install_llvm

#!/usr/bin/env bash
# Usage: bash install_llvm
# N.B.: VERSION_NUMBER is used only on Linux. On other platforms, the latest version will be installed.
# The script will determine the version number based on the current date and the LLVM release
# schedule, which is updated every six months as of 2026, in March and September. The version number
# in September 2025 is 21.

set -euo pipefail

echo OSTYPE: "$OSTYPE"

# N.B.: Do not quote darwin* etc., or the pattern will be interpreted as a literal string without
# expanding the wildcard *. See https://www.gnu.org/software/bash/manual/bash.html#Double-Quotes-1 :
# Enclosing characters in double quotes (‘"’) preserves the literal value of all characters within
# the quotes, with the exception of ‘$’, ‘`’, ‘\’, and, when history expansion is enabled, ‘!’.
if [[ "$OSTYPE" == darwin* ]] ; then
    echo "We are on macOS."
    brew update || true
    # clang is included in llvm, but flang and lld are separate packages.
    brew install llvm flang lld  # The latest version will be installed, which is 21 in September 2025.
elif [[ "$OSTYPE" == cygwin* || "$OSTYPE" == win* || "$OSTYPE" == msys* ]]; then
    echo "We are on Windows."
    choco upgrade chocolatey -y --no-progress || true
    # flang, clang, and lld are supposed to be included in llvm.
    choco install llvm -y --no-progress
    choco upgrade llvm -y --no-progress  # Update to the latest version
else  # Assume Ubuntu

    echo "We assume that we are on Ubuntu."

    # Install prerequisites.
    sudo apt update || true && sudo apt install -y curl gnupg

    # According to "LLVM Release Schedule" at https://llvm.org, we assume that the version is
    # updated every six months, in March and September. The version number in September 2025 is 21.
    CURRENT_YEAR=$(date +%Y)
    CURRENT_MONTH=$(date +%m)
    CURRENT_MONTH=${CURRENT_MONTH#0} # Remove leading zero if any
    echo "Current year: ${CURRENT_YEAR}, current month: ${CURRENT_MONTH}"
    if [[ ${CURRENT_MONTH} -ge 9 ]]; then
        VERSION_NUMBER=$((21 + (CURRENT_YEAR - 2025) * 2))
    elif [[ ${CURRENT_MONTH} -ge 3 ]]; then
        VERSION_NUMBER=$((21 + (CURRENT_YEAR - 2025) * 2 - 1))
    else
        VERSION_NUMBER=$((21 + (CURRENT_YEAR - 2026) * 2 ))
    fi
    echo "Using the version number ${VERSION_NUMBER} based on the current date."

    # Add the LLVM repository and the GPG key.
    # See https://apt.llvm.org/
    UBUNTU_CODENAME=$( (lsb_release -sc 2>/dev/null || (. /etc/os-release && echo "$VERSION_CODENAME")) )  # e.g., "noble" for Ubuntu 24.04
    curl -fsSL https://apt.llvm.org/llvm-snapshot.gpg.key \
        | gpg --dearmor --yes \
        | sudo tee /usr/share/keyrings/apt.llvm.org.gpg > /dev/null
    echo "deb [signed-by=/usr/share/keyrings/apt.llvm.org.gpg] https://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-${VERSION_NUMBER} main" \
        | sudo tee /etc/apt/sources.list.d/llvm-toolchain-${VERSION_NUMBER}.list

    # Install the packages (clang is needed even if we use only flang).
    sudo apt update
    sudo apt install -y llvm-${VERSION_NUMBER} flang-${VERSION_NUMBER} clang-${VERSION_NUMBER}  lld-${VERSION_NUMBER}

    # Verify the installation
    for TOOL in flang clang lld; do
        type "${TOOL}-${VERSION_NUMBER}" > /dev/null 2>&1 || \
            { echo "${TOOL}-${VERSION_NUMBER} not found after the installation, which probably failed." >&2 ; exit 1; }
    done

    # Set up symlinks pointing to tool-${VERSION_NUMBER}
    for TOOL in flang clang lld; do
        BIN_DIR="/usr/local/bin"  # This usually comes before /usr/bin in PATH
        if [[ -f "${BIN_DIR}/${TOOL}" || -L "${BIN_DIR}/${TOOL}" ]]; then
            echo "${TOOL} already exists in ${BIN_DIR}:"
            ls -al "${BIN_DIR}/${TOOL}"
            echo "Backing ${TOOL} to ${TOOL}.bak"
            sudo mv "${BIN_DIR}/${TOOL}" "${BIN_DIR}/${TOOL}.bak"
        fi
        echo "Creating a symbolic link named ${TOOL} pointing to ${TOOL}-${VERSION_NUMBER} in ${BIN_DIR}."
        sudo ln -sf "$(command -v ${TOOL}-${VERSION_NUMBER})" "${BIN_DIR}/${TOOL}"
    done

fi

# Verify installation
for TOOL in flang clang lld; do
    type $TOOL > /dev/null 2>&1 || \
        { echo "$TOOL not found after the installation, which probably failed." >&2 ; exit 1; }
done

# Final messages
echo "LLVM installation completed."
for TOOL in flang clang lld; do
    echo -e "\nThe path to $TOOL is:\n"
    command -v "$TOOL"
    echo -e "\nThe $TOOL version is:\n"
    "$TOOL" --version || true
done