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MERGE STRATEGIES

The merge mechanism (git merge and git pull commands) allows the backend merge strategies to be chosen with -s option. Some strategies can also take their own options, which can be passed by giving -X<option> arguments to git merge and/or git pull.

ort

This is the default merge strategy when pulling or merging one branch. This strategy can only resolve two heads using a 3-way merge algorithm. When there is more than one common ancestor that can be used for 3-way merge, it creates a merged tree of the common ancestors and uses that as the reference tree for the 3-way merge. This has been reported to result in fewer merge conflicts without causing mismerges by tests done on actual merge commits taken from Linux 2.6 kernel development history. Additionally this strategy can detect and handle merges involving renames. It does not make use of detected copies. The name for this algorithm is an acronym ("Ostensibly Recursive’s Twin") and came from the fact that it was written as a replacement for the previous default algorithm, recursive.

The ort strategy can take the following options:

ours

This option forces conflicting hunks to be auto-resolved cleanly by favoring our version. Changes from the other tree that do not conflict with our side are reflected in the merge result. For a binary file, the entire contents are taken from our side.

This should not be confused with the ours merge strategy, which does not even look at what the other tree contains at all. It discards everything the other tree did, declaring our history contains all that happened in it.

theirs

This is the opposite of ours; note that, unlike ours, there is no theirs merge strategy to confuse this merge option with.

ignore-space-change
ignore-all-space
ignore-space-at-eol
ignore-cr-at-eol

Treats lines with the indicated type of whitespace change as unchanged for the sake of a three-way merge. Whitespace changes mixed with other changes to a line are not ignored. See also git-diff[1] -b, -w, --ignore-space-at-eol, and --ignore-cr-at-eol.

  • If their version only introduces whitespace changes to a line, our version is used;

  • If our version introduces whitespace changes but their version includes a substantial change, their version is used;

  • Otherwise, the merge proceeds in the usual way.

renormalize

This runs a virtual check-out and check-in of all three stages of a file when resolving a three-way merge. This option is meant to be used when merging branches with different clean filters or end-of-line normalization rules. See "Merging branches with differing checkin/checkout attributes" in gitattributes[5] for details.

no-renormalize

Disables the renormalize option. This overrides the merge.renormalize configuration variable.

find-renames[=<n>]

Turn on rename detection, optionally setting the similarity threshold. This is the default. This overrides the merge.renames configuration variable. See also git-diff[1] --find-renames.

rename-threshold=<n>

Deprecated synonym for find-renames=<n>.

subtree[=<path>]

This option is a more advanced form of subtree strategy, where the strategy makes a guess on how two trees must be shifted to match with each other when merging. Instead, the specified path is prefixed (or stripped from the beginning) to make the shape of two trees to match.

recursive

This can only resolve two heads using a 3-way merge algorithm. When there is more than one common ancestor that can be used for 3-way merge, it creates a merged tree of the common ancestors and uses that as the reference tree for the 3-way merge. This has been reported to result in fewer merge conflicts without causing mismerges by tests done on actual merge commits taken from Linux 2.6 kernel development history. Additionally this can detect and handle merges involving renames. It does not make use of detected copies. This was the default strategy for resolving two heads from Git v0.99.9k until v2.33.0.

The recursive strategy takes the same options as ort. However, there are three additional options that ort ignores (not documented above) that are potentially useful with the recursive strategy:

patience

Deprecated synonym for diff-algorithm=patience.

diff-algorithm=[patience|minimal|histogram|myers]

Use a different diff algorithm while merging, which can help avoid mismerges that occur due to unimportant matching lines (such as braces from distinct functions). See also git-diff[1] --diff-algorithm. Note that ort specifically uses diff-algorithm=histogram, while recursive defaults to the diff.algorithm config setting.

no-renames

Turn off rename detection. This overrides the merge.renames configuration variable. See also git-diff[1] --no-renames.

resolve

This can only resolve two heads (i.e. the current branch and another branch you pulled from) using a 3-way merge algorithm. It tries to carefully detect criss-cross merge ambiguities. It does not handle renames.

octopus

This resolves cases with more than two heads, but refuses to do a complex merge that needs manual resolution. It is primarily meant to be used for bundling topic branch heads together. This is the default merge strategy when pulling or merging more than one branch.

ours

This resolves any number of heads, but the resulting tree of the merge is always that of the current branch head, effectively ignoring all changes from all other branches. It is meant to be used to supersede old development history of side branches. Note that this is different from the -Xours option to the recursive merge strategy.

subtree

This is a modified ort strategy. When merging trees A and B, if B corresponds to a subtree of A, B is first adjusted to match the tree structure of A, instead of reading the trees at the same level. This adjustment is also done to the common ancestor tree.

With the strategies that use 3-way merge (including the default, ort), if a change is made on both branches, but later reverted on one of the branches, that change will be present in the merged result; some people find this behavior confusing. It occurs because only the heads and the merge base are considered when performing a merge, not the individual commits. The merge algorithm therefore considers the reverted change as no change at all, and substitutes the changed version instead.

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