1 Apr
2022
1 Apr
'22
9:41 p.m.
Jon Steinhart wrote in
<202204011726.231HQFm03349496(a)darkstar.fourwinds.com>:
|David Barto writes:
|>> On Apr 1, 2022, at 8:59 AM, Douglas McIlroy <douglas.mcilroy(a)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)