Notes on i80386 string assembly routines. Author: Kees J. Bot
2 Jan 1994
Remarks.
All routines set up proper stack frames, so that stack traces can be
derived from core dumps. String routines are often the ones that
get the bad pointer.
Flags are often not right in boundary cases (zero string length) on
repeated string scanning or comparing instructions. This has been
handled in sometimes nonobvious ways.
Only the eax, edx, and ecx registers are not preserved, all other
registers are. This is what GCC expects. (ACK sees ebx as scratch
too.) The direction byte is assumed to be wrong, and left clear on
exit.
Assumptions.
The average string is short, so short strings should not suffer from
smart tricks to copy, compare, or search large strings fast. This
means that the routines are fast on average, but not optimal for
long strings.
It doesn't pay to use word or longword operations on strings, the
setup time hurts the average case.
Memory blocks are probably large and on word or longword boundaries.
No unaligned word moves are done. Again the setup time may hurt the
average case. Furthermore, the author likes to enable the alignment
check on a 486.
String routines.
They have been implemented using byte string instructions. The
length of a string it usually determined first, followed by the
actual operation.
Strcmp.
This is the only string routine that uses a loop, and not
instructions with a repeat prefix. Problem is that we don't know
how long the string is. Scanning for the end costs if the strings
are unequal in the first few bytes.
Strchr.
The character we look for is often not there, or at some distance
from the start. The string is scanned twice, for the terminating
zero and the character searched, in chunks of increasing length.
Memory routines.
Memmove, memcpy, and memset use word or longword instructions if the
address(es) are at word or longword boundaries. No tricks to get
alignment after doing a few bytes. No unaligned operations.
