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  • starry night thinks this is interesting:

Set the mark on the file you want diff to compare, put the cursor on the other file, then type =. Emacs compares the two files and opens a window with a *diff* buffer containing the output from the command.

From

Cover of Learning GNU Emacs, 3rd Edition

Note

You must use C-SPC to mark the region to use diff, the current point is the new file(if compare with old file...), use C-x C-x to change mark and point