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Commitc60e520

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Use memcpy instead of a byte loop in pglz_decompress
The byte loop used in pglz_decompress() because of possible overlap maybe quite inefficient, so this commit replaces it with memcpy. The gainsdo depend on the data (compressibility) and hardware, but seem to bequite significant.Author: Andrey BorodinReviewed-by: Michael Paquier, Konstantin Knizhnik, TelsDiscussion:https://postgr.es/m/469C9ED9-348C-4FE7-A7A7-B0FA671BEE4C@yandex-team.ru
1 parent6d61c3f commitc60e520

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1 file changed

+41
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‎src/common/pg_lzcompress.c

Lines changed: 41 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -714,11 +714,13 @@ pglz_decompress(const char *source, int32 slen, char *dest,
714714
if (ctrl&1)
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{
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/*
717-
* Otherwise it contains the match length minus 3 and the
718-
* upper 4 bits of the offset. The next following byte
719-
* contains the lower 8 bits of the offset. If the length is
720-
* coded as 18, another extension tag byte tells how much
721-
* longer the match really was (0-255).
717+
* Set control bit means we must read a match tag. The match
718+
* is coded with two bytes. First byte uses lower nibble to
719+
* code length - 3. Higher nibble contains upper 4 bits of the
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* offset. The next following byte contains the lower 8 bits
721+
* of the offset. If the length is coded as 18, another
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* extension tag byte tells how much longer the match really
723+
* was (0-255).
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*/
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int32len;
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int32off;
@@ -731,16 +733,44 @@ pglz_decompress(const char *source, int32 slen, char *dest,
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/*
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* Now we copy the bytes specified by the tag from OUTPUT to
734-
* OUTPUT. It is dangerous and platform dependenttouse
735-
*memcpy() here, because the copied areas could overlap
736-
*extremely!
736+
* OUTPUT (copy len bytes from dp - offtodp). The copied
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*areas could overlap, to preven possible uncertainty, we
738+
*copy only non-overlapping regions.
737739
*/
738740
len=Min(len,destend-dp);
739-
while (len--)
741+
while (off<len)
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{
741-
*dp=dp[-off];
742-
dp++;
743+
/*---------
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* When offset is smaller than length - source and
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* destination regions overlap. memmove() is resolving
746+
* this overlap in an incompatible way with pglz. Thus we
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* resort to memcpy()-ing non-overlapping regions.
748+
*
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* Consider input: 112341234123412341234
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* At byte 5 here ^ we have match with length 16 and
751+
* offset 4. 11234M(len=16, off=4)
752+
* We are decoding first period of match and rewrite match
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* 112341234M(len=12, off=8)
754+
*
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* The same match is now at position 9, it points to the
756+
* same start byte of output, but from another position:
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* the offset is doubled.
758+
*
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* We iterate through this offset growth until we can
760+
* proceed to usual memcpy(). If we would try to decode
761+
* the match at byte 5 (len=16, off=4) by memmove() we
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* would issue memmove(5, 1, 16) which would produce
763+
* 112341234XXXXXXXXXXXX, where series of X is 12
764+
* undefined bytes, that were at bytes [5:17].
765+
*---------
766+
*/
767+
memcpy(dp,dp-off,off);
768+
len-=off;
769+
dp+=off;
770+
off+=off;
743771
}
772+
memcpy(dp,dp-off,len);
773+
dp+=len;
744774
}
745775
else
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{

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