Toolchain: Add support for building the userland with the mold linker

This commit adds support for building the SerenityOS userland with the
new [mold linker].

This is not enabled by default yet; to link using mold, run the
`Toolchain/BuildMold.sh` script to build the latest release of mold, and
set the `ENABLE_MOLD_LINKER` CMake variable to ON. This option relies on
toolchain support that has been added just recently, so you might need
to rebuild your toolchain for mold to work.

[mold linker]: https://github.com/rui314/mold

CMakeLists.txt

@@ -267,6 +267,9 @@ endif()
 add_subdirectory(AK)
 add_subdirectory(Kernel)
 if(NOT "${SERENITY_ARCH}" STREQUAL "aarch64")
+    if (ENABLE_MOLD_LINKER)
+        add_link_options(-fuse-ld=mold)
+    endif()
     add_subdirectory(Userland)
     add_subdirectory(Tests)
 endif()

Documentation/AdvancedBuildInstructions.md

@@ -53,6 +53,7 @@ There are some optional features that can be enabled during compilation that are
 - `ENABLE_PCI_IDS_DOWNLOAD`: downloads the [`pci.ids` database](https://pci-ids.ucw.cz/) that contains information about PCI devices at build time, if not already present. Enabled by default.
 - `BUILD_LAGOM`: builds [Lagom](../Meta/Lagom/ReadMe.md), which makes various SerenityOS libraries and programs available on the host system.
 - `ENABLE_KERNEL_LTO`: builds the kernel with link-time optimization.
+- `ENABLE_MOLD_LINKER`: builds the userland with the [`mold` linker](https://github.com/rui314/mold). `mold` can be built by running `Toolchain/BuildMold.sh`.
 - `INCLUDE_WASM_SPEC_TESTS`: downloads and includes the WebAssembly spec testsuite tests. In order to use this option, you will need to install `prettier` and `wabt`. wabt version 1.0.23 or higher is required to pre-process the WebAssembly spec testsuite.
 - `SERENITY_TOOLCHAIN`: Specifies whether to use the established GNU toolchain, or the experimental Clang-based toolchain for building SerenityOS. See the [Clang-based toolchain](#clang-based-toolchain) section below.
 - `SERENITY_ARCH`: Specifies which architecture to build for. Currently supported options are `i686` and `x86_64`. `x86_64` requires a separate toolchain build from `i686`.

Meta/CMake/serenity_options.cmake

@@ -11,3 +11,4 @@ serenity_option(ENABLE_KERNEL_ADDRESS_SANITIZER OFF CACHE BOOL "Enable kernel ad
 serenity_option(ENABLE_KERNEL_COVERAGE_COLLECTION  OFF CACHE BOOL "Enable KCOV and kernel coverage instrumentation in gcc/clang")
 serenity_option(ENABLE_KERNEL_LTO OFF CACHE BOOL "Build the kernel with link-time optimization")
 serenity_option(ENABLE_EXTRA_KERNEL_DEBUG_SYMBOLS  OFF CACHE BOOL "Enable -Og and -ggdb3 options for Kernel code for easier debugging")
+serenity_option(ENABLE_MOLD_LINKER OFF CACHE BOOL "Link the SerenityOS userland with the mold linker")

Meta/CMake/utils.cmake

@@ -57,7 +57,9 @@ function(serenity_libc target_name fs_name)
         target_link_libraries(${target_name} clang_rt.builtins)
         # FIXME: Implement -static-libstdc++ in the next toolchain update.
         target_link_options(${target_name} PRIVATE -nostdlib++ -Wl,-Bstatic -lc++ -Wl,-Bdynamic)
-        target_link_options(${target_name} PRIVATE -Wl,--no-dependent-libraries)
+        if (NOT ENABLE_MOLD_LINKER)
+            target_link_options(${target_name} PRIVATE -Wl,--no-dependent-libraries)
+        endif()
     endif()
     target_link_directories(LibC PUBLIC ${CMAKE_CURRENT_BINARY_DIR})
     serenity_generated_sources(${target_name})

Toolchain/BuildClang.sh

@@ -294,6 +294,10 @@ pushd "$DIR/Build/clang"
     done
 popd
 
+pushd "$DIR/Local/clang/bin/"
+    buildstep "mold_symlink" ln -s ../../mold/bin/mold ld.mold
+popd
+
 # === SAVE TO CACHE ===
 
 pushd "$DIR"

Toolchain/BuildIt.sh

@@ -399,6 +399,10 @@ pushd "$DIR/Build/$ARCH"
 
 popd
 
+pushd "$DIR/Local/$ARCH/$ARCH-pc-serenity/bin"
+    buildstep "mold_symlink" ln -s ../../../mold/bin/mold ld.mold
+popd
+
 
 # == SAVE TO CACHE ==
 

Toolchain/BuildMold.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+
+# This script builds the mold linker that can optionally be used for linking
+# the SerenityOS userland.
+set -e
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+NPROC="nproc"
+SYSTEM_NAME="$(uname -s)"
+
+if [ "$SYSTEM_NAME" = "OpenBSD" ]; then
+    NPROC="sysctl -n hw.ncpuonline"
+elif [ "$SYSTEM_NAME" = "FreeBSD" ]; then
+    NPROC="sysctl -n hw.ncpu"
+elif [ "$SYSTEM_NAME" = "Darwin" ]; then
+    NPROC="sysctl -n hw.ncpu"
+fi
+
+[ -z "$MAKEJOBS" ] && MAKEJOBS=$($NPROC)
+
+mkdir -p "$DIR"/Tarballs
+pushd "$DIR"/Tarballs
+
+if [ "$1" = "--git" ]; then
+    [ ! -d mold ] && git clone https://github.com/rui314/mold.git
+
+    cd mold
+
+    git pull
+else
+    VERSION=1.0.2
+    [ ! -e mold-$VERSION.tar.gz ] && curl -L "https://github.com/rui314/mold/archive/refs/tags/v$VERSION.tar.gz" -o mold-$VERSION.tar.gz
+    [ ! -e mold-$VERSION ] && tar -xzf mold-$VERSION.tar.gz
+    cd mold-$VERSION
+fi
+
+make clean
+export DESTDIR="$DIR"/Local/mold
+export PREFIX=
+make -j "$MAKEJOBS"
+make install
+
+popd