LLVM Upgrade Process

This page documents how to update the Zig source code to a new LLVM version.

master branch is always the latest LLVM release. The llvmX branch is kept up to date with the next LLVM release. The day that LLVM is released, we merge the llvmX branch into master, and start the next llvmX branch, following this upgrade process.

Before the LLVM Release

Start the llvmX branch.

Update the version numbers in:

• cmake/Findllvm.cmake
• cmake/Findclang.cmake
• cmake/Findlld.cmake

Build LLVM, Clang, and LLD in Release mode with assertions on.

Update src/zig_clang_*.cpp to the new versions. Before updating, replace each file with the original file from the old LLVM version and do a git diff to see what patches have been applied by Zig. Then update each file to the versions from the new LLVM, and re-apply the patches as necessary.

cp $LLVM_SRC/clang/tools/driver/driver.cpp     src/zig_clang_driver.cpp
cp $LLVM_SRC/clang/tools/driver/cc1_main.cpp   src/zig_clang_cc1_main.cpp
cp $LLVM_SRC/clang/tools/driver/cc1as_main.cpp src/zig_clang_cc1as_main.cpp
cp $LLVM_SRC/llvm/tools/llvm-ar/llvm-ar.cpp    src/zig_llvm-ar.cpp

Be sure not to accidentally regress #3292 when updating zig_clang_driver.cpp.

Update lib/include/ to the latest clang_install_prefix/lib/clang/$VER/include/.

rm -rf lib/include
cp -r $LLVM_INSTALL_PREFIX/lib/clang/$VER/include/ lib/

Update lib/libcxx/, lib/libcxxabi, and lib/libunwind to the latest llvm-project/ respective directory. Only the include/ directory, src/ directory, and LICENSE.txt are copied. CMake, README, shell scripts files are not copied. Update src/libunwind.zig and src/libcxx.zig to have a correct list of files.

• libunwind: gcc_personality_v0.c is not provided by libunwind and should remain.
• libcxx: additionally cp $LLVM_SRC/libcxx/vendor/llvm/default_assertion_handler.in lib/libcxx/include/__assertion_handler.
• libcxx: retain the zig patch

Update tsan to the latest llvm-project/compiler-rt/tsan directory. Only the necessary files are copied. Update src/libtsan.zig to have a correct list of files.

Run tools/update_cpu_features.zig with the new LLVM version. Unless you have a beefy machine, you'll probably want --single-threaded to limit the CPU and memory resource usage of this script. After running the tool, inspect the git diff. Think carefully about deletions, since this could represent breaking changes. Modifications to the script might be needed in order to clean up the output.

Update the CI scripts to the new version.

Update the static asserts at the bottom of src/zig_llvm.cpp

Run tools/update_clang_options.zig and use it to update the file src-self-hosted/clang_options_data.zig. Note that it uses the standard library of the zig compiler used to compile the tool to detect -m arguments correctly, which means you must use the new compiler with updated CPU target features rather than a master branch build.

Build zig in debug mode with asserts-enabled llvm, clang, lld, and run the full Zig test suite.

After the LLVM Release

• Merge the llvmX branch into master.
• Update How to build LLVM, libclang, and liblld from source.
• Update Building Zig on Windows.

llvm-reduce Template

In case you need to debug a regression in LLVM, here are some steps.

First obtain the crash.ll file using --verbose-llvm-ir. Verify that it crashes with clang:

$ $HOME/local/llvm18-assert/bin/clang -O3 -c crash.ll
zig2: /home/andy/dev/llvm-project-18/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:991: void {anonymous}::SelectionDAGLegalize::LegalizeOp(llvm::SDNode*): Assertion `(TLI.getTypeAction(*DAG.getContext(), Op.getValueType()) == TargetLowering::TypeLegal || Op.getOpcode() == ISD::TargetConstant || Op.getOpcode() == ISD::Register) && "Unexpected illegal type!"' failed.

Next make the interesting.sh script:

#!/bin/sh

$HOME/local/llvm18-assert/bin/clang -O3 -c $1 2>&1 | \
  grep -F 'LegalizeDAG.cpp:991: void {anonymous}::SelectionDAGLegalize::LegalizeOp(llvm::SDNode*' || \
  exit 1

exit 0

Run llvm-reduce like this:

$HOME/local/llvm18-assert/bin/llvm-reduce --test=interesting.sh crash.ll

Using opt and llc instead of clang may result in faster reduction.