GNU @command{diff} runs quite efficiently; however, in some circumstances you can cause it to run faster or produce a more compact set of changes.
One way to improve @command{diff} performance is to use hard or symbolic links to files instead of copies. This improves performance because @command{diff} normally does not need to read two hard or symbolic links to the same file, since their contents must be identical. For example, suppose you copy a large directory hierarchy, make a few changes to the copy, and then often use `diff -r' to compare the original to the copy. If the original files are read-only, you can greatly improve performance by creating the copy using hard or symbolic links (e.g., with GNU `cp -lR' or `cp -sR'). Before editing a file in the copy for the first time, you should break the link and replace it with a regular copy.
You can also affect the performance of GNU @command{diff} by giving it options that change the way it compares files. Performance has more than one dimension. These options improve one aspect of performance at the cost of another, or they improve performance in some cases while hurting it in others.
The way that GNU @command{diff} determines which lines have changed always comes up with a near-minimal set of differences. Usually it is good enough for practical purposes. If the @command{diff} output is large, you might want @command{diff} to use a modified algorithm that sometimes produces a smaller set of differences. The @option{-d} or @option{--minimal} option does this; however, it can also cause @command{diff} to run more slowly than usual, so it is not the default behavior.
When the files you are comparing are large and have small groups of changes scattered throughout them, you can use the @option{--speed-large-files} option to make a different modification to the algorithm that @command{diff} uses. If the input files have a constant small density of changes, this option speeds up the comparisons without changing the output. If not, @command{diff} might produce a larger set of differences; however, the output will still be correct.
Normally @command{diff} discards the prefix and suffix that is common to both files before it attempts to find a minimal set of differences. This makes @command{diff} run faster, but occasionally it may produce non-minimal output. The @option{--horizon-lines=lines} option prevents @command{diff} from discarding the last lines lines of the prefix and the first lines lines of the suffix. This gives @command{diff} further opportunities to find a minimal output.
Suppose a run of changed lines includes a sequence of lines at one end and there is an identical sequence of lines just outside the other end. The @command{diff} command is free to choose which identical sequence is included in the hunk. In this case, @command{diff} normally shifts the hunk's boundaries when this merges adjacent hunks, or shifts a hunk's lines towards the end of the file. Merging hunks can make the output look nicer in some cases.
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