This is a bit of a holy grail, that in practice isn't actually that important. https://stackoverflow.com/questions/523307/semantic-diff-uti... has a good overview of some of the possibilities.

Makes for amazing demos. Might some day be what we are all using. My hopes have been dampened severely by how long this has taken to make any head way. (And, honestly, it couples your diff tool to the rest of your tool chain rather heavily. I can see why it has a hard time taking off.)