From 6a5fb966720fffaae28bbd9408c55414c3b8c813 Mon Sep 17 00:00:00 2001 From: Elijah Newren Date: Wed, 4 Aug 2021 05:38:01 +0000 Subject: [PATCH 1/2] Change default merge backend from recursive to ort There are a few reasons to switch the default: * Correctness * Extensibility * Performance I'll provide some summaries about each. === Correctness === The original impetus for a new merge backend was to fix issues that were difficult to fix within recursive's design. The success with this goal is perhaps most easily demonstrated by running the following: $ git grep -2 KNOWN_FAILURE t/ | grep -A 4 GIT_TEST_MERGE_ALGORITHM $ git grep test_expect_merge_algorithm.failure.success t/ $ git grep test_expect_merge_algorithm.success.failure t/ In order, these greps show: * Seven sets of submodule tests (10 total tests) that fail with recursive but succeed with ort * 22 other tests that fail with recursive, but succeed with ort * 0 tests that pass with recursive, but fail with ort === Extensibility === Being able to perform merges without touching the working tree or index makes it possible to create new features that were difficult with the old backend: * Merging, cherry-picking, rebasing, reverting in bare repositories... or just on branches that aren't checked out. * `git diff AUTO_MERGE` -- ability to see what changes the user has made to resolve conflicts so far (see commit 5291828df8 ("merge-ort: write $GIT_DIR/AUTO_MERGE whenever we hit a conflict", 2021-03-20) * A --remerge-diff option for log/show, used to show diffs for merges that display the difference between what an automatic merge would have created and what was recorded in the merge. (This option will often result in an empty diff because many merges are clean, but for the non-clean ones it will show how conflicts were fixed including the removal of conflict markers, and also show additional changes made outside of conflict regions to e.g. fix semantic conflicts.) * A --remerge-diff-only option for log/show, similar to --remerge-diff but also showing how cherry-picks or reverts differed from what an automatic cherry-pick or revert would provide. The last three have been implemented already (though only one has been submitted upstream so far; the others were waiting for performance work to complete), and I still plan to implement the first one. === Performance === I'll quote from the summary of my final optimization for merge-ort (while fixing the testcase name from 'no-renames' to 'few-renames'): Timings Infinite merge- merge- Parallelism recursive recursive of rename merge-ort v2.30.0 current detection current ---------- --------- ----------- --------- few-renames: 18.912 s 18.030 s 11.699 s 198.3 ms mega-renames: 5964.031 s 361.281 s 203.886 s 661.8 ms just-one-mega: 149.583 s 11.009 s 7.553 s 264.6 ms Speedup factors Infinite merge- merge- Parallelism recursive recursive of rename v2.30.0 current detection merge-ort ---------- --------- ----------- --------- few-renames: 1 1.05 1.6 95 mega-renames: 1 16.5 29 9012 just-one-mega: 1 13.6 20 565 And, for partial clone users: Factor reduction in number of objects needed Infinite merge- merge- Parallelism recursive recursive of rename v2.30.0 current detection merge-ort ---------- --------- ----------- --------- mega-renames: 1 1 1 181.3 Signed-off-by: Elijah Newren Signed-off-by: Junio C Hamano --- builtin/merge.c | 10 ++++++++-- builtin/rebase.c | 2 +- sequencer.c | 4 ++-- 3 files changed, 11 insertions(+), 5 deletions(-) diff --git a/builtin/merge.c b/builtin/merge.c index d7b14bf4a7..fbe21d413c 100644 --- a/builtin/merge.c +++ b/builtin/merge.c @@ -88,9 +88,9 @@ static int autostash; static int no_verify; static struct strategy all_strategy[] = { - { "recursive", DEFAULT_TWOHEAD | NO_TRIVIAL }, + { "recursive", NO_TRIVIAL }, { "octopus", DEFAULT_OCTOPUS }, - { "ort", NO_TRIVIAL }, + { "ort", DEFAULT_TWOHEAD | NO_TRIVIAL }, { "resolve", 0 }, { "ours", NO_FAST_FORWARD | NO_TRIVIAL }, { "subtree", NO_FAST_FORWARD | NO_TRIVIAL }, @@ -1484,6 +1484,12 @@ int cmd_merge(int argc, const char **argv, const char *prefix) fast_forward = FF_NO; } + if (!use_strategies && !pull_twohead && + remoteheads && !remoteheads->next) { + char *default_strategy = getenv("GIT_TEST_MERGE_ALGORITHM"); + if (default_strategy) + append_strategy(get_strategy(default_strategy)); + } if (!use_strategies) { if (!remoteheads) ; /* already up-to-date */ diff --git a/builtin/rebase.c b/builtin/rebase.c index 12f093121d..587a2f1d29 100644 --- a/builtin/rebase.c +++ b/builtin/rebase.c @@ -1713,7 +1713,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix) int i; if (!options.strategy) - options.strategy = "recursive"; + options.strategy = "ort"; strbuf_reset(&buf); for (i = 0; i < strategy_options.nr; i++) diff --git a/sequencer.c b/sequencer.c index a4e5c43fcf..d08ddae52f 100644 --- a/sequencer.c +++ b/sequencer.c @@ -636,7 +636,7 @@ static int do_recursive_merge(struct repository *r, for (i = 0; i < opts->xopts_nr; i++) parse_merge_opt(&o, opts->xopts[i]); - if (opts->strategy && !strcmp(opts->strategy, "ort")) { + if (!opts->strategy || !strcmp(opts->strategy, "ort")) { memset(&result, 0, sizeof(result)); merge_incore_nonrecursive(&o, base_tree, head_tree, next_tree, &result); @@ -3988,7 +3988,7 @@ static int do_merge(struct repository *r, o.branch2 = ref_name.buf; o.buffer_output = 2; - if (opts->strategy && !strcmp(opts->strategy, "ort")) { + if (!opts->strategy || !strcmp(opts->strategy, "ort")) { /* * TODO: Should use merge_incore_recursive() and * merge_switch_to_result(), skipping the call to From f5a3c5e6377b806c320e4220ba26e467b4c4e992 Mon Sep 17 00:00:00 2001 From: Elijah Newren Date: Wed, 4 Aug 2021 05:38:02 +0000 Subject: [PATCH 2/2] Update docs for change of default merge backend Make it clear that `ort` is the default merge strategy now rather than `recursive`, including moving `ort` to the front of the list of merge strategies. Signed-off-by: Elijah Newren Signed-off-by: Junio C Hamano --- Documentation/git-rebase.txt | 28 ++++----- Documentation/gitfaq.txt | 2 +- Documentation/merge-options.txt | 2 +- Documentation/merge-strategies.txt | 93 ++++++++++++++++-------------- Documentation/user-manual.txt | 2 +- 5 files changed, 67 insertions(+), 60 deletions(-) diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt index 73d49ec8d9..3f1030df70 100644 --- a/Documentation/git-rebase.txt +++ b/Documentation/git-rebase.txt @@ -352,8 +352,8 @@ See also INCOMPATIBLE OPTIONS below. -s :: --strategy=:: - Use the given merge strategy, instead of the default - `recursive`. This implies `--merge`. + Use the given merge strategy, instead of the default `ort`. + This implies `--merge`. + Because 'git rebase' replays each commit from the working branch on top of the branch using the given strategy, using @@ -366,7 +366,7 @@ See also INCOMPATIBLE OPTIONS below. --strategy-option=:: Pass the through to the merge strategy. This implies `--merge` and, if no strategy has been - specified, `-s recursive`. Note the reversal of 'ours' and + specified, `-s ort`. Note the reversal of 'ours' and 'theirs' as noted above for the `-m` option. + See also INCOMPATIBLE OPTIONS below. @@ -527,7 +527,7 @@ The `--rebase-merges` mode is similar in spirit to the deprecated where commits can be reordered, inserted and dropped at will. + It is currently only possible to recreate the merge commits using the -`recursive` merge strategy; different merge strategies can be used only via +`ort` merge strategy; different merge strategies can be used only via explicit `exec git merge -s [...]` commands. + See also REBASING MERGES and INCOMPATIBLE OPTIONS below. @@ -1216,16 +1216,16 @@ successful merge so that the user can edit the message. If a `merge` command fails for any reason other than merge conflicts (i.e. when the merge operation did not even start), it is rescheduled immediately. -By default, the `merge` command will use the `recursive` merge -strategy for regular merges, and `octopus` for octopus merges. One -can specify a default strategy for all merges using the `--strategy` -argument when invoking rebase, or can override specific merges in the -interactive list of commands by using an `exec` command to call `git -merge` explicitly with a `--strategy` argument. Note that when -calling `git merge` explicitly like this, you can make use of the fact -that the labels are worktree-local refs (the ref `refs/rewritten/onto` -would correspond to the label `onto`, for example) in order to refer -to the branches you want to merge. +By default, the `merge` command will use the `ort` merge strategy for +regular merges, and `octopus` for octopus merges. One can specify a +default strategy for all merges using the `--strategy` argument when +invoking rebase, or can override specific merges in the interactive +list of commands by using an `exec` command to call `git merge` +explicitly with a `--strategy` argument. Note that when calling `git +merge` explicitly like this, you can make use of the fact that the +labels are worktree-local refs (the ref `refs/rewritten/onto` would +correspond to the label `onto`, for example) in order to refer to the +branches you want to merge. Note: the first command (`label onto`) labels the revision onto which the commits are rebased; The name `onto` is just a convention, as a nod diff --git a/Documentation/gitfaq.txt b/Documentation/gitfaq.txt index afdaeab850..8c1f2d5675 100644 --- a/Documentation/gitfaq.txt +++ b/Documentation/gitfaq.txt @@ -275,7 +275,7 @@ best to always use a regular merge commit. [[merge-two-revert-one]] If I make a change on two branches but revert it on one, why does the merge of those branches include the change?:: - By default, when Git does a merge, it uses a strategy called the recursive + By default, when Git does a merge, it uses a strategy called the `ort` strategy, which does a fancy three-way merge. In such a case, when Git performs the merge, it considers exactly three points: the two heads and a third point, called the _merge base_, which is usually the common ancestor of diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt index f819bd8dd6..72b5318850 100644 --- a/Documentation/merge-options.txt +++ b/Documentation/merge-options.txt @@ -112,7 +112,7 @@ With --squash, --commit is not allowed, and will fail. Use the given merge strategy; can be supplied more than once to specify them in the order they should be tried. If there is no `-s` option, a built-in list of strategies - is used instead (`recursive` when merging a single head, + is used instead (`ort` when merging a single head, `octopus` otherwise). -X