The Diff Tool

As the XOR tool, the Diff tool performs a comparison of two layouts. In contrast to the XOR tool, it does a cell-by-cell and object-by-object comparison and reports differing cells, instances and geometrical objects. In effect, the comparison is more strict and not purely geometry-related. It does not verify the identity of the layouts on mask level but rather the exact identity of the objects that comprise the layout file. On the other hand, the Diff tool usually detects the actual changes rather than their effect on geometry.

Usually, that kind of comparison is very sensitive to "cosmetic" changes, i.e. cell renaming. KLayout's Diff tool tries to mitigate this effect with these features:

Before it does the cell-by-cell comparison it tries to detect cells which have been renamed by comparing their instantiation. That way, it can compare the right cells even though their names may be different. The basis of that functionality is a cell matching algorithm. This algorithm compares cells by taking into account their bounding boxes, shape counts per layer, number of instances and other parameters. The algorithm will choose a partner cell which matches closest with respect to these parameters. If that scheme fails, it is possible to revert to name matching by unchecking the option "Don't use names to match cells".
It allows some level of control over the strictness of the compare. For example, cell arrays can be expanded before the individual instances are compared. By default, some second-order information like users properties or certain text properties is not compared.
The diff tool can also work in "XOR" more. In that mode, the differences found are used to provide input for a subsequent, polygon-only XOR step. The result is a fair approximation of a true, as-if-flat XOR which delivers a superset of the true XOR's results. It may report some locations as being different which if fact are not, but it will not fail to report differences where there are some. Compared with the XOR tool's functionality, some options are missing (i.e. tolerance), but the performance is much better.

The Diff tool is found in the "Tools" menu. In this dialog:

Select layout A and B in the "Input" section.
Uncheck "Don't use names to match cells" to revert to pure name matching. Cells which have been renamed will not be compared against then.
Check "Run XOR on differences" to select the "XOR mode".
Check "Summarize missing layers" to have missing layers reported as one difference instead of one per shape.
Check "Detailed information" to receive detailed information about every difference. Without that option, only the number of differing shapes or instances is reported.
Check "Expand cell arrays" to compare individual instances of array instances.
Check "Exact compare" to include second-order information (i.e. user properties, text orientation) in the compare.

The Diff tool will create a marker database and show the results in the marker database browser.