schwab_sort.h

@@ -21,7 +21,7 @@
 
 /** Below this many elements we do insertion sort */
 #ifndef SCHWAB_SELECTION_THRESHOLD
-#define SCHWAB_SELECTION_THRESHOLD 8
+#define SCHWAB_SELECTION_THRESHOLD 16
 #endif /* SCHWAB_SELECTION_THRESHOLD */
 
 typedef uint32_t sch_rand_state;
@@ -51,75 +51,8 @@ static inline void schwab_swap(uint32_t *a, uint32_t *b) {
 	*b = t;
 }
 
-/** Branchless conditional swap */
-#define SCH_CSWAP(a, b) do {              \
-	uint32_t _x = (a), _y = (b);          \
-	uint32_t _gt = -(_x > _y);            \
-	uint32_t _tmp = (_x ^ _y) & _gt;      \
-	(a) = _x ^ _tmp;                      \
-	(b) = _y ^ _tmp;                      \
-} while (0)
-
-/**
- * Tiny unrolled register-heapsort of exactly 5 elements.
- *
- * This heap:
- *
- *           a[0]
- *          /    \
- *      a[1]     a[2]
- *     /    \
- *  a[3]   a[4]
- *
- * @param a The array
- */
-void sch_hsort5(uint32_t* a) {
-	/* Build max heap */
-	SCH_CSWAP(a[1], a[3]);
-	SCH_CSWAP(a[1], a[4]);
-
-	/* Heapify(0) */
-	/* Max child = (a[2] > a[1]) ? 2 : 1; */
-	/* SCH_CSWAP(a[0], a[maxChild]) */
-	uint32_t cmp = -(a[2] > a[1]);
-	uint32_t i = (1 ^ 2) & cmp ^ 1;
-	/* Right selects a[2] if cmp==1, else 1 */
-	SCH_CSWAP(a[0], a[i]);
-
-	/* Sort phase */
-
-	/* 1st max to end */
-	SCH_CSWAP(a[0], a[4]);
-	/* heapify a[0..3] */
-	cmp = -(a[2] > a[1]);
-	i = (1 ^ 2) & cmp ^ 1;
-	SCH_CSWAP(a[0], a[i]);
-
-	/* 2nd max to end */
-	SCH_CSWAP(a[0], a[3]);
-	/* heapify a[0..2] */
-	cmp = -(a[2] > a[1]);
-	i = (1 ^ 2) & cmp ^ 1;
-	SCH_CSWAP(a[0], a[i]);
-
-	/* 3rd max to end */
-	SCH_CSWAP(a[0], a[2]);
-
-	/* Final two */
-	SCH_CSWAP(a[0], a[1]);
-}
-
 /** Simple insertion sort for small cases */
 inline void sch_insertion_sort(uint32_t *arr, int low, int high) {
-	/* Dual heapsort5 probably helps insertion speed */
-	/* This is sane, because insertion benefits from */
-	/* data being "basically nearly sorted" as input */
-	if(high - low > 10) {
-		sch_hsort5(&arr[0]);
-		sch_hsort5(&arr[5]);
-	}
-
-	/* "Real" insertion sort part comes here */
 	for(int i = low + 1; i <= high; ++i) {
 		uint32_t key = arr[i];
 
@@ -135,6 +68,24 @@ inline void sch_insertion_sort(uint32_t *arr, int low, int high) {
 	}
 }
 
+/** Simple insertion sort for small cases v2 - not necessarily better */
+inline void sch_insertion_sort2(uint32_t* arr, int low, int high) {
+	for(int i = low + 1; i <= high; ++i) {
+		uint32_t key = arr[i];
+
+		/* Separate load and compare to expose ILP */
+		int j = i;
+		#pragma GCC unroll 2
+		while(j > 0) {
+			uint32_t prev = arr[j - 1];
+			if (prev <= key) break;
+			arr[j] = prev;
+			--j;
+		}
+		arr[j] = key;
+	}
+}
+
 /** Simple SELECTION sort for small cases - not necessarily better */
 inline void sch_selection_sort(uint32_t* arr, int low, int high) {
 	#pragma GCC unroll 2