[TUHS] A Reiser tour de force

[Douglas McIlroy <douglas.mcilroy@dartmouth.edu>]

The recent discussion about Research choosing BSD's paging over
Reiser-London's brought to mind a stunning program by Reiser that
Research did adopt.

A critical primitive in the Blit terminal was bitblt (block transfer
of a rectangular area). It was used ubiquitously, for example to
refresh data when window-stacking changed, to move data within a
window, or to pop up a menu.. The display memory was word-oriented, so
bitblt was fraught with niggling details about bit alignment and
overlap of source and destination. A general bitblt subroutine was a
rats' nest of conditionals--grossly inefficient for important special
cases like scrolling.

Bitblt got refined (i.e. elaborated) several times before Reiser did
away with it entirely. Instead he wrote a just-in-time generator of
optimal code. Thousands of distinct variants, which varied in size
from 16 to 72 bytes, could be produced by the same 400 lines of
assembler code.

Doug