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unpack_trees(): skip trees that are the same in all input

unpack_trees() merges two trees (the current HEAD and the destination commit) when switching to another branch, checking and updating the index entry where the destination differs from the current HEAD. It merges three trees (the common ancestor, the current HEAD and the other commit) when performing a three-way merge, checking and updating the index entry when the merge result differs from the current HEAD. It does so by walking the input trees in parallel all the way down to the leaves. One common special case is a directory is identical across the trees involved in the merge. In such a case, we do not have to descend into the directory at all---we know that the end result is to keep the entries in the current index. This optimization cannot be applied in a few special cases in unpack_trees(), though. We need to descend into the directory and update the index entries from the target tree in the following cases: - When resetting (e.g. "git reset --hard"); and - When checking out a tree for the first time into an empty working tree (e.g. "git read-tree -m -u HEAD HEAD" with missing .git/index). Signed-off-by: Junio C Hamano <gitster@pobox.com>

Junio C Hamano committed Dec 22, 2010 at 14:13 UTC 83c90314aa27ae3768c04375d02e4f3fb12b726d

1 file changed +56

unpack-trees.c

+56

@@ -436,6 +436,10 @@ static int switch_cache_bottom(struct traverse_info *info)

436 return ret;

437 }

438

439 +static int fast_forward_merge(int n, unsigned long dirmask,

440 + struct name_entry *names,

441 + struct traverse_info *info);

442 +

443 static int traverse_trees_recursive(int n, unsigned long dirmask,

444 unsigned long df_conflicts,

445 struct name_entry *names,

@@ -447,6 +451,11 @@ static int traverse_trees_recursive(int n, unsigned long dirmask,

451 struct traverse_info newinfo;

452 struct name_entry *p;

453

454 + if (!df_conflicts) {

455 + int status = fast_forward_merge(n, dirmask, names, info);

456 + if (status)

457 + return status;

458 + }

459 p = names;

460 while (!p->mode)

461 p++;

@@ -694,6 +703,53 @@ static struct cache_entry *find_cache_entry(struct traverse_info *info,

703 return NULL;

704 }

705

706 +static int fast_forward_merge(int n, unsigned long dirmask,

707 + struct name_entry *names,

708 + struct traverse_info *info)

709 +{

710 + int i;

711 + struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };

712 + struct unpack_trees_options *o = info->data;

713 +

714 + /* merging two or more trees with an identical subdirectory? */

715 + if ((n < 2) || ((1UL << n) - 1) != dirmask ||

716 + !o->merge || o->reset || o->initial_checkout)

717 + return 0;

718 + for (i = 1; i < n; i++)

719 + if (hashcmp(names[i-1].sha1, names[i].sha1))

720 + return 0;

721 +

722 + /*

723 + * Instead of descending into the directory, keep the contents

724 + * of the current index.

725 + */

726 + while (1) {

727 + struct cache_entry *ce;

728 + ce = next_cache_entry(o);

729 + if (!ce)

730 + break;

731 + /* Is the entry still in that directory? */

732 + if (do_compare_entry(ce, info, names))

733 + break;

734 + /*

735 + * Note: we do not just run unpack_index_entry() here,

736 + * as the callback may want to compare what is in the

737 + * index with what are from the HEAD and the other tree

738 + * and reject the merge. We pretend that ancestors, the

739 + * HEAD and the other tree all have the same contents as

740 + * the current index, which is a lie, but it works.

741 + */

742 + for (i = 0; i < n + 1; i++)

743 + src[i] = ce;

744 + mark_ce_used(ce, o);

745 + if (call_unpack_fn(src, o) < 0)

746 + return unpack_failed(o, NULL);

747 + if (ce_stage(ce))

748 + mark_ce_used_same_name(ce, o);

749 + }

750 + return dirmask;

751 +}

752 +

753 static void debug_path(struct traverse_info *info)

754 {

755 if (info->prev) {