llvm-mingw, ready to fetch

llvm-mingw is a clang toolchain that builds Windows programs for aarch64, armv7, i686 and x86_64. This gets you release 20260826 of it, unpacked into a directory, without downloading and expanding a release archive on every machine and every CI runner.

Only the parts you do not already have are downloaded, so a second toolchain and the next release both cost much less than the first.

Before you start

Fetching is done by desync, one binary, kept in this repository so there is nothing to install or build. Take the one for your machine:

# Linux, including WSL
curl -fsSLo desync https://github.com/V-Sekai/casync-llvm-mingw/raw/main/desync/linux_amd64/desync
chmod +x desync

# macOS, Apple silicon. For an Intel Mac use darwin_amd64.
curl -fsSLo desync https://github.com/V-Sekai/casync-llvm-mingw/raw/main/desync/darwin_arm64/desync
chmod +x desync

On Windows, in PowerShell:

curl.exe -fsSLo desync.exe https://github.com/V-Sekai/casync-llvm-mingw/raw/main/desync/windows_amd64/desync.exe

linux_arm64 and windows_arm64 are there too. If you would rather build it, go install github.com/folbricht/desync/cmd/desync@latest gets you the same tool.

Take the toolchain for the machine that will run the compiler. The Windows one unpacks and runs on Windows as it is. The Linux and macOS ones carry symlinks that a Windows filesystem cannot hold, so if you want one of those on a Windows machine, fetch it under WSL and into your WSL home directory rather than /mnt/c.

Get the toolchain

Nothing needs to be cloned.

mkdir -p ~/.local/casync-store

# Linux, including WSL
./desync untar --no-same-owner --cache ~/.local/casync-store \
  --store https://v-sekai.github.io/casync-llvm-mingw/store \
  --index https://github.com/V-Sekai/casync-llvm-mingw/raw/main/llvm-mingw-20260826-ucrt-ubuntu-22.04-x86_64.caidx \
  llvm-mingw

# macOS
./desync untar --no-same-owner --cache ~/.local/casync-store \
  --store https://v-sekai.github.io/casync-llvm-mingw/store \
  --index https://github.com/V-Sekai/casync-llvm-mingw/raw/main/llvm-mingw-20260826-ucrt-macos-universal.caidx \
  llvm-mingw

On Windows, in PowerShell:

New-Item -ItemType Directory -Force $HOME\.local\casync-store
.\desync.exe untar --no-same-owner --cache $HOME\.local\casync-store --store https://v-sekai.github.io/casync-llvm-mingw/store --index https://github.com/V-Sekai/casync-llvm-mingw/raw/main/llvm-mingw-20260826-ucrt-x86_64.caidx llvm-mingw

Expect 600 to 750 MB in the llvm-mingw directory. It takes a few seconds on Linux or macOS, and a minute or two on Windows, which writes the 9317 files more slowly. Keep the --cache directory: the next toolchain you fetch reuses what is already in it.

Use it

export PATH="$PWD/llvm-mingw/bin:$PATH"
x86_64-w64-mingw32-clang --version

Then build for Windows from wherever you are:

echo 'int main(void){return 0;}' > hello.c
x86_64-w64-mingw32-clang hello.c -o hello.exe

Swap the triple for aarch64-, armv7- or i686-w64-mingw32-clang to target the other architectures. clang++, ld, ar and the rest are in the same bin directory under the same prefixes. On Windows the compilers are also there under their plain names, so llvm-mingw\bin\clang.exe works.

If something goes wrong

What you see	What it means
`desync: command not found`	Run it as `./desync`, from the directory you downloaded it into.
`The system cannot find the file specified`, naming your cache	The `--cache` directory has to exist first. Run the `mkdir` line above.
`operation not permitted` on the Linux or macOS toolchain	You are on a Windows filesystem. Fetch those two under WSL, into your WSL home directory.
The download starts from nothing every time	Pass the same `--cache` directory on every run.

Rebuild it for a newer release

Put the upstream tag in VERSION, replace sources.sha256 with the checksums of that release’s assets, and run ./build.sh on Linux. It fetches each asset, fails if a checksum does not match, extracts it, and rebuilds the indexes. ./check_store.sh then verifies the result and ./extract.sh unpacks the toolchains from the checked-out store, both of which CI runs on every pull request. All three scripts pick the right desync/ binary for the machine they are on.

Why the toolchains are kept this way, and how the chunk size was chosen, are recorded in the decision 20260828-prebuilt-toolchains-as-casync-stores in manuals.