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