Hi Alban, On Fri, 24 Jan 2020, Alban Gruin wrote: > One of the first things done when using a sequencer-based > rebase (ie. `rebase -i', `rebase -r', or `rebase -m') is to make a todo > list. This requires knowledge of the commit range to rebase. To get > the oid of the last commit of the range, the tip of the branch to rebase > is checked out with prepare_branch_to_be_rebased(), then the oid of the > head is read. After this, the tip of the branch is not even modified. > The `am' backend, on the other hand, does not check out the branch. > > On big repositories, it's a performance penalty: with `rebase -i', the > user may have to wait before editing the todo list while git is > extracting the branch silently, and "quiet" rebases will be slower than > `am'. > > Since we already have the oid of the tip of the branch in > `opts->orig_head', it's useless to switch to this commit. > > This removes the call to prepare_branch_to_be_rebased() in > do_interactive_rebase(), and adds a `orig_head' parameter to > get_revision_ranges(). prepare_branch_to_be_rebased() is removed as it > is no longer used. At this point, I am a bit puzzled as a reader: why can we just drop this? My immediate reaction was: isn't this required to switch to a new branch when `switch_to` is non-`NULL`? So I went digging a little. The `prepare_branch_to_be_rebased()` call was introduced in 53bbcfbde7c (rebase -i: implement the main part of interactive rebase as a builtin, 2018-09-27). And looking at the `git-rebase--interactive` part of that patch, it becomes relatively obvious that we inherited this behavior from the shell scripting days. 2c58483a598 (rebase -i: rewrite setup_reflog_action() in C, 2018-08-10) converted the `setup_reflog_action` function (which oddly enough not only set up the reflog action, but also switched to a new branch if so configured). That function was introduced in d48f97aa854 (rebase: reindent function git_rebase__interactive, 2018-03-23), but that was _still_ not the commit that introduced that "let's check out the upstream" behavior. It goes _all_ the way back to 1b1dce4bae7 (Teach rebase an interactive mode, 2007-06-25). Except that back then, it was only done when a branch name was provided (`git rebase -i `). So it behaved correctly. The problem was introduced in 71786f54c41 (rebase: factor out reference parsing, 2011-02-06), as it substituted the `if test ! -z "$1"` with `if test ! -z "$switch_to"`, relying on the command-line parsing of `git-rebase.sh`. But wait! Wait, wait, wait! `switch_to` is still only set in that incantation where we provide a branch name _in addition to_ an upstream commit. Ah, I think I slowly see where this is going. The problem is actually 2ec33cdd19b (rebase--interactive: don't require what's rebased to be a branch, 2010-03-14) which failed to realize that essentially the entire `git checkout` was necessary to accommodate the subsequent call a mere 8 lines further down from that `checkout`: git symbolic-ref HEAD > "$DOTEST"/head-name 2> /dev/null || echo "detached HEAD" > "$DOTEST"/head-name So that 2ec33cdd19b commit could have saved itself a lot of trouble by realizing what the role of that `git checkout` is, and should have pulled that `head-name` logic into that conditional instead of _actually_ switching to a new branch. Now, let's see what the C code does to determine "head-name". Indeed, it is already handled in the option parsing, in that monster of a function called `cmd_rebase()`. And yes, I think that `head_name` should also be mentioned in this commit message, as something like git rebase -i should eventually indeed switch to the specified branch, and this here patch does _not_ break that promise. This is my only concern with this patch, though, therefore: Acked-by: Johannes Schindelin Thanks, Dscho > This introduces a visible change: as we do not switch on the tip of the > branch to rebase, no reflog entry is created at the beginning of the > rebase for it. > > Unscientific performance measurements, performed on linux.git, are as > follow: > > Before this patch: > > $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50 > > real 0m8,940s > user 0m6,830s > sys 0m2,121s > > After this patch: > > $ time git rebase -m --onto v4.18 463fa44eec2fef50~ 463fa44eec2fef50 > > real 0m1,834s > user 0m0,916s > sys 0m0,206s > > Reported-by: SZEDER Gábor > Signed-off-by: Alban Gruin > --- > > Added a line in the first paragraph to make it clear that the `am' > backend is not affected. > > builtin/rebase.c | 18 +++++------------- > sequencer.c | 14 -------------- > sequencer.h | 3 --- > 3 files changed, 5 insertions(+), 30 deletions(-) > > diff --git a/builtin/rebase.c b/builtin/rebase.c > index 8081741f8a..6154ad8fa5 100644 > --- a/builtin/rebase.c > +++ b/builtin/rebase.c > @@ -246,21 +246,17 @@ static int edit_todo_file(unsigned flags) > } > > static int get_revision_ranges(struct commit *upstream, struct commit *onto, > - const char **head_hash, > + struct object_id *orig_head, const char **head_hash, > char **revisions, char **shortrevisions) > { > struct commit *base_rev = upstream ? upstream : onto; > const char *shorthead; > - struct object_id orig_head; > - > - if (get_oid("HEAD", &orig_head)) > - return error(_("no HEAD?")); > > - *head_hash = find_unique_abbrev(&orig_head, GIT_MAX_HEXSZ); > + *head_hash = find_unique_abbrev(orig_head, GIT_MAX_HEXSZ); > *revisions = xstrfmt("%s...%s", oid_to_hex(&base_rev->object.oid), > *head_hash); > > - shorthead = find_unique_abbrev(&orig_head, DEFAULT_ABBREV); > + shorthead = find_unique_abbrev(orig_head, DEFAULT_ABBREV); > > if (upstream) { > const char *shortrev; > @@ -314,12 +310,8 @@ static int do_interactive_rebase(struct rebase_options *opts, unsigned flags) > struct replay_opts replay = get_replay_opts(opts); > struct string_list commands = STRING_LIST_INIT_DUP; > > - if (prepare_branch_to_be_rebased(the_repository, &replay, > - opts->switch_to)) > - return -1; > - > - if (get_revision_ranges(opts->upstream, opts->onto, &head_hash, > - &revisions, &shortrevisions)) > + if (get_revision_ranges(opts->upstream, opts->onto, &opts->orig_head, > + &head_hash, &revisions, &shortrevisions)) > return -1; > > if (init_basic_state(&replay, > diff --git a/sequencer.c b/sequencer.c > index b9dbf1adb0..4dc245d7ec 100644 > --- a/sequencer.c > +++ b/sequencer.c > @@ -3715,20 +3715,6 @@ static int run_git_checkout(struct repository *r, struct replay_opts *opts, > return ret; > } > > -int prepare_branch_to_be_rebased(struct repository *r, struct replay_opts *opts, > - const char *commit) > -{ > - const char *action; > - > - if (commit && *commit) { > - action = reflog_message(opts, "start", "checkout %s", commit); > - if (run_git_checkout(r, opts, commit, action)) > - return error(_("could not checkout %s"), commit); > - } > - > - return 0; > -} > - > static int checkout_onto(struct repository *r, struct replay_opts *opts, > const char *onto_name, const struct object_id *onto, > const char *orig_head) > diff --git a/sequencer.h b/sequencer.h > index 9f9ae291e3..74f1e2673e 100644 > --- a/sequencer.h > +++ b/sequencer.h > @@ -190,9 +190,6 @@ void commit_post_rewrite(struct repository *r, > const struct commit *current_head, > const struct object_id *new_head); > > -int prepare_branch_to_be_rebased(struct repository *r, struct replay_opts *opts, > - const char *commit); > - > #define SUMMARY_INITIAL_COMMIT (1 << 0) > #define SUMMARY_SHOW_AUTHOR_DATE (1 << 1) > void print_commit_summary(struct repository *repo, > -- > 2.24.1 > >