The only documentation about debugging the redisplay of emacs I did find in ./etc/DEBUG
** Debugging Emacs Redisplay problems
The src/.gdbinit file defines many useful commands for dumping redisplay related data structures in a terse and user-friendly format:
`ppt' prints value of PT, narrowing, and gap in current buffer. `pit' dumps the current display iterator `it'. `pwin' dumps the current window 'win'. `prow' dumps the current glyph_row `row'.
`pg' dumps the current glyph `glyph'. `pgi' dumps the next glyph. `pgrow' dumps all glyphs in current glyph_row `row'. `pcursor' dumps current output_cursor.
However, this tells me nothing about an efficient debugging of redisplay_internal, and about display in general.