Compile with LLVM

Compile Kernel with WLLVM

make CC=wllvm HOSTCC=wllvm defconfig
make CC=wllvm HOSTCC=wllvm -j$(nproc)

extract-bc vmlinux

Get vmlinux.bc bitcode file.

(Optional) Cannot Handle Assembly Instructions

Using lower version LLVM may cause compilation error about assembly code. Add LLVM_IAS=0 to force LLVM to skip these assembly instructions.

make CC=wllvm HOSTCC=wllvm LLVM_IAS=0 defconfig
make CC=wllvm HOSTCC=wllvm LLVM_IAS=0 -j192

Generate Unoptimized Kernel Bitcode

Certain programs cannot be compiled with customized optimization. This script is to re-compile the given programs and emit unoptimized bitcode.

#!/bin/bash


llvm_linker=llvm-link

function get_cmd
{
    path="$2"

    # change output file name
    awk_cmd='{ 
    s = ""; 
    ofile = 0;
    for (i = 3; i <= NF; i++)
    {
      if (ofile == 1)
      {
        s = s "'"$path"'" " ";
        ofile = 0;
      }
      else
      {
        if ($i == "-o")
           ofile = 1;
        s = s $i " ";
      }
    }
    print s }'
    cmd_line=`head -n 1 $1 | awk "$awk_cmd"`

    # output LLVM IR
    cmd_line="${cmd_line} -emit-llvm"
    # disable opt
    cmd_line="${cmd_line} -mllvm -disable-llvm-optzns"
    echo $cmd_line
    return
}

function compile_opt
{
    cmd_path="$1"
    basepath="$2"
    tmp_path="${basepath}.tmp.bc"
    output_path="${basepath}.opt.bc"
    cmd=`get_cmd "$cmd_path" "$tmp_path"`
    eval "$cmd" 2> /dev/null
    cmd_result=$?
    if [ ! -e "$tmp_path" ] || [ $cmd_result -ne 0 ]
    then
        exit 1
    fi
    # for legacy pass manager
    opt -always-inline -inline -mem2reg -simplifycfg -instcombine "$tmp_path" -o "$output_path"

    # for new pass manager
    opt -passes=always-inline,inline,mem2reg,simplifycfg,instcombine "$tmp_path" -o "$output_path"
    cmd_result=$?
    if [ ! -e "$output_path" ] || [ $cmd_result -ne 0 ]
    then
        exit 1
    fi
}

while getopts ":l:" opt; do
    case $opt in
        l) llvm_linker=$OPTARG
           ;;
        \?) echo "Usage: -l <llvm-link>" 1>&2
            exit 1
            ;;
    esac
done
shift $((OPTIND-1))
from_file=$1
if [ -z "$from_file" ]
then
    echo "no input file" 1>&2
    exit 1
fi

extract-bc -l false -m $from_file
manifest="${from_file}.llvm.manifest"
if [ ! -r "$manifest" ]
then
    echo "no manifest file" 1>&2
    exit 1
fi

tmpmanifest="${from_file}.tmp.manifest"
echo -n "" > "$tmpmanifest"

cat "$manifest" |
    while read o_bc_name;
    do
        basename=`basename "${o_bc_name%.o.bc}"`
        # basename="${basename#.}"
        dir="${o_bc_name%/*}"
        basepath="${dir}/${basename}"
        if [ -z "$dir" ] && [ -z "$basename" ]
        then
            continue
        fi

        cmd_path="${basepath}.o.cmd"
        if [ ! -r "${cmd_path}" ]
        then
            echo "${cmd_path} not found"
            continue
        fi

        while [ `jobs | wc -l` -ge 8 ]
        do
            sleep 0.1
        done
        output_path="${basepath}.opt.bc"
        compile_opt "$cmd_path" "$basepath" &
        echo "$output_path"
        echo "$output_path" >> "${tmpmanifest}"
    done

while [ `jobs | wc -l` -ne 0 ]
do
    sleep 1
done

outputfile="${from_file}.opt.bc"
$llvm_linker -o "$outputfile" `cat "${tmpmanifest}"`

Take linux kernel as an example:

make CC=wllvm HOSTCC=wllvm menuconfig
make CC=wllvm HOSTCC=wllvm -j192
./unopt.sh vmlinux

Generate TBAA Metadata

Replace fno-strict-aliasing to fstrict-aliasing in top-level Makefile.

- KBUILD_CFLAGS += -fno-strict-aliasing
+ KBUILD_CFLAGS += -fstrict-aliasing

Then go again all the process to generate TBAA metadata.

Reference

Compile Linux Kernel to LLVM Bitcode

Generate Linux Kernel LLVM Bitcode with Custom Optimization