GCC GIT mirror

For the gcc svn repository exists also an read only git mirror. So you can use git for your daily gcc development.

The git repository is available at:

http://gcc.gnu.org/git/gcc.git
git://gcc.gnu.org/git/gcc.git
git+ssh://gcc.gnu.org/git/gcc.git

Simple (Read-Only) Usage

WARNING: Apparently for some users git clone ends up pulling down the whole repository, not just the objects referenced from the repository heads, with unfortunate results for git gc. See this thread for more details. It's probably best for even read-only users to use the "Advanced" procedure below; although the setup is a bit complex, usage is not.

If you don't have commit access to the GCC SVN repository, and you are happy with getting the default set of trunk and 4.x branches, you can just do

git clone git://gcc.gnu.org/git/gcc.git
git config branch.trunk.rebase true

and then

git pull

any time you want to update your tree. You will probably also want to set up git-merge-changelog, as described later. To extract patches for submission, use

git format-patch origin/trunk

Advanced Usage

The above works fine for browsing the repository and simple hacking, but doesn't really mesh well with the SVN model. Here's what seems to me to be the optimal way of using git-svn with the GCC repository. Please email me (jason) if you have any questions/issues with these instructions.

Setup

mkdir gcc-git
cd gcc-git
git init
# Need to use this exact SVN url for gcc.git and git-svn to work together.  Adding an SSH username to the url is ok.
git svn init -s svn+ssh://gcc.gnu.org/svn/gcc
git remote add origin git://gcc.gnu.org/git/gcc.git
# Get the trunk.
git config remote.origin.fetch refs/remotes/trunk:refs/remotes/trunk
# Also get the various release branches so we can refer to them as needed.
for f in 2_95 3_0 3_1 3_2 3_3 3_4 4_0 4_1 4_2 4_3 4_4; do git config --add remote.origin.fetch refs/remotes/gcc-$f-branch:refs/remotes/gcc-$f-branch; done
for f in 1_00 1_1; do git config --add remote.origin.fetch refs/remotes/egcs_${f}_branch:refs/remotes/egcs_${f}_branch; done
# Add more branches if you want, e.g.
# git config --add remote.origin.fetch refs/remotes/lto:refs/remotes/lto
# If you want all branches, do
# git config remote.origin.fetch refs/remotes/*:refs/remotes/*
# Actually pull down the stuff.
git fetch
# Tell pulls in tracking branches to rebase in order to work better with SVN.
git config branch.autosetuprebase remote
# Create master branch tracking SVN trunk
git checkout -b master trunk
# Ignore the same files that SVN does.
git svn show-ignore >> .git/info/exclude

and you're all set to start hacking. Note that the first remote.origin.fetch line doesn't use --add because we aren't interested in origin's heads, just its remotes that track SVN. The uncommented lines pull down the SVN trunk and release branches; look at the commented lines if you want other branches. Pulling all branches increases the size of the .git directory by about 30% over just trunk and release branches.

If you want a branch that lives in a subdirectory of branches, such as redhat/gcc-4_4-branch, you can't get it from the gcc.git mirror for some reason, so leave out the remote.origin.fetch line for that branch (and don't use the wildcard fetch commented out above), and use git svn fetch to fetch the branch from SVN:

git config --unset svn-remote.svn.branches
git config --unset svn-remote.svn.tags
git config --add svn-remote.svn.fetch branches/redhat/gcc-4_4-branch:refs/remotes/redhat/gcc-4_4-branch
git svn fetch
git svn init -s svn+ssh://gcc.gnu.org/svn/gcc

clearing branches and tags across the fetch so it doesn't pull the whole world down from SVN (and take forever).

git-merge-changelog

Before long you'll get frustrated with git's handling of ChangeLog merges, which is just as bad as SVN's. But there's a fix for that!

git clone git://git.savannah.gnu.org/gnulib.git
cd gnulib
./gnulib-tool --create-testdir --dir=/tmp/testdir123 git-merge-changelog
cd /tmp/testdir123
./configure
make
make install
git config --global merge.merge-changelog.name "GNU-style ChangeLog merge driver"
git config --global merge.merge-changelog.driver "/usr/local/bin/git-merge-changelog %O %A %B"
echo "ChangeLog   merge=merge-changelog" >> ~/gcc-git/.git/info/attributes

Unfortunately, for some reason the arguments to git-merge-changelog are in a different order when rebasing, and it isn't smart enough to notice that we've set our branches up to rebase automatically. So we explicitly say "git pull --rebase" below so it will put our ChangeLog entries at the top of the file. Alternatively, you could set the environment variable GIT_UPSTREAM.

Usage

Pull changes from the git mirror as desired using

git pull --rebase

or from the SVN repository (e.g. if you need a change that isn't in the git mirror yet, or are working on a branch that isn't in the git mirror) using

git svn rebase

You probably don't want to use git svn fetch other than for the initial import of a branch, as shown above.

To look at your local changes, use

git log -p trunk..

To push all of them to the SVN server, use

git svn dcommit

Making Patches

git diff produces unified diffs, while the GCC project prefers context diffs. To make a context diff, create a wrapper script somewhere:

cat > ~/bin/git_diff_wrapper <<EOF
#! /bin/sh
exec diff -p -L "$1" "$2" "$5" | cat
EOF
chmod +x ~/bin/git_diff_wrapper

and when you want to make a context diff for submitting a patch, do

GIT_EXTERNAL_DIFF=git_diff_wrapper git diff trunk

Working with other branches

To switch your current working directory to a different branch, first set up a local tracking branch:

git checkout -b gcc44 gcc-4_4-branch

Then you can switch between branches with

git checkout master
git checkout gcc44

To make a separate working directory for hacking on another branch at the same time, you can do

mkdir lto
cd lto
git init
# Tell git not to re-fetch shared objects in your main repository
echo ~/gcc-git/.git/objects >> .git/objects/info/alternates
cp ~/gcc-git/.git/info/{attributes,exclude} .git/info/
# Force local fetch of trunk; see below
git fetch ~/gcc-git refs/remotes/trunk:refs/remotes/trunk
git svn init -s svn+ssh://gcc.gnu.org/svn/gcc
git remote add origin git://gcc.gnu.org/git/gcc.git
git config remote.origin.fetch refs/remotes/trunk:refs/remotes/trunk
git config --add remote.origin.fetch refs/remotes/lto:refs/remotes/lto
git fetch
git config branch.autosetuprebase remote
git checkout -b master lto

And then do pull/rebase/dcommit as with your main repository. This repository should be significantly smaller than the main one as it only contains the changes on the branch itself; it uses your main repository for anything on the trunk. In fact, if you do this for a branch that's already in your main repository, the .git directory in the new repository will be less than 10MB. If you pull in this repository sooner than in the main one, you'll get a few duplicate objects here, but you can clean them up with 'git gc'. You can use this repository without doing anything in your trunk repository as long as you don't delete the trunk repository or remove branches that this one is using.

The double fetch is to work around an apparent shortcoming of the "alternates" functionality (at least with 1.6.2.2): if the alternate repository doesn't have the objects at the heads of *all* the branches you are fetching with the initial fetch, git will download everything. So if your main repository is even one commit behind the head, git will re-download the whole shebang all over again. After the first fetch git is smarter about only downloading deltas, so the second fetch will only download the differences between trunk and the branch.

Note that you shouldn't use git cherry-pick to copy changes between SVN branches, as that brings along svn metadata which could confuse git-svn into thinking you're on a different SVN branch. Better I think to use git show/git apply, or do the merge in SVN directly (which also handles new testcases better).

Stacked Git

I'm also finding Stacked Git useful for hacking on GCC, since it makes it easy to pop off patches that I'm still working on so I can apply the one I've just finished testing. See the StGit webpage for information on general usage; for using it with git-svn what I find works best is to do

Update to current git head:

stg pull

Commit patch FOO to SVN:

# Pop all current patches
stg pop -a
# Make sure I'm up-to-date with SVN head
git svn rebase
# Push back the patch I want to commit to SVN
stg push FOO
# And commit it so that it's a normal git commit rather than an stg patch
stg commit
# Finally, push it out to SVN
git svn dcommit
# And reapply my other patches to my working tree
stg push -am

Useful aliases

Here are some aliases I'm finding useful (to add to your ~/.gitconfig):

[alias]
        # The current branch.  Surely there's an easier way to get this?
        cbr = "!git branch 2>/dev/null | sed -n 's/^[*] \\(.*\\)/\\1/p'"
        # The branch being tracked by the current branch.
        track = "!git config --get branch.`git cbr`.merge"
        # Show all the local commits on this branch.
        lg = "!git log -p `git track`.."
        # Show all the local changes on this branch as one big diff.
        df = "!git diff `git track`"
        # Reorganize the local commits on this branch.
        rb = "!git rebase -i `git track`"

Moving changes between machines

I tend to do development on my laptop, and then send patches off to a hefty compile server to do full regression testing. Before git, I did this by rsyncing diffs, but git simplifies the process. First, add to your gcc-git/.git/config:

[remote "testbox"]
        url = testbox.foo.bar:gcc-git
        push = +master:master

Then to push your current changes to the testbox, just "git push testbox". Note that you'll then need to "git co -f" or "git reset --hard" on the testbox to update the working copy from the git repository. This could be another alias:

        tup = "!git push testbox && ssh testbox.foo.bar 'cd gcc-git; git co -f'"

Further Reading

An introduction to git-svn for Subversion/SVK users and deserters

git-svn workflow