Use the existing C-level line number cache within #'line-number. src/ChangeLog addition: 2015-05-15 Aidan Kehoe <kehoea@parhasard.net> * buffer.c: * buffer.c (Fline_number): New C implementation, using the line number cache of line-number.c, with a new optional BUFFER argument. * buffer.c (syms_of_buffer): Make it available to Lisp. * line-number.c (buffer_line_number): New argument, RESPECT-NARROWING, describing whether to count from the beginning of the visible region or from the beginning of the buffer. * line-number.h: * line-number.h (buffer_line_number): Update its declaration. * redisplay.c (window_line_number): Call it with the new argument. lisp/ChangeLog addition: 2015-05-15 Aidan Kehoe <kehoea@parhasard.net> * simple.el: * simple.el (line-number): Moved to buffer.c; we have an existing line number cache in C, it's a shame not to have it available.