its members
<https://riscv.org/membership/?action=viewlistings>. You can find much more
information at the RISC-V website <https://riscv.org/>, including the current
ISA specification <https://riscv.org/specifications/>. You might note that
RISC-V defines 32-bit and 64-bit variants and also supports a compressed
variant, allowing 16-bit instructions to be freely intermingled with the
standard 32-bit representations. The standard is structured to allow
implementers to choose appropriate subsets to support, for instance a
micro-controller might support 'RV32I' (32-bit RISC-V with the integer
instructions) and an application core running Linux might implement RV64IMAFD
(commonly shortened to RV64G: 64-bit with integer instructions, the multiply
extension, atomics, and single and double precision floating point). A
generous portion of the opcode space is left reserved for implementers or
researchers to add their own instructions.

In line with the proposed policy for adding a new target
(https://reviews.llvm.org/D23162), RISC-V has a clear specification, multiple
software models, and multiple FPGA implementations as well as prototype ASICs
from various groups. At lowRISC (http://www.lowrisc.org/), inspired by our
previous experience with the Raspberry Pi project, we are working towards
creating a completely open source RISC-V SoC and producing low-cost
development boards around it. Feel free to contact me off-list to discuss
lowRISC further. LLVM is a key part of our development plan, and with
community approval I would like to act as maintainer for the backend. The vast
majority of my LLVM work over the past 6 years has sadly been out-of-tree, but
I'm far from new to the project.

In the RISC-V community right now, GCC is by some way the more stable compiler
port. We've discussed best way of moving forward with LLVM at the last couple
of RISC-V Workshops and a number of us concluded a fresh codebase may be the
best way to move forwards. Producing a series of patches that introduce RISC-V
support incrementally in easy-to-review chunks with associated test cases at
every point also allows us to get the maximum benefit from LLVM's code review
procedure. It also provides a good basis for more detailed documentation on
writing an LLVM backend (and making modifications to an existing one, e.g.
making it much easier for a research group wanting to explore RISC-V changes).
This is an area I also hope to contribute to. The approach of small,
incremental patches is somewhat similar to what is being done with the AVR
backend. I'm grateful to David Chisnall who suggested that starting with the
MC layer may be a productive way to go about developing this backend, and so
far this seems to be working well.

The current status is that I have submitted a series of 10 patches
implementing assembler support and an initial set of relocations and fixups.
Help reviewing these would be very welcome, do let me know if you'd like to be
CCed in or added as a reviewer to future patches. I'd ultimately like the
RISC-V backend to be considered a "reference" backend, and as such
I specifically welcome reviews you might worry are pedantic.

Please find the current set of patches for your review here:
* <https://reviews.llvm.org/differential/?authors=asb>

I've obviously spent a lot of time with the MC layer recently, and I'd be
happy to put that to use in helping review MC patches for other archs.

Mini development roadmap:
* Complete MC layer (supporting up to RV32+RV64G at least)
  * There is currently no specification for supported RISC-V assembly syntax,
  mnemonics etc. The ideal solution may not always be "whatever the GCC port
  currently does", so some aspect of this will involve discussions with the
  wider RISC-V software community.
* Codegen
* Compressed instruction set support (RVC)
* Benchmarking and comparison to GCC RISC-V (and potentially other archs)

Finally I'd like to give a prominent mention to Colin Schmidt, the UC Berkeley
student who has been maintaining the current out-of-tree RISC-V LLVM port
<https://github.com/riscv/riscv-llvm>. The RISC-V community owes him a debt of
gratitude.

All comments very welcome,

Alex