Initial support for rustc_codegen_gcc

rustc can use different backend and this new script builds all the needed parts
to have GCC as the code generator.

It needs (at the moment) a patched version of GCC and builds it with libjit support.
Some glue code (in Rust) is added in the mix to finally be able to use the rustc
compiler with this dynamic backend.

Fixes compiler-explorer/compiler-explorer#2683

Author: dkm

Dockerfile

@@ -11,6 +11,7 @@ RUN apt update -y -q && apt upgrade -y -q && apt update -y -q && \
     gcc \
     gcc-multilib \
     git \
+    flex \
     libc6-dev-i386 \
     libc6-dev:i386 \
     linux-libc-dev \

build/build-rustc-cg-gcc.sh

@@ -0,0 +1,190 @@
+#!/bin/bash
+
+## $1 : version, currently rustc_cg_gcc does not have any and only uses master branch.
+## $2 : destination: a directory or S3 path (eg. s3://...)
+
+set -ex
+
+ROOT=$PWD
+VERSION=$1
+
+if [[ "${VERSION}" != "master" ]]; then
+    echo "Only support building master"
+    exit 1
+fi
+
+FULLNAME=rustc-cg-gcc-${VERSION}-$(date +%Y%m%d)
+
+OUTPUT=${ROOT}/${FULLNAME}.tar.xz
+S3OUTPUT=
+if [[ $2 =~ ^s3:// ]]; then
+    S3OUTPUT=$2
+else
+    if [[ -d "${2}" ]]; then
+        OUTPUT=$2/${FULLNAME}.tar.xz
+    else
+        OUTPUT=${2-$OUTPUT}
+    fi
+fi
+
+## From now, no unset variable
+set -u
+
+OUTPUT=$(realpath "${OUTPUT}")
+
+rm -rf  build-rustc-cg-gcc
+mkdir -p build-rustc-cg-gcc
+
+pushd build-rustc-cg-gcc
+PREFIX=$(pwd)/gcc-install
+
+export CARGO_HOME=$PWD/rustup
+export RUSTUP_HOME=$PWD/rustup
+
+export PATH=$RUSTUP_HOME/bin:$PATH
+
+## Download rustc_cg_gcc
+git clone --depth 1 https://github.com/antoyo/rustc_codegen_gcc.git
+
+## Download rustup and install it in a local dir
+## Installs :
+## - minimal profile
+## - the required build-deps from RUSTUP_COMP_BUILD_DEPS (-c *)
+## - version taken from rust-toolchain file
+
+RUSTUP_COMP_BUILD_DEPS=(rust-src rustc-dev llvm-tools-preview)
+
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- --profile minimal \
+                                                               -c "$(printf '%s\n' "$(IFS=,; printf '%s' "${RUSTUP_COMP_BUILD_DEPS[*]}")")" \
+                                                               --no-modify-path \
+                                                               -y \
+                                                               --default-toolchain  "$(cat rustc_codegen_gcc/rust-toolchain)"
+
+source  "$PWD/rustup/env"
+
+##
+## Prepare rustc_cg_gcc
+##
+pushd rustc_codegen_gcc
+
+# where the libgccjit.so will be installed
+echo "$PREFIX/lib"  > gcc_path
+
+./build_sysroot/prepare_sysroot_src.sh
+popd
+
+##
+## Build customized GCC with libgccjit
+##
+
+git clone --depth 1 https://github.com/antoyo/gcc.git
+
+GCC_REVISION=$(cd gcc; git ls-remote --heads "${URL}" "refs/heads/${BRANCH}" | cut -f 1)
+
+# clean
+rm -rf gcc-build  gcc-install
+mkdir -p gcc-build gcc-install
+
+echo "Downloading prerequisites"
+pushd gcc
+./contrib/download_prerequisites
+popd
+
+pushd gcc-build
+LANGUAGES=jit,c++
+PKGVERSION="Compiler-Explorer-Build-libgccjit-${GCC_REVISION}"
+
+CONFIG+=" --enable-checking=release"
+CONFIG+=" --enable-host-shared"
+CONFIG+=" --build=x86_64-linux-gnu"
+CONFIG+=" --host=x86_64-linux-gnu"
+CONFIG+=" --target=x86_64-linux-gnu"
+CONFIG+=" --disable-bootstrap"
+CONFIG+=" --enable-multiarch"
+CONFIG+=" --with-abi=m64"
+CONFIG+=" --with-multilib-list=m32,m64,mx32"
+CONFIG+=" --enable-multilib"
+CONFIG+=" --enable-clocale=gnu"
+CONFIG+=" --enable-languages=${LANGUAGES}"
+CONFIG+=" --enable-ld=yes"
+CONFIG+=" --enable-gold=yes"
+CONFIG+=" --enable-libstdcxx-debug"
+CONFIG+=" --enable-libstdcxx-time=yes"
+CONFIG+=" --enable-linker-build-id"
+CONFIG+=" --enable-lto"
+CONFIG+=" --enable-plugins"
+CONFIG+=" --enable-threads=posix"
+CONFIG+=" --with-pkgversion=\"${PKGVERSION}\""
+
+## this is needed for all libs (gmp, mpfr, …) to be PIC so as to not cause issue
+## when they are statically linked in the libgccjit.so
+CONFIG+=" --with-pic"
+
+../gcc/configure --prefix="${PREFIX}" ${CONFIG}
+
+make -j"$(nproc)"
+make -j"$(nproc)" install-strip
+popd
+
+##
+## Back to rustc_cg_gcc for building
+##
+pushd rustc_codegen_gcc
+./build.sh --release
+popd
+
+##
+## Everything should be correctly build
+##
+
+##
+## Before packaging, remove build deps
+##
+
+for c in "${RUSTUP_COMP_BUILD_DEPS[@]}"
+do
+    rustup component remove "$c"
+done
+
+##
+## Package everything together:
+## - the rustc toolchain from rustup
+## - libgccjit
+## - librustc_codegen_gcc
+## - sysroot
+
+mkdir -p toolroot
+
+# rustc toolchain
+mv rustup/toolchains/*/* toolroot/
+
+# libgccjit
+mv ./gcc-install/lib/libgccjit.so* toolroot/lib
+
+# cg_gcc backend
+mv ./rustc_codegen_gcc/target/release/librustc_codegen_gcc.so toolroot/lib
+
+# sysroot
+mv ./rustc_codegen_gcc/build_sysroot/sysroot toolroot/
+
+##
+## Simple sanity checks:
+## - check for assembly output
+## - check for correct exec output
+
+echo "fn main() -> Result<(), &'static str> { Ok(()) }" > /tmp/test.rs
+./toolroot/bin/rustc -Cpanic=abort -Zcodegen-backend=librustc_codegen_gcc.so  --sysroot toolroot/sysroot --emit asm -o test.s  /tmp/test.rs
+test test.s
+
+./toolroot/bin/rustc -Cpanic=abort -Zcodegen-backend=librustc_codegen_gcc.so --sysroot toolroot/sysroot /tmp/test.rs
+./test
+
+# Don't try to compress the binaries as they don't like it
+pushd toolroot
+
+export XZ_DEFAULTS="-T 0"
+tar Jcf "${OUTPUT}" --transform "s,^./,./${FULLNAME}/," ./
+
+if [[ -n "${S3OUTPUT}" ]]; then
+    s3cmd put --rr "${OUTPUT}" "${S3OUTPUT}"
+fi