[TUHS] A Reiser tour de force

[Steffen Nurpmeso]

Jon Steinhart wrote in
 <202204011726.231HQFm03349496 at darkstar.fourwinds.com>:
 |David Barto writes:
 |>> On Apr 1, 2022, at 8:59 AM, Douglas McIlroy <douglas.mcilroy at dartmouth.e\
 |>> du> wrote:
 |>> 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
 ...
 |>> 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.
 ...
 |> Does this exist for the rest of us to study?
 ...
 |It's not insanely complicated by modern standards.  Without any knowledge
 |of other work, I did the same thing for a 68020 based graphics system where
 |the JIT code went into the I-cache and was amazingly fast for its day.
 |
 |If I remember correctly, things started with an outer-loop test to see
 |if there were overlapping regions to determine whether to go forward
 |to backwards to avoid having the destination trash the source.
 ...

Only to add that "modern standard" C libraries define
"overlapping" by means of exclusivity, meaning that memcpy(x,
&x[1], 1) is an invalid overlapping that requires memmove() to be
used.

 |#?132(0.02 1/76)|alp-2022:tmp$ cat t.c
 |#include <string.h>
 |void doit(char *cp);
 |int main(void){
 |  char buf[3] = {'1', '2', '\0'};
 |  doit(buf);
 |  return buf[0];
 |}
 |void doit(char *cp){
 |  memcpy(cp, &cp[1], 2);
 |}
 |#?0(0.02 1/76)|alp-2022:tmp$ gcc -D_FORTIFY_SOURCE=2 -O -o zt t.c
 |#?0(0.02 1/76)|alp-2022:tmp$ ./zt
 |Illegal instruction
 |#?132(0.02 1/76)|alp-2022:tmp$ gcc --version
 |gcc (Alpine 11.2.1_git20220219) 11.2.1 20220219
 |Copyright (C) 2021 Free Software Foundation, Inc.
 |This is free software; see the source for copying conditions.  There is NO
 |warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 |
 |#?0(0.02 1/76)|alp-2022:tmp$

Your mileage will vary though.

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)