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quicksort pivoting strategy changes when slowdown is recognized (works well against worst cases)

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This commit is contained in:
Richard Thier 2023-07-01 06:06:03 +02:00
parent 83c79f4832
commit 4436c79821
1 changed files with 26 additions and 11 deletions
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37
thiersort.h
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@ -234,6 +234,12 @@
*/ */
#include <stdint.h> #include <stdint.h>
#ifndef TS_NO_RAND
#include <stdlib.h> /* For rand() call for pivoting in quicksort */
#define TS_RAND 1 /* For use in branches */
#else
#define TS_RAND 0 /* For use in branches */
#endif /* TS_NO_RAND */
#ifdef TS_SSE2 #ifdef TS_SSE2
#include <emmintrin.h> #include <emmintrin.h>
@ -539,7 +545,7 @@ static inline void ts_quicksort_inplace_impl(
const TSU32 bi, const TSU32 bi,
void *reent_data), void *reent_data),
void *reent_data, void *reent_data,
TSBOOL const_check); TSBOOL slowdown_detected);
/** Simple inplace quicksort for tselems */ /** Simple inplace quicksort for tselems */
static inline void ts_quicksort_inplace( static inline void ts_quicksort_inplace(
@ -575,7 +581,7 @@ static inline void ts_quicksort_inplace_impl(
const TSU32 bi, const TSU32 bi,
void *reent_data), void *reent_data),
void *reent_data, void *reent_data,
TSBOOL const_check) { TSBOOL slowdown_detected) {
/* Must do this early exit! */ /* Must do this early exit! */
// TODO: TS_UNLIKELY // TODO: TS_UNLIKELY
if(to == 0) return; if(to == 0) return;
@ -586,9 +592,18 @@ static inline void ts_quicksort_inplace_impl(
/* Pivoting */ /* Pivoting */
TSU32 len = (to - from); TSU32 len = (to - from);
TSU32 mid = from + len / 2; TSU32 pivi;
const union tskey pivotkey = arr[mid].key; /* Compiler should optimize this branch out compile time! */
TSU32 pivoti = arr[mid].i; if(!slowdown_detected || !TS_RAND) {
/* Also if TS_NO_RAND */
pivi = from + len / 2;
} else {
/* Only if there is no TS_NO_RAND and only when slowdown in progress */
/* This can help a bit to alleviate the worst case scenarios */
pivi = from + (rand() % len);
}
const union tskey pivotkey = arr[pivi].key;
TSU32 pivoti = arr[pivi].i;
/* Main loop */ /* Main loop */
TSU32 left = from; TSU32 left = from;
@ -609,7 +624,7 @@ static inline void ts_quicksort_inplace_impl(
TSU32 righti = arr[right].i; TSU32 righti = arr[right].i;
while((left < right) && !lt(rightkey, pivotkey, righti, pivoti, reent_data)) { while((left < right) && !lt(rightkey, pivotkey, righti, pivoti, reent_data)) {
/* Compiler should optimize this branch out compile time! */ /* Compiler should optimize this branch out compile time! */
if(const_check) { if(slowdown_detected) {
/* (**): Check for stepping over everything because its all the same.. */ /* (**): Check for stepping over everything because its all the same.. */
/* This with the above if gives two GE so an equality check */ /* This with the above if gives two GE so an equality check */
if(!lt(pivotkey, rightkey, pivoti, righti, reent_data)) { if(!lt(pivotkey, rightkey, pivoti, righti, reent_data)) {
@ -658,8 +673,8 @@ static inline void ts_quicksort_inplace_impl(
/* Swap */ /* Swap */
const struct tselem tmp = arr[left]; const struct tselem tmp = arr[left];
arr[left] = arr[mid]; arr[left] = arr[pivi];
arr[mid] = tmp; arr[pivi] = tmp;
/* XXX: Instead of (**) above, we could so some other sort here as a fallback (like a heapsort or merge) */ /* XXX: Instead of (**) above, we could so some other sort here as a fallback (like a heapsort or merge) */
/* This ignores constantly the same arrays */ /* This ignores constantly the same arrays */
@ -691,9 +706,9 @@ static inline void ts_quicksort_fromto(
if(from >= to) return; if(from >= to) return;
TSU32 len = (to - from); TSU32 len = (to - from);
TSU32 mid = from + len / 2; TSU32 pivi = from + len / 2;
const union tskey pivotkey = src[mid].key; const union tskey pivotkey = src[pivi].key;
TSU32 pivoti = src[mid].i; TSU32 pivoti = src[pivi].i;
TSU32 li = from; TSU32 li = from;
TSU32 ri = to - 1; TSU32 ri = to - 1;