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Author : Colin Schmidt (colins@eecs.berkeley.edu)
Date : February 24, 2014
Version : (under version control)

This repository contains a new target for LLVM RISC-V. It supports the latest version of the ISA 2.0. This backend currently only supports assembly generation and riscv64-unknown-*-gcc must be used to assemble and link the executable. The RISCV branch is based on LLVM 3.3 and, the riscv-trunk branch is following upstream LLVM master.

The backend is structured similarly to most other LLVM backends and tries to use the tablegen format as much as possible. The description of the instructions are found in RISCVInstFormats.td, and RISCVInstrInfo*.td. The registers are described in RISCVRegisterInfo.td and the calling convention is described in RISCVCallingConv.td.

The instructions are defined using the LLVM IR DAG format, and simple instructions that use pre-existing LLVM IR operations should be very easy to add. The instructions are divided into separate files based on their extension, e.g. atomic operations are defined in RISCVInstInfoA.td. Instructions implemented with these patterns are simply matched against the programs LLVM IR DAG for selection. More complicated instructions can use C++ to perform custom lowering of the LLVM IR in RISCVISelLowering.cpp. Combining of multiple LLVM IR nodes into single target instructions is also possible using C++ in the same file. In general RISCVISelLowering.cpp sets up the lowering based on the ISA and the specific subtargets features.

This backend does not include all features of a backend but is focused on generating assembly in an extensible way such that adding new ISA extensions and using them should be relatively painless. As the RISC-V support develops the backend may provide more features.

The compiler is fairly robust with similar performance to riscv64-unknown-*-gcc, so it use in any and all projects is encouraged.

Feedback and suggestions are welcome.

The LLVM RISCV backend is built just as the normal LLVM system.

$ git clone -b RISCV https://github.com/riscv/riscv-llvm.git
$ git submodule update --init
$ mkdir build
$ cd build

$ cmake -DCMAKE_INSTALL_PREFIX=/opt/riscv -DLLVM_TARGETS_TO_BUILD="RISCV" ../ $ make $ make install

Now if /opt/riscv is on your path you should be able to use clang and LLVM with RISC-V support.

Using the llvm-riscv is fairly simple to build a full executable however you need riscv64-unknown-*-gcc to do the assembling and linking. An example of compiling hello world:

$ cat hello.c
#include <stdio.h>
int main() {
    printf("Hello World!\n");
}
$ clang -target riscv64 -mriscv=RV64IAMFD -S hello.c -o hello.S
$ riscv64-unknown-elf-gcc -o hello.riscv hello.S