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The current qsort(3) implementation ignores the sizes of partitions, and

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always perform recursion on the left partition, then use a tail call to
handle the right partition.  In the worst case this could require O(N)
levels of recursions.

Reduce the possible recursion level to log2(N) by always recursing on the
smaller partition instead.

Obtained from:	PostgreSQL 9d6077abf9d6efd992a59f05ef5aba981ea32096
This commit is contained in:
Xin LI 2017-05-19 04:59:12 +00:00
parent a3f893fc61
commit ca1578f0c0
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=318515
1 changed files with 38 additions and 15 deletions
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53
lib/libc/stdlib/qsort.c
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@ -117,7 +117,7 @@ qsort(void *a, size_t n, size_t es, cmp_t *cmp)
#endif
{
char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
size_t d, r;
size_t d1, d2;
int cmp_result;
int swaptype_long, swaptype_int, swap_cnt;
@ -137,7 +137,8 @@ loop: SWAPINIT(long, a, es);
pl = a;
pn = (char *)a + (n - 1) * es;
if (n > 40) {
d = (n / 8) * es;
size_t d = (n / 8) * es;
pl = med3(pl, pl + d, pl + 2 * d, cmp, thunk);
pm = med3(pm - d, pm, pm + d, cmp, thunk);
pn = med3(pn - 2 * d, pn - d, pn, cmp, thunk);
@ -182,21 +183,43 @@ loop: SWAPINIT(long, a, es);
}
pn = (char *)a + n * es;
r = MIN(pa - (char *)a, pb - pa);
vecswap(a, pb - r, r);
r = MIN(pd - pc, pn - pd - es);
vecswap(pb, pn - r, r);
if ((r = pb - pa) > es)
d1 = MIN(pa - (char *)a, pb - pa);
vecswap(a, pb - d1, d1);
d1 = MIN(pd - pc, pn - pd - es);
vecswap(pb, pn - d1, d1);
d1 = pb - pa;
d2 = pd - pc;
if (d1 <= d2) {
/* Recurse on left partition, then iterate on right partition */
if (d1 > es) {
#ifdef I_AM_QSORT_R
qsort_r(a, r / es, es, thunk, cmp);
qsort_r(a, d1 / es, es, thunk, cmp);
#else
qsort(a, r / es, es, cmp);
qsort(a, d1 / es, es, cmp);
#endif
if ((r = pd - pc) > es) {
/* Iterate rather than recurse to save stack space */
a = pn - r;
n = r / es;
goto loop;
}
if (d2 > es) {
/* Iterate rather than recurse to save stack space */
/* qsort(pn - d2, d2 / es, es, cmp); */
a = pn - d2;
n = d2 / es;
goto loop;
}
} else {
/* Recurse on right partition, then iterate on left partition */
if (d2 > es) {
#ifdef I_AM_QSORT_R
qsort_r(pn - d2, d2 / es, es, thunk, cmp);
#else
qsort(pn - d2, d2 / es, es, cmp);
#endif
}
if (d1 > es) {
/* Iterate rather than recurse to save stack space */
/* qsort(a, d1 / es, es, cmp); */
n = d1 / es;
goto loop;
}
}
/* qsort(pn - r, r / es, es, cmp);*/
}