F18 is now part of LLVM and it is called Flang in the LLVM repository.

The code from this repository can now be found at https://github.com/llvm/llvm-project/tree/main/flang/

If you have a local fork of this repository or pull-requests that need to be migrated to the LLVM monorepo, the following recipe may help you:

# From your local F18 clone:
$ git clone https://github.com/newren/git-filter-repo /tmp/git-filter-repo
$ /tmp/git-filter-repo/git-filter-repo --path-rename :flang/ --force  --message-callback 'return re.sub(b"(#[0-9]+)", b"flang-compiler/f18\\1", message)' --refs <branch name>

After this, all the commits from the previous upstream F18 should match the ones in the monorepo now. If you don't provide the --refs option, this will rewrite all the branches in your repo.

From there you should be able to rebase any of your branch/commits on top of the LLVM monorepo:

$ git remote set-url origin git@github.com:llvm/llvm-project.git
$ git fetch origin
$ git rebase origin/main -i

Cherry-picking commits should also work, if you checkout the main branch from the monorepo you can git cherry-pick <sha1> from your (rewritten) branches.

You can also export patches with git format-patch <range> and re-apply it on the monorepo using git am <patch file>.