Low Level Virtual Machine (LLVM)

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RISC-V LLVM Support Build Status

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.

Installation

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.

Use

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