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.

--- a/src/redisplay.c	Mon May 11 18:04:58 2015 +0100
+++ b/src/redisplay.c	Fri May 15 18:11:47 2015 +0100
@@ -7320,7 +7320,7 @@
      : marker_position (w->pointm[type]));
   EMACS_INT line;
 
-  line = buffer_line_number (b, pos, 1);
+  line = buffer_line_number (b, pos, 1, 1);
 
   {
     static Ascbyte window_line_number_buf[DECIMAL_PRINT_SIZE (long)];