Coding Illness: December 2021

Update(s):

- Jan 16th 2022 adding section on Objdump

RISC-V Vector Extension (RVV) has recently been ratified in its version 1.0 (announcement and specification pdf). The 1.0 milestone is key, it means RVV maturity has reached a stable state: numerous commercial and free implementation of the standard are appearing and software developers can now dedicate significant effort to port and develop library on top of RVV without fear of seeing the specification rug being pulled under their feet. In this article we will review how to build a version of the clang compiler compatible with RVV (v0.10) and to develop, build and execute our first RVV program.

Building the compiler

Before building a compiler for RVV with need a basic riscv toolchain. This toolchain will provide the standard library and some basic tools require to build a functioning binary. The toolchain will be installed under ~/RISCV/ (feel free to adapt this directory to your setup).

# update to the your intended install directory
export RISCV=~/RISCV-TOOLS/

# downloading basic riscv gnu toolchain, providing:
# - runtime environement for riscv64-unknown-elf (libc, ...)
# - spike simulator
git clone https://github.com/riscv-collab/riscv-gnu-toolchain
./configure --prefix=$RISCV
make -j3

Compiling for RVV requires a recent version of clang (this was tested with clang 14).

# downloading llvm-project source from github
git clone https://github.com/llvm/llvm-project.git
cd llvm-project
# configuring build to build llvm and clang in Release mode
# using ninja
# and to use gold as the linker (less RAM required)
# limiting targets to RISCV, and using riscv-gnu-toolchian
# as basis for sysroot
cmake -G Ninja -DLLVM_ENABLE_PROJECTS="clang;lld;" \
      -DCMAKE_BUILD_TYPE=Release \
      -DDEFAULT_SYSROOT="$RISCV/riscv64-unknown-elf/" \
      -DGCC_INSTALL_PREFIX="$RISCV" \
      -S llvm -B build-riscv/ -DLLVM_TARGETS_TO_BUILD="RISCV"
# building clang/llvm using 4 jobs (can be tuned to your machine)
cmake --build build/ -j4

Building clang/llvm require a large amount of RAM (8GB seems to be the bare minimum, 16GB is best) and will consume a lot of disk space. Those requirements can be reduced by selecting Release build type (rather than the default Debug) and by using gold linker.

This process will generate clang binary in llvm-project/build/bin/clang .

More information on how to download and build clang/llvm can be found on the project github page.
Development.

The easiest way to develop software directly is for RVV is to rely on the rvv intrinsics. This project offers intrinsics for most of the instruction of the extension. The documentation is accessible on github and support is appearing in standard compilers (most notably clang/llvm).

As a first exercise, we will use the SAXPY example from rvv-intrinsic-doc rvv_saxpy.c.

Building the simulator

Let's first build an up-to-date proxy kernel pk:

# downloading and install proxy-kernel for riscv64-unknown-elf
git clone https://github.com/riscv-software-src/riscv-pk.git
cd riscv-v
mkdir build && cd build
make -j4 && make install
../configure --prefix=$RISCV --host=riscv64-unknown-elf

Let's now build the simulator (directly from the top of the master branch, why not !).

git clone https://github.com/riscv-software-src/riscv-isa-sim
cd riscv-isa-sim
mkdir build && cd build
../configure --prefix=$RISCV
make -j4 && make install

Building the program

RVV is supported as part of the experimental extensions of clang. Thus it must be enabled explicitly when executing clang, and it must be associated with a version number, the current master of clang only support v0.10 of the RVV specification.

clang -L $RISCV/riscv64-unknown-elf/lib/ --gcc-toolchain=$RISCV/ \

       rvv_saxpy.c -menable-experimental-extensions -march=rv64gcv0p10 \
      -target riscv64 -O3 -mllvm --riscv-v-vector-bits-min=256 \

       -o test-riscv-clang

Executing

To execute the program we are going to use the spike simulator and the riscv-pk proxy kernel.

Spike is part of the riscv-gnu-toolchain available at https://github.com/riscv-collab/riscv-gnu-toolchain , riscvv-pk is also available on github. https://github.com/riscv-software-src/riscv-pk

the binary image of pk must be the first unnamed argument to spike before the main elf.

$RISCV/bin/spike --isa rv64gcv $RISCV/riscv64-unknown-elf/bin/pk \

                  test-riscv-clang

NOTES: I tried to use riscv-tools (https://github.com/riscv-software-src/riscv-tools) does not seem actively maintain and several issue poped up when I tried building it.

Objdump

Not all objdump support RISC-V vector extension. If you have built llvm has indicated above, you should be able to use the llvm-objdump program built within to disassemble a program with vector instructions.

llvm-objdump -d --mattr=+experimental-v <binary_file>

References

IREE (MLIR dialect) page of riscv-v cross compilation https://google.github.io/iree/building-from-source/riscv/
Official llvm github project https://github.com/llvm/llvm-project
RVV intrinsic documentation on github https://github.com/riscv-non-isa/rvv-intrinsic-doc
https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/GiTkPw-9r8A?pli=1
Properly configuring clang/llvm build https://stackoverflow.com/questions/68580399/using-clang-to-compile-for-risc-v

Coding Illness

Programming with RISC-V Vector extension: how to build and execute a basic RVV test program (emulation/simulation)